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Hypothesis The drop deposition technique can impact contact angle measurements. We hypothesized that the drop pinch-off, during the traditionally used pendant drop technique, significantly alters the static contact angle. The capillary waves and dynamic wetting pressure generated during the pendant drop deposition are the source for forced spreading, which can be circumvented by alternative liquid-needle drop deposition techniques. Experiments To compare the role of drop pinch-off and resultant dynamic wetting pressure, we meticulously observed and quantified the entire drop deposition process using high speed imaging until the drop attains the static contact angle in both cases, namely pendant drop and liquid needle deposition technique. Conventionally used standard substrates are compared using both techniques and further compared using literature data. The capillary waves and corresponding drop shape variations are analysed for quantifying the dynamic wetting pressure by measuring drop base diameter, contact angle and centre of mass. Findings We compared three parameters - drop pinch-off, spreading behaviour and respective static contact angles along with the resultant dynamic wetting pressure for both the techniques, i.e., pendant drop and liquid-needle. For the pendant drop technique we observed a pronounced drop volume dependency of these parameters even though the corresponding Bond numbers are less than unity. In contrast, for the liquid needle there is no such dependency. With a theoretical argument corroborating experimental observations, this work highlights the importance of a well controlled drop deposition, with a minimum wetting pressure, in order to guarantee contact angle data that is independent of drop deposition effects, thereby only reflecting the substrate properties.

Background We use inkjet printing and film adhesion methods to economically develop paper-based electrowetting-on-dielectric (EWOD) mixing device. Because the periodic shrinking and stretching of droplet contact line induces surface capillary wave, the coalesced droplet on EWOD device vibrates to accelerate solute mixing on the merged spot. Methods An office inkjet printer fabricates pads of conducting silver nanoparticles to compose EWOD activation electrodes on a piece of glossy label paper, covered with one oil-coated dielectric film. Using the triggering relays of each pad, two droplets of thirty microliter individually loaded on the electrode ends are driven to coalesce. Alloy nanorod-catalyzed chemiluminescence assay can be performed on this mixing device. Findings When ac voltage of 100 Hz is applied on the electrode flakes, compared with solute transport efficiency in static condition, solute diffusivity is enhanced by 4 times using this mixing device. Linear relation (R2 = 0.99) between luminescence intensity acquired with smartphone camera and luminol concentration of aforementioned chemiluminescence assay is obtained, demonstrating reproducibility improvement for accurate determination.

The relaxation dynamics of thermal capillary waves for nanoscale liquid films on anisotropic-slip substrates are investigated using both molecular dynamics (MD) simulations and a Langevin model. The anisotropy of slip on substrates is achieved using a specific lattice plane of a face-centered cubic lattice. This surface’s anisotropy breaks the simple scalar proportionality between slip velocity and wall shear stress and requires the introduction of a slip-coefficient tensor. The Langevin equation can describe both the growth of capillary wave spectra and the relaxation of capillary wave correlations, with the former providing a time scale for the surface to reach thermal equilibrium. Temporal correlations of interfacial Fourier modes, measured at thermal equilibrium in MD, demonstrate that (i) larger slip lengths lead to a faster decay in wave correlations and (ii) unlike isotropic-slip substrates, the time correlations of waves on anisotropic-slip substrates are wave-direction-dependent. These findings emerge naturally from the proposed Langevin equation, which becomes wave-direction-dependent, agrees well with MD results, and allows us to produce experimentally verifiable predictions.\ud \ud

The physicochemical processes occurring during laser ablation of glassy carbon by femto- and nanosecond laser pulses have been studied. It is shown that laser irradiation of a material in the mode of laser drilling with various numbers of pulses leads to the formation of a homogeneous array of blind holes decorated with periodic structures of different scales and morphologies. The formation mechanism of the latter is related to the occurring thermocapillary instabilities of the melt. It is found that the depth and diameter of blind holes are determined by not only the thickness of the melt layer but also its chemical etching due to the oxidation by air oxygen. It is shown that, based on the obtained array of blind holes, one can design polymer replicas repeating the blind-hole morphology. Possible mechanisms of laser etching of glassy carbon and subsequent applications of polymer replicas are discussed.

High-resolving photo and videorecording was used to visualize the fine structure of the flow pattern upon the coalescence of a droplet, $$0.39 < D < 0.43$$ cm in diameter, falling at a velocity $$3.1 < U < 3.9$$ m/s with water in a reservoir. The spreading of droplets of water, water solution of ink, and copper and iron sulphates is studied. For all pairs of substances, in addition to cavities, crowns, and sheets with shaped outer edges, capillary waves radiated by the annular region of fluid coalescence and thin jetlets at the cavity bottom and the crown walls were observable in the splash formation regime. The jetlets penetrate the sheet and emerge from the tooth tips in the form of spikes. Sequences of small droplets fly out from the spike tips, whose positions change with time. The fluid acceleration is favored by the conversion of the available potential energy upon the annihilation of the free surfaces of the coalescing fluids.

This paper conducted an experimental study of droplet behavior characteristics occurring within a burning aviation kerosene droplet and on the droplet interface. The droplet combustion was performed in a quiescent high-temperature environment by means of the droplet suspension technique. The soot particles were used as tracers to visualize the fluid motion in the droplet. The shape variation of droplet, internal flow pattern in the burning droplet, and instabilities at gas-liquid phase interface were recorded by a high speed camera. The internal fluid flow in the droplet was revealed to be caused by the Marangoni effect, which was generated because of the surface tension gradient induced by the temperature gradient on the droplet surface. The studies showed that the Marangoni effect has a significant impact on the internal flow. The amplitude of the capillary waves on the surface of the droplet was non-uniform and presented the amplitude characteristics of large-small-some. The wavelength of the capillary waves for burning aviation kerosene droplet ranged from 110 µm to 120 µm.

Dimensionality often strongly affects material properties and phase transition behaviors, but its effects on crystal surfaces, such as roughening and premelting, have been poorly studied. Our simulation revealed that these surface behaviors are distinct in monolayer and multilayer Lennard-Jones (LJ) crystals. Solid surfaces fluctuate as capillary waves during the roughening process, but complete roughening is preempted by premelting. As the melting temperature is approached, the thickness of the premelted liquid layer approaches a constant (i.e., blocked premelting) for monolayer crystals, but diverges as a power law (i.e., complete premelting) for bilayer and trilayer crystals. The surface liquids of monolayer crystals contain crystalline patches and exhibits rough liquid-vapour and liquid-crystal interfaces, in contrast to the normal surface liquids of bilayer and trilayer crystals. Monolayer crystals melt heterogeneously from the surface without forming a hexatic phase and produce many vacancies.

Soft surfaces impacted by liquid droplets trap more air underneath than their rigid counterparts. The extended lifetime of the air film not only facilitates bouncing behaviours of the impacting droplets but also increases the possibility of interactions between the air film itself and the air cavity formed inside the droplets by capillary waves. Such interactions may cause rupture of the trapped air film by a so-called dimple inversion phenomenon and suppress bouncing. In this work, we systematically investigate the relationship between air cavity collapse and air film rupture for water droplets impacting soft, hydrophobic surfaces. By constructing a bouncing-to-wetting phase diagram based on the rupturing dynamics of the trapped air film, we observe that the regime in which air film rupture is induced by dimple inversion consistently separates the bouncing regime and the one in which wetting is caused by random rupture. We also found that air film rupture by dimple inversion in-turn affects both the collapsing dynamics of the air cavity and the resulting high-speed jet. We then provide a detailed characterisation of the collapsing dynamics of the air cavity and subsequent jetting. Agency for Science, Technology and Research (A*STAR) Nanyang Technological University This study is supported by the Nanyang Technological University (NTU) and A*STAR (SERC Grant No. 1523700102). S.M. is supported by NTU Research Scholarship.

Combined high-resolution optic and acoustic methods are used for the first time to synchronously detect the fine structure of processes initiated by submersion of a freely falling water drop in a tank with degassed liquid. The emphasis is on analyzing the evolution of the fine structure of flow pattern, the accompanying capillary waves, and the amplitude–frequency characteristics of sound packets. The sequence of acoustic signals includes a shock pulse, which is caused by the initial contact of the drop with the surface of the liquid at rest, and a series of consecutive sound packets. The shock pulse being reliably reproduced in experiments has a complex structure, which includes short segments of high-frequency oscillations and longer segments of low-frequency oscillations. The number of emitted sound packets and their parameters vary in an arbitrary way from one experiment to another while the size of drop and the distance of fall remain invariable. A series of experiments reveals cases of single and secondary packet emission with different spectral compositions of packets, as well as cases of their total absence. Analysis of data is performed with allowance for the effect of the “double layer”, which replaces the coalescence surface of liquids and retains the disturbances caused by transformation of available potential surface energy to other forms, such as thermal energy, pressure disturbance, and energy of fine currents actively affecting the gas cavities.

A charged conducting drop suspended in an insulating medium shows non-coalescence with an interface under high strength of an externally applied electric field. We perform numerical simulations of the non-coalescence phenomenon to understand the underlying physical mechanisms and the effect of electric field strength and fluid conductivity on the coalescence behavior of a conducting drop with a conducting liquid pool under highly viscous conditions. We show that two factors primarily govern the coalescence or non-coalescence of the drop with the interface. First, the magnitude of the charge transfer time scale (which governs the rate of charge transfer during contact between the drop and the pool) relative to the time scale of the capillary waves. Second, the strength of the electric forces compared to the viscous forces. We further show that for the case of macro drops ( D ∼ 2 mm), charge transfer by fluid convection dominates charge conduction at lower electric conductivities ( σ ∼ 1 0 − 8 S/m) only. Finally, we explain the non-dependence of secondary droplet’s size and charge on the fluid’s electric conductivity as observed in the experiments.

The dispersion equation is derived analytically and in a linear approximation using the dimensionless amplitude of oscillations for capillary waves on the charged surface of a layer of viscous conductive liquid on a metal wire. The metal wire is shown to stabilize the capillary waves. The wire effect becomes substantial only at high values of the wire radius. The appearance of the electric charge on the liquid layer complicates the spectrum of the realizing motions of the liquid and quantitatively changes the values of the frequencies, increments, and decrements.

A review of non-contact methods for determining the surface tension of liquids along with analysis of the physical effects underlying them is presented. A review of non-contact methods for determining the surface tension of liquids along with analysis of the physical effects underlying them is presented. The advantages, disadvantages, possible ranges of measurements and scope of non-contact methods are discussed. Passive methods do not require any stimulation of the measurement object whereas active methods, when implemented, affect the liquid under study. Electrical, electromagnetic, acoustic, jet, and mechanical methods used for excitation of capillary waves are described in analysis of the methods based on capillary waves intended to study the properties of surface-active substances. We also considered methods based on the processes occurred in an oscillating or rotating levitating drop of liquid (containerless technology) used to study the properties of melts and methods based on local deformation of the liquid surface by the electric field, acoustic pulse, and gas jet. It is shown that aerodynamic methods based on deformation of the liquid surface under control by a gas stream are the most promising for standard laboratory conditions. It is noted that containerless methods should be used in zero gravity tests, whereas the methods based on capillary waves or deformation of the liquid surface with a focused acoustic pulse appeared useful in testing inviscid liquids. Methods based on electrical deformation of the liquid surface under control are limited with the thickness of the liquid layer, relatively high sensitivity to the density and dielectric constant of the liquid, and some other uncontrolled factors. It was also noted that aerodynamic non-contact method of the surface tension determination is characterized by low sensitivity to the density and viscosity of the controlled liquid. The presented review of non-contact methods of the surface tension determination provides optimal choice of the method most suited for solving specific measurement problems.

We characterize the assembly of microscopic particles in patterned beds. The particles are immersed in a liquid template generated by vibration. We study, at the same time, the waves in the liquid surface, and the topography of the bed. Results show that the surface waves are travelling, harmonic and capillary waves, in contrast with previous studies that examined Faraday subharmonic waves. The patterned bed presents an alternation of annular ridges and troughs, with a characteristic lengthscale similar to the surface wavelength. The amplitude of the variation of the bed thickness increases as the mean bed thickness increases, and as the vibration frequency decreases; this is explained in terms of a Shields number that depends on both variables. In turn, the localization efficiency of the particles decreases with the bed thickness but increases with the pattern characteristic lengthscale. We discuss assembly design and compare with dry granular systems.

Experimental data are used to consider the behavior of gravity–capillary waves in the regions of a sharp change in the surface tension (phase transitions in films). The classical theory of surface waves is applied to systems with a model dependence of the surface tension on the surfactant concentration. The frequency shift in the regions of a sharp change in the surface tension is shown to remain small at all wavelengths and surface tensions used in experiments. However, the attenuation in these regions increases noticeably as compared to constant surface tension, which can be used to detect such transitions experimentally.

We explore the thermal field related to the self-propulsion of floating liquid marbles filled with aqueous ethanol. Cherenkov-like thermal waves arising from self-propulsion are registered. The opening angle of the thermal field Cherenkov triangle is governed by the inter-relation between the velocity of self-propulsion and the phase velocity of the capillary waves. The self-propulsion is driven by soluto-capillarity accompanied by thermo-capillarity. A semiquantitative analysis of the effect is presented. The empirical selection rule for capillary waves responsible for the mass, momentum, and heat transfer is outlined. The soluto-capillarity leads to much stronger spatial variations of the surface tension than the thermo-capillarity.

Analysis of thermal capillary waves on the surface of a liquid usually assumes incompressibility of the bulk fluid. However, for droplets or bubbles with submicronic size, or for epithelial cells whose out-of-plane elongation can be modeled by an effective 2D bulk modulus, compressibility of the internal fluid must be taken into account for the characterization of their shape fluctuations. We present a theoretical analysis of the fluctuations of a two-dimensional compressible droplet. Analytical expressions for area, perimeter and energy fluctuations are derived and compared with Cellular Potts Model (CPM) simulations. This comparison shows a very good agreement between theory and simulations, and offers a precise calibration method for CPM simulations.

Direct measurements of surface tension, viscosity and surface elasticity under the action of external forces are often impossible. In many tasks, the magnetic fluid surface tension is considered to be independent of the magnitude of the applied magnetic field and is determined by the properties of the base fluid. The anisotropy of the magnetic properties at the interface due to the jump in the fluid’s magnetization suggests the dependence of the surface tension tensor on the magnetic field. In this paper, we propose a new experimental method for studying the surface tension of a magnetic fluid in an external uniform magnetic field, depending on the orientation of the magnetic field intensity towards the liquid-gas interface. The study was carried out with the help of modified capillary wave method in a magnetic field orthogonal to the liquid surface and with the ring detachment method in the case of a longitudinal field. It was shown experimentally that the surface tension of a ferrocolloid fluid base (kerosene) does not depend either on the frequency of capillary waves excitation or on intensity of an applied external magnetic field, and corresponds to the value determined with the help of a commercial tensiometer by the standard method of ring detachment. The surface tension of the ferrofluid decreases with increasing intensity of the orthogonal magnetic field interface and with increasing frequency of acoustic vibrations. However, an increase in the field strength longitudinally directed to the interface, provokes an increase in the surface tension of the magnetic fluid. The experimental results are in qualitative agreement with theoretical predictions by A. V. Zhukov: the eigenvalues of the surface tension tensor monotonically increase with the tangential magnetic field component and monotonically decrease with an increase in its normal component.

Active matters exhibit interesting collective behaviors and novel phases, which provide an important platform for the study of nonequilibrium physics. Mixtures of active and passive particles have been intensively studied in motility-induced phase separation, but the morphology of the active–passive interface has been poorly explored. In this work, we investigate the interface morphology in two-dimensional mixtures of active and passive particles using Brownian dynamics simulations. By systematically changing the Peclet number (Pe) and area fraction (ρ), we obtain the phase diagram of the active–passive interface, including rough sharp, rough invasive and flat interdiffusive interfaces. For a sharp interface, dynamic scaling analysis in the propagation stage shows that the roughness exponent α, the growth exponent β, the time exponent κ, and the dynamic exponent z satisfy z = α/(β − κ). Such anomalous scaling indicates that the roughening behavior does not belong to the conventional universality classes with Family–Vicsek scaling for the growth of passive interfaces. On the other hand, the interface in the middle-wavelength regime during the morphology relaxation stage can be described by capillary wave theory. The mean interface position propagates with time as t1/2, which is robust at different ρ and Pe values in the propagation stage and exhibits superdiffusion in the morphology relaxation stage. These similarities and differences between the active–inactive interfaces and passive interfaces cast light on the interfacial growth of active matter.

The novel drug delivery system refers to the formulations and technologies for transporting a pharmaceutical compound in the body as it is needed to safely achieve its desired therapeutic effects. In this study, the onset vibrational amplitude of capillary surface waves for ultrasonic atomization spray is explained based on Faraday instability. Using ultrasonic frequency, the vibrational amplitude approached a critical point, and the liquid surface broke up into tiny drops. The micro-droplets were are steadily and continuously formed after the liquid feeding rate was optimized. The simulation study reported a minimum vibrational amplitude or onset value of 0.38 μm at 500 kHz frequency. The required minimum energy to atomize the drops was simulated by COMSOL Multiphysics simulation software. The simulation result agreed well with the numerical results of a subharmonic vibrational model that ocurred at 250 kHz frequency on the liquid surface. This newly designed single frequency ultrasonic atomizer showed its true physical characteristic of resonance on the fluid surface plane. Hence, this research will contribute to the future development of a single-frequency ultrasonic nebulizer and mechatronics for the generation of uniform atomized droplets.

Liquid atomization and fountain formation by focused ultrasound was first published by Wood and Loomis [1]. Since then, the cavitation-wave hypothesis emerged to explain atomization in a fountain, which states atomization arises from a combination of surface capillary waves and the collapse of cavitation bubbles. More recently, high intensity focused ultrasound (HIFU) has been shown to fractionate tissue through either pulsed-cavitation or millisecond boiling histotripsy therapies; however it is unclear how millimeter-size boiling bubbles or cavitation bubble clouds fractionate tissue into submicron-size fragments. The objective of this work is to test the hypothesis experimentally that atomization and fountain formation occurs similarly in liquids and tissues and results in tissue erosion. A 2-MHz HIFU transducer operating at peak in situ pressures of 50 MPa and −11 MPa (linear intensity = 14,000 W/cm(2)) was focused at the interface between a liquid or tissue and air. A high-speed camera was used to monitor atomization and fountain formation in water, ethanol, glycerol, bovine liver, and porcine blood clots. The in situ linear intensity threshold for consistent atomization in one 10-ms pulse increased in the order: ethanol (180 W/cm(2)) < blood clot (250 W/cm(2)) < water (350 W/cm(2)) < liver (6200 W/cm(2)); glycerol did not atomize. Average jet velocities for the initial spray at the maximum acoustic intensity were similar for all materials and on the order of 20 m/s. The tissue erosion rate of liver approached saturation at around 300 10-ms pulses repeated at 1 Hz, which had an average erosion volume of 25.7±10.9 mm(3). While tissue atomization and fountain formation does not completely mimic what is observed in liquids, atomization provides a plausible explanation of how tissue is fractionated in millisecond boiling and possibly even cavitation cloud histotrispy therapies.

Over a thousand animal species are capable of walking on the interface between air and water. These speciesinclude water striders, a family of insects from the order Hemiptera that has an almost unique ability to walk on the surface of the water without penetrating it. They achieve this outstanding feat by making use of the surface tension and their long hydrophobic legs. Experiments have revealed that water striders transfer some momentum to the underlying fluid through capillary waves and hemi-spherical vortices and that both waves and vortices contribute to the mechanism of propulsion. However, the exact momentum and energy carried by waves and vortices have never been quantified together, and only the energy of the surface waves has been quantified to date. An analysis of the complete energy balance between the interface and the body of the water requires measurement of the free surface topography, together with the three-dimensional (3D) flow field in the water under the surface. The ultimate aim of this work was to develop a method capable of doing this.

The long-range transmission of sound waves in water has been widely discussed, where the surface of water is often considered as smooth and rigid. In theory, tiny capillary waves can be induced when mechanic waves hit the interface of two substances with different wave speeds. In this paper, the shape of water-air interface is analyzed when a far-field plane acoustic wave from underwater reaches water surface. In conclusion, the amplitude of water surface wave is at submicron level, which decreases in the exponential form, and the attenuation coefficient increases greatly with the increase of frequency. Based on theories discussed, a fusion sensing system is proposed for the detection of underwater sound source from air. To build up this system, a laser vibrometer device connected to a computer with picometer resolution, is needed. At 2nm detection threshold, this detection method is able to find a 10Hz,140dB underwater sound source at 2.45m away. It is hard for underwater structures to eliminate noise in low frequency, thus monitoring water surface capillary wave is a promising new way for underwater noise detection.

Emulsified oil slicks can not only damp short gravity and capillary waves on the sea surface, but also reduce the permittivity of the contaminated area. This paper simulates the backscattering coefficients of emulsion oil covered sea surfaces based on AIEM, with the damping model described by model of local balance (MLB). The sea surface covered by emulsion oil with finite thickness is modeled as a layered-medium to calculate the composite reflection coefficients. The simulation results of oil-covered sea surface are compared to those of clean sea surfaces and discussed in terms of incidence angles and frequencies of EM waves, oil thickness and wind speeds.

In this study, the onset amplitude of the initial capillary surface wave for ultrasonic atomization of fluids has been implemented. The design and characterization of 485 kHz microfabricated silicon-based ultrasonic nozzles are presented for the concept of economic energy development. Each nozzle is composed of a silicon resonator and a piezoelectric drive section consisting of three Fourier horns. The required minimum energy to atomize liquid droplets is verified by COMSOL Multiphysics simulation software to clarify experimental data. The simulation study reports a minimum vibrational amplitude (onset) of 0.365 μm at the device bottom under the designated frequency of 485 kHz. The experimental study agrees well with the suggested frequency and the amplitude concerning the corresponding surface vibrational velocity in simulation. While operating, the deionized water was initially atomized into microdroplets at the given electrode voltage of 5.96 V. Microdroplets are steadily and continuously formed after the liquid feeding rate is optimized. This newly designed ultrasonic atomizer facilitates the development of capillary surface wave resonance at a designated frequency. A required vibrational amplitude and finite electric driving voltage promote not only the modern development in the green energy industry, but also the exploration of noninvasive, microencapsulated drug delivery and local spray needs.

International audience; A new type of instability of standing gravity—capillary waves has been detected under the irradiation of the liquid surface by a continuous plane ultrasonic beam with a constant amplitude. At an intensity of 1.03-MHz ultrasound above a certain threshold, the excitation of one of the eigenmodes of gravity—capillary waves at a frequency of 6.56 Hz has been observed. The time of establishment of the stationary amplitude of oscillations of the surface significantly exceeds the intrinsic time of relaxation of the mode and decreases with an increase in the threshold excess. The scenario of development of instability is similar to an inverse energy cascade.

Bubble-film extraction as an improved form of bubbling flotation allows to reduce the concentration of surface-active impurities in water to the level of tenths of a milligram per liter. The use of a polydisperse stream of air bubbles during bubble-film extraction significantly accelerates the process of water purification, when the proportion of large and small air bubbles reaches a certain ratio in their mixed stream. The best case is when air bubbles of different sizes form an integrated structure inside the collecting funnel of the discharge channel of the bubble-film extractor. In the built-in structure, capillary waves accompanying the rupture of large bubbles in the upper part of the critical gasfilling zone inside the funnel of the bubble film extractor transmit their pulses to small-sized bubbles. These pulses overcome the electrostatic repulsion in the boundary menisci of the contacting bubbles and thereby initiate their fusion with the release of more adsorbed surface-active substances into that part of the discharge channel in the bubble-film extractor where a stream of flat liquid films of flotation concentrate appears. As a result, a significant increase in the rate of water purification from its surface-active pollution occurs. The features of bubble-film extraction (enhanced bubble flotation) for water purification are considered from the standpoint of the equilibrium and dynamics of adsorption of surface-active admixtures of water (surfactants) on air bubbles with various sizes. It is shown, that within a certain ratio of dimensions and quantity of air bubbles in their stream transferring surfactants from the water bulk into the bubble-film extractor, the productivity of bubble-film extraction process increases many times. The reason consists in the collective fusion of big and small air bubbles in their embedded structure inside of the bubble-films extractor under the action of capillary waves appearing at the bursting of large bubbles.

Capillary ripples in thin viscous films are important features of coating and lubrication flows. Here, we present experiments based on digital holographic microscopy, measuring with nanoscale resolution the morphology of capillary ripples ahead of a viscous drop spreading on a prewetted surface. Our experiments reveal that upon increasing the spreading velocity, the amplitude of the ripples first increases and subsequently decreases. Above a critical spreading velocity, the ripples even disappear completely and this transition is accompanied by a divergence of the ripple wavelength. These observations are explained quantitatively using linear wave analysis, beyond the usual lubrication approximation, illustrating that new phenomena arise when the capillary number becomes of the order of unity.

Thin viscous liquid films sitting on a solid substrate support nonlinear capillary waves, driven by surface shear stresses at a liquid–gas interface. When surface tension is spatially dependent other mechanisms, such as the thermocapillary effect, influence the dynamics of thin films. In this article we show that in liquids with broken time-reversal symmetry the character of the aforementioned waves and of the thermocapillary effect are significantly modified due to the presence of odd or Hall viscosity in the liquid. This is because odd viscosity gives rise to new terms in the pressure gradient of the flow thus modifying the evolution equation of the liquid–gas interface accordingly. These terms in turn break the reflection symmetry of the evolution equation leading the system to evolve from a pitchfork to a Hopf bifurcation. The odd-viscosity incipient waves can stabilize unstable thin liquid films. For instance, we show that they can suppress the thermocapillary instability. We establish the parameter ranges that odd viscosity has to satisfy in order to initiate those waves that will lead to stability.

The theoretically calculated dependence of the sizes of the main and satellite droplets on the dimensionless wave number of the perturbation of the surface of the Newtonian fluid jet is proved experimentally. It is also shown that an increase in the fluid viscosity significantly changes the dynamics of development of capillary waves in the jet and the mechanism of formation of satellite droplets.

Direct-contact condensation of a vapor bubble train rising in a subcooled liquid bath is accelerated using sonic [O(1 kHz)] acoustic actuation of the liquid-vapor interface. The actuation couples with the characteristic wavelength of the capillary (Faraday) surface instability to form waves which disrupt the interfacial thermal boundary layer, thereby enhancing heat transfer from the bubble to the surrounding subcooled liquid and accelerating vapor condensation, significantly reducing vapor volume. It is shown that the acoustically-induced surface waves lead to bi-directional penetration of fingers of subcooled liquid and vapor across the interface and the small-scale interfacial mixing caused by this motion disrupts the nominally conduction-limited heat transfer and leads to a pronounced, rapid reduction in vapor volume. It is also shown that the acoustic actuation is effective at reduced ambient pressures (e.g., in power cycle condensers) and during co-located boiling and condensation in pool boiling over submerged heater surfaces. Such acoustically-enhanced condensation may enable reduction in the volume of the subcooled liquid bath and therefore in the overall scale of direct-contact condensers in various configurations.

This study investigates the interaction between an impacting liquid drop and a static, axisymmetric splat generated on a solid prior to impact, focusing mainly on the wave production and propagation on the splat with low impact velocity. Using high-speed video, the wave generation is observed and analyzed, including a first wave caused by drop kinetic energy, and secondary and tertiary capillary waves subjected to surface tension. Also, the wave propagation magnitude is discussed concerning the effects of Weber number and fluid viscosity, the purpose of which is to provide a suitable method for predicting the wave diameter. Results show that increasing Weber number decreases the wave propagation slightly, but this effect becomes less significant at higher Weber number. In particular to the fluid of butanol with high viscosity, the wave propagation is very insensitive to Weber number. The formula based on the kinematic discontinuity theory can be modified to predict the wave propagation magnitude with a fairly good agreement, for both the kinematic wave and the capillary wave. In addition, the dynamic wavelength situated between the first and secondary waves, and the propagation speed of the wave during the drop–splat interaction are also analyzed. This study advances the fundamental understanding of the spray evaporative cooling with low volumetric flux.

The surface flows of a liquid in the presence of a partially submerged vibrating plate are investigated. In the case of low-frequency plate vibrations, two vortices are formed on the surface of the liquid on each side of the plate in which liquid particles move away from plate surfaces. Under parametric excitation of transverse capillary waves near the boundary of the liquid layer wetting the plate surface, the flow direction in vortices becomes opposite. High-frequency vibrations of the plate create additional secondary vortices on the liquid surface.

International audience; A method dedicated to the measurement of low surface tension coefficients in binary fluids is proposed. The method which is not intrusive and does not interfere with the volume/weight proportion of liquid composing the binary mixture utilizes excitation of progressive capillary waves by pulsed radiation pressure force of focused ultrasound beam. A known dispersion relationship for gravity-capillary waves is used to evaluate the surface tension coefficient. The experiments are carried out in microgravity conditions in order to make the capillary term of the relationship strongly dominant and thus to reduce artifacts in acquired data.

Gravitational–capillary waves, arising in a system of thick layers of two aggregate states of a substance, are studied using the example of water and its vapour. Their initially flat interface is in hydrostatic and thermodynamic equilibrium at atmospheric pressure and the corresponding saturation temperature $100\,^{\circ }\text{C}$, provided that a balance is maintained for heat fluxes transported through immovable phases in the process of heat conduction. From the problem of linear stability for small perturbations, estimation relations for their damping coefficient and eigenfrequency shift are obtained, including the factors of viscosities of the media and phase transition. The first two contributions, proportional to the kinematic viscosity of the liquid and the square root of the kinematic viscosity of its vapour, respectively, are consistent, accurate to additionally accounted for capillary forces, with the result of Dore’s analysis, which is a development of the weakly viscous Lamb theory for ‘deep water’. Numerical calculations have shown that, with increasing wavelength, the accuracy of the proposed approximation increases, especially for the wave damping coefficient, and the contribution of the viscosity of the light phase increases in percentage terms. As for the phase transition effect, which was previously overestimated using the quasi-equilibrium approximation, it remains insignificant at the level of heat fluxes acceptable in the model of thick layers of homogeneous media. The factor of thermocapillarity is found to be very weak; it becomes qualitatively noticeable only on the background of disappearing heating. Here, together with the factor of dependence of the saturation temperature on the phase pressure, it determines a non-zero correction to the wave damping coefficient.

The results of experimental studies of wave turbulence in a system of capillary waves formed on the surface of liquid hydrogen at a temperature of 15 K in a cylindrical cell with monochromatic radially symmetric pumping are presented. The local maximum formation with a decrease in the pumping amplitude was observed for the first time in the high-frequency region of the inertial range of the stationary turbulent spectrum at the direct cascade edge. The occurrence of a local maximum can be associated with a significant manifestation of viscous damping in the high-frequency region of the spectrum.

Recently, the applications of ultrasonic vibration assistance to laser-based manufacturing processes are rapidly proliferating. Ultrasonic vibration-assisted laser drilling (UVLD) process involves simultaneous application of high frequency vertical vibrations to the workpiece while being irradiated with a continuous wave laser beam. In UVLD, the ultrasonic vibration assistance causes expulsion of droplets from the laser melted surface, resulting in the formation of deep holes. In this paper, systematic analysis of the effects of ultrasonic vibration frequency (20–40 kHz) and displacement (16–32 µm) on melt expulsion characteristics in early stages of drilling and geometric/quality features of the holes for UVLD of AISI 316 is presented. Based on the analysis of initiation of droplet ejection from the melt pool and particle size of the ejected droplets, mechanisms of droplet ejection based on capillary wave theory are proposed. It was observed that while increasing both ultrasonic vibration frequency and displacement resulted in reduction in droplet ejection initiation time and the formation of deeper holes for the given laser irradiation time (100 ms), the effect of vibration displacement was much more pronounced than the frequency on the variation.

The wetting of atomic/molecular liquid under the ultrasonic vibration were studied for probing of the spreading driving force. Under the ultrasonic vibration, the wettability can be improved significantly. The momentum transfer at the liquid/solid interface can be a driving force for spreading, but it is invalid for the contact angles in a range near 90° for the non-wetting systems. The ultrasonic vibration induced the liquid/vapor interface roughening (i.e., the capillary wave at liquid/vapor interface) also can be a source of driving force. The spreading depended on the physic property of liquid, especially depended on the capillary wave on the surface of liquid. Therefore, in the application of ultrasonic-assisted brazing, the improved wettability by the ultrasonic vibration may be mainly caused by the mentioned physical factors rather than the chemical reaction at the liquid/solid interface.

The Mulliken charge embedded newly developed OPLS force field has been applied for computation of various structural and dynamical properties of water–dodecane biphasic system. The interface thickness has been investigated in the presence of finite concentration of third component, a lipophilic ligand, tri-isoamyl phosphate (TiAP) and acid employing molecular dynamics simulation. The physical properties at the interface like interface structure, interfacial tension, interface thickness have been studied for a wide range of mole fraction of TiAP and with various aqueous phase acidity. The interfacial tension is found to be decreased with increasing mole fraction of TiAP whereas increased with the acid concentration. The inverse relation between interface thickness and interfacial tension has been established. The surface tension computed from pressure tensor was found to be in excellent agreement with the experimental results, which is then used to determine the interface thickness employing capillary wave theory (CWT) using a simple scheme of finding the bulk correlation length by adopting conductor like screening model for real solvents. Furthermore, a simple equation has been obtained by fitting the MD results of total interface thickness for a wide range of composition of third component. The value of interface thickness calculated from the model equation was found to be in good agreement with the available experimental and MD results for a wide variety of oil-water biphasic systems. Additionally, the stability of the dimer of TiAP with different orientation was established by computing the dimerization energy using DFT which was also captured in the MD simulation. Furthermore, the present MD data shows that the water extraction is enhanced at higher mole fraction of TiAP because of wider interface thickness at higher composition of TiAP which induced the water transport to the organic phase. Again, the calculated values reveal that the water extractability is decreased with increasing aqueous phase acidity as the interface thickness is reduced at higher acidity. The present results might be useful in the frontier area of interfacial science and engineering as well as in partitioning of drug molecules at the biological interfaces.

Electrohydrodynamic (EHD) instabilities can be induced in polymers by placing a polymer film above its Tg in a strong electric field between two capacitor plates or electrodes. The polymer experiences an electrostatic stress at the interface between the polymer and air due to a mismatch in their dielectric constants. This stress, along with thermal fluctuations, induces small magnitude capillary waves in the polymer film, and the minima and maxima of those waves experience slightly different electric field strengths. In a sufficiently strong electric field, the capillary wave maxima, where the distances between the polymer film and the top electrode(s) are the smallest, are eventually drawn up to the top electrode. The wavelength of the instabilities in the film and the ability of the polymer to be drawn upward is a dependent on the competition between surface tension forces and the electrostatic stress imparted on the polymer. While EHD instabilities are typically used to pattern polymer surfaces on a nanometer-scale, instabilities have been induced in polymer films with air gaps up to 500 μm. Upper electrodes with non-planar structures have also been used to induce instabilities in polymer films, resulting in patterned polymer surfaces without contact. Size, shape, arrangement, and placement of the upper electrode relative to the polymer film and lower electrode, as well as the processing conditions such as temperature and applied voltage, can all be modified to produce a desired array of structures with tailored performance characteristics. Patterned polymer surfaces can provide high-index contrast over a periodic matrix with 3-dimensional element shapes. The dielectric contrast and array pitch and height can be tuned to control specular reflection and achieve specific scattering characteristics. Surfaces with tailored scattering characteristics in the aforementioned ranges could be useful in producing frequency-selective windows for glare reduction, anti-reflective solar cells with enhanced efficiency, surface waveguides and whispering gallery-mode resonator arrays for integrated photonics and sensors, and surfaces with controlled emissivity for directed heat dissipation.

The interface between two fluids is roughened by thermally excited capillary waves. By using colloid-polymer systems which exhibit liquid-gas phase separation, the time and length scales of capillary waves become accessible to optical microscopy methods. Here, we study such a system using bright-field optical microscopy combined with a novel extension of differential dynamic microscopy. With differential dynamic microscopy, we analyze images in order to determine the decay time of interfacial fluctuations spanning wavevectors from 0.1 to 1 μm-1. We find capillary velocities on the order of 0.1 μm s-1 that depend on the sample composition in expected ways and that match values from the literature. This work demonstrates the first application of differential dynamic microscopy to the study of interfacial dynamics.

We study the wetting critical behavior of the three-state ($s=\ifmmode\pm\else\textpm\fi{}1,0$) Blume-Emery-Griffiths model using numerical simulations. This model provides a suitable scenario for the study of the role of vacancies on the wetting behavior of a thin magnetic film. To this aim we study a system confined between parallel walls with competitive short-range surface magnetic fields (${h}_{L}=\ensuremath{-}|{h}_{1}|$). We locate relevant critical curves for different values of the biquadratic interaction and use a thermodynamic integration method to calculate the surface tension as well as the interfacial excess energy and determine the wetting transition. Furthermore, we also calculate the local position of the interface along the film and its fluctuations (capillary waves), which are a measure of the interface width. To characterize the role played by vacancies on the interfacial behavior we evaluate the excess density of vacancies, i.e., the density difference between a system with and without interface. We also show that the temperature dependence of both the local position of the interface and its width can be rationalized in term of a finite-size scaling description, and we propose and successfully test the same scaling behavior for the average position of the center of mass of the vacancies and its fluctuations. This shows that the excess of vacancies can be associated to the presence of the interface that causes the observed segregation. This segregation phenomena is also evidenced by explicitly evaluating the interfacial free energy.

Here we provide a unified theoretical description of two different physical situations in which liquid jets are expelled out of the bulk of a liquid as a consequence of the capillary collapse of a void. We demonstrate that the velocity field giving rise to the emergence of these jets can be calculated as the flow generated by a line of sinks with a length and an intensity that can be expressed in terms of the initial cavity radius and the wavelength and velocity of the capillary waves propagating along the cavity walls. The predicted jet speeds, which are expressed through algebraic equations, are in good quantitative agreement with those obtained from experiments and from the simulations of bubbles bursting on a free surface or after the implosion of the crater formed when a drop impacts a liquid pool.

Drop impact with a smooth surface can produce a dimple mode of contact due to a combined effect of a capillary wave and a thin film instability.

We present an experimental study of bubble coalescence at an air–water interface and characterize the evolution of both the underwater neck and the surface bridge. We explore a wide range of Bond number, , the bubbles are non-spherical – residing predominantly above the free surface – and, while an inertial–capillary scaling for the underwater neck growth is still observed, the controlling length scale is defined by the curvature of the bubbles near their contact region. With it, an inertial–capillary scaling collapses the neck contours across all Bond numbers to a universal shape. Finally, we characterize the upper surface with a simple oscillatory model which balances capillary forces and the inertia of liquid trapped at the centre of the liquid-film surface.

IF=3.557; International audience; In this paper, we propose an experimental and numerical investigation for the impact of the surface tension and the continuous phase viscosity on the dynamics of the liquid bridge during the coalescence process in liquid–liquid systems. A specific configuration of a sessile drop in direct contact with another drop placed over it has been studied. Calculating the redefined Reynolds number Re, it is found that for all studied cases, the coalescence process is dominated by the inertial force. The first step of the work was the validation of the numerical model that has been performed in an axisymmetric coordinate system. This has been done by the comparison between numerical and experimental results obtained in the framework of experimental series realized in parallel for two different liquid–liquid (LL) systems: water drops in silicone oil (SilOil M40.165) and water drops in sunflower oil. A good agreement was found between different results for numerous parameters used for comparisons. It is found that for the first stages of the coalescence (at the start of the drops merging), for a given drop’s viscosity, the dynamics of the dimensionless liquid bridge is conducted by the viscosity of the continuous phase where it is illustrated that the more the surrounded viscosity is large, the lower the rate of the liquid bridge growth, the lower the earlier radial velocity of the bridge, and the higher the external capillary pressure generated around the bridge. Moreover, it is depicted that the impact of the surface tension starts appearing after the complete development of the liquid bridge where it is observed that for the same surrounding phase viscosity, the propagation of the capillary wave is faster for a LL system with higher surface tensions than those of lower surface tensions.

The well-known thermal capillary wave theory, which describes the capillary spectrum of the free surface of a liquid film, does not reveal the transient dynamics of surface waves, e.g. the process through which a smooth surface becomes rough. Here, a Langevin model is proposed that can capture this dynamics, goes beyond the long-wave paradigm which can be inaccurate at the nanoscale, and is validated using molecular dynamics simulations for nanoscale films on both planar and cylindrical substrates. We show that a scaling relation exists for surface roughening of a planar film and the scaling exponents belong to a specific universality class. The capillary spectra of planar films are found to advance towards a static spectrum, with the roughness of the surface $W$ increasing as a power law of time $W\sim t^{1/8}$ before saturation. However, the spectra of an annular film (with outer radius $h_0$ ) are unbounded for dimensionless wavenumber $qh_0 due to the Rayleigh–Plateau instability.

Capillary waves, surface waves whose restoring force is dominated by surface tension, have been an important part of experimental fluid mechanics for many years. Capillary waves are typically generated by a vibrating wave generator which may be the walls of the tank containing the liquid or a separate object. In both cases, a meniscus exists at the interface between the liquid and the wave generator. Though this meniscus is due to the same surface tension that controls capillary waves, its effect on capillary wave formation has not been studied. Herein we present experimental results showing that such a meniscus is needed to form capillary waves, that the lack of a meniscus (a flat interface at the wall) prevents the formation of these waves, and that a meniscus of either positive or negative curvature serves equally well in enabling capillary wave formation.