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In this study, the photothermal effect of an infrared laser induced evaporation and Marangoni convection of a liquid column in microchannels is numerically studied. The volumetric Gaussian heat source is used to model the laser beam and the shear stress at the vapor–liquid interface due to the surface tension gradient is accounted. The results show that under the same laser spot position and laser power, the evaporation mass flow rate in the hydrophilic microchannel is greater than that in the hydrophobic microchannel due to the small thermal resistance. The direction of flow patterns in liquid column are totally different since the relative magnitudes of the horizontal and vertical temperature gradients are opposite for these two surfaces. The effects of the laser power and spot position are also studied. It is found that as the laser power increases, the evaporation mass flow rate, evaporation heat ratio and strength of the Marangoni convection are all increased because more heat can be generated. Regarding the laser spot position, the evaporation mass flow rate and evaporation heat ratio both decrease with increasing the distance between the laser spot and interface because of increased thermal resistance. Moreover, the laser spot position can change the Marangoni convection roll both by the direction and magnitude. The results obtained are helpful for the design and operation of the microdevices with the photothermal effect induced phase change. [ABSTRACT FROM AUTHOR]

In this work, the characteristics of the water droplet evaporation on the hydrophobic surface was visually studied, which was actuated by the photothermal effect of an infrared laser with the wavelength of 1550 nm. The interface temperature was measured by the infrared temperature measurement technique. The variations of the droplet geometrical parameters were captured, with which the evaporation rate was determined by the image process technique. Experimental results indicated that once the IR laser was applied, accompanying with the rapid increase of the interface temperature, the droplet evaporation was instantly induced. Generated vapor was condensed to form satellite droplets near the triple-phase contact line due to such a tiny local heating source that would not significantly change the surrounding temperature. After an increase in the contact radius and corresponding quick decrease of the droplet height, the original droplet gradually shrank. In this process, there existed a period with a relatively stable interface temperature and evaporation rate. In addition, the effects of the laser power and initial droplet volume were also explored. It was shown that both the interface temperature rising and average evaporation rates almost linearly increased with the laser power because more heat was generated by absorbing the light energy. Regarding the initial droplet volume, increasing the droplet volume firstly resulted in an increase in the interface temperature rising and average evaporation rates and then caused both of them to be reduced. The results obtained in this work are helpful for the future applications of droplet-based devices utilizing the photothermal effect. [ABSTRACT FROM AUTHOR]

• The superhydrophobic surface with hydrophilic patterns is designed and fabricated. • Photothermally induced interface dynamics of neighboring droplets is investigated. • The irradiated droplet is rapidly collapsed due to photothermally induced phase change. • The adjacent droplet's outboard TCL shows more significant extension. • Complex and multistep droplet sorting with high throughput and simplicity is enabled. The droplets sorting in the well-defined microdroplets array is significantly important in fields including biomedicine, pharmacy, primary diagnosis and chemosynthesis due to its pivotal role in the follow-up analysis, collection and other applications. Recently, the light control of droplets using the interactions between light and fluids allows for the contactless droplets sorting with prominent features. Among the interactions, the photothermal effect has shown significant potential. In this work, we investigated the photothermal effect-induced interface dynamics of neighboring droplets in the microdroplets array on the superhydrophobic surface with hydrophilic patterns. It was found that the irradiated droplet was rapidly collapsed by the photothermal effect-induced evaporation-condensation-extension while the adjacent droplet's outboard triple-phase contact line was gradually extended and showed more significant extension due to the limited vapor condensation in the space between the droplets. The light-induced interface behaviors of neighboring droplets under different droplet gaps and laser powers were also analyzed. Based on the findings, a proof of concept of the droplets sorting platform manipulated by the light was demonstrated. With the assistance of the hydrophilic patterns on the superhydrophobic surface and the laser heating, the target droplet reached the hydrophilic patterns could spontaneously move to the designated position via the imposed asymmetrical surface tension, realizing the droplets sorting. The results indicated that the light-induced interface migration of the droplets in the microdroplets array could sort the target droplets with high yields and remarkable simplicity and show promising potential in many biochemical analysis applications but not limited. [ABSTRACT FROM AUTHOR]

Highlights • Gas consumption characteristics in a G-L-S microreactor is studied. • Variation in the gas phase is used to determine the detailed reaction process. • Discrete gas consumption process is found and it is mainly in the waiting stage. • Effect of operation condition on the gas consumption characteristics is discussed. • The corresponding microreactor performances are also obtained. Abstract In this work, the gas consumption characteristics were investigated by gas-liquid two-phase flow in a capillary microreactor using the catalytic hydrogenation of nitrobenzene as a reference gas-liquid reaction. Image processing technique was used to measure the gas consumption from the variation in the gas phase through the recorded images. The influence of the inlet nitrobenzene concentration and both the gas and liquid flow rates on the gas consumption and the microreactor performance was investigated. The experimental results showed that there existed a discrete hydrogen consumption process, i.e. the waiting stage and inburst stage. Hydrogen consumption was mainly attributed to the waiting stage in each bubble formation cycle. Minimal hydrogen was consumed in the inburst stage. In addition, there was a clear difference between the H 2 consumption rates under each operating condition. As the gas flow rate and nitrobenzene concentration increased, the H 2 consumption rate was gradually decreased with each successive formation cycle. Although the consumption rate was gradually reduced with each successive formation cycle with increasing liquid flow rate, the downward trend became increasingly gradual and clearly stabilized at 20 μL/min. The microreactor performances under different operating conditions were also investigated. The results showed that the nitrobenzene conversion was first increased and then gradually decreased with increasing the gas flow rate, whereas it was gradually decreased with increasing both the liquid flow rate and nitrobenzene concentration. The results obtained in this work are beneficial for not only understanding process intensification but also promoting the use of microreactor technology in additional industrial applications. [ABSTRACT FROM AUTHOR]

The photothermal effect induced evaporation in microchannels is a typical phenomenon in optofluidics, which greatly affects the performance of this type microdevice. In this study, the dynamic flow and heat and mass transfer characteristics of a liquid column induced by an IR laser with the wavelength of 1550 nm in a microchannel is numerically studied. The results show that the Marangoni convection is caused by the non-uniform temperature distribution arising from a local heating source of the IR laser. The intensity of the Marangoni convection is weak at the beginning because the spontaneous evaporation is dominant. With the laser heating going on, the intensity of the Marangoni convection becomes strong and then weak again due to the competition between the photothermal effect and increased vapor pressure. It is also found that the heat transfer coefficient at the interface and Biot number decrease with the laser heating time because the increased vapor pressure in the gas phase resists the evaporation. Due to the same reason, although the evaporation mainly relies on the spontaneous evaporation at the beginning, the evaporation rate is the highest and then gradually decreases with the laser heating time and finally becomes relatively stable. Correspondingly, the mass transfer coefficient at the interface is also gradually decreased with the laser heating time. Besides, the effects of the laser power and air humidity are also studied. It is shown that the evaporation rate increases with increasing the laser power and decreasing the air humidity. [ABSTRACT FROM AUTHOR]

• The effect of flow rate and electric field intensity on electro-spray morphology is investigated. • The formation and transformation mechanism of each electro-spray jet mode are clarified. • The enhanced heat transfer strategy of electro-spray phase change cooling technology at elevated flow rates is proposed. • At a flow rate of Q = 800 ml/h and an applied voltage of V = 6 KV, the critical heat flux ( CHF ) can reach 100.8 W/cm2. This study has enhanced the application prospects of electro-spray in the cooling field. The advancement of semiconductor and microelectronics technologies has underscored the increasing importance of efficient heat management in electronic devices. To overcome the shortcoming of low flow rates of electro-spray, the present study is motivated by using visualization and heat transfer performance experiments to reveal the morphology evolution and heat transfer characteristics of electro-sprays with high flow rates. The results indicate that the morphology of electro-sprays is closely related to the electric field intensity and the flow rate, and can be classified into six modes: dripping mode, spindle mode, cone jet mode, multi-cone jet mode, simple jet mode, and oscillating jet mode. Furthermore, the formation mechanisms and transition rules of these modes are discussed in detail. In addition, the effects of the flow rate, voltage, and spray morphology on the heat transfer performance of electro-sprays was investigated, and strategies for enhancing heat transfer were subsequently proposes. At a flow rate is 800 ml/h and an applied voltage is 6 KV, the critical heat flux ( CHF ) can reach 100.8 W/cm2, and the maximum heat transfer coefficient ( h max ) can reach 14416.55 W/m2K. This study not only enhances the comprehension of high flow rate electro-spray technology, but serves as a vital reference for the design and application of electro-spray cooling devices. [ABSTRACT FROM AUTHOR]

In the microalgae cultivation process, CO 2 in the bubble form is supplied into photobioreactors as carbon source for microalgae growth. The coalescence of CO 2 bubbles greatly affects the mass transport and thus the biomass production rate. In this work, therefore, the dynamic behaviors of CO 2 bubbles coalescing at two parallel capillary orifices in microalgae suspension were visually studied and compared with pure water. The movement and distribution of microalgae at gas–liquid interface were also observed. The effects of the center distance of capillary orifices and gas flow rate were also analyzed. It was found that many microalgae and microalgae aggregation adsorbed on the surface of the bubble and tended to distribute between the two bubbles when two bubbles coalesced. Besides, it was also found that in microalgae suspension more time was needed for the coalescence of bubbles, and the coalesced bubbles were more easily detached as a result of low surface tension but with lower terminal velocity after detachment than that in pure water. When the center distance of capillary orifices was 1.5 mm ( S = 3 d 0 ), the coalesced bubbles had less growth time and smaller detachment diameter. A critical gas flow rate of 15 mL/min was obtained, at which the minimum detachment diameter and terminal velocity of coalesced bubble was achieved. These findings will be beneficial for the design of aeration system of photobioreactors for microalgae cultivation to enhance the mass transport at the interface. [ABSTRACT FROM AUTHOR]

Abstract: Understanding the coupled two-phase dynamics in the cathode gas diffusion layer (GDL) and gas channel (GC) is of vital importance to optimize water management and to improve performance of proton exchange membrane fuel cell (PEMFC). However, it is difficult to directly observe the two-phase flow in the cathode of PEMFC because the GDL is opaque and has complex structures. To circumvent this issue, a similarity experiment model is developed in the present study. Based on the soft lithography technology, we fabricate a microfluidic device composed of a duct representing the GC and a bundle of tubes representing the GDL. Emergence of liquid droplets from these tubes into the duct gas stream is conducted to mimic the two-phase dynamics in the GDL and the GC. With this similarity experiment, we explore the influences of the gas flow rate on the liquid water flow in the cathode of PEMFC as well as the effects of the GDL surface defects on the two-phase dynamics in the GC. The two-phase flows in the hydrophilic and hydrophobic systems are also compared to elucidate the effects of wettability on water management of PEMFCs. [Copyright &y& Elsevier]

Abstract: A three-dimensional pore network model is developed in this paper to disclose the slow evaporation characteristics in various hydrophobic porous media. The constructed network is composed of large cubic pores connected by small throats of square cross section. During evaporation in the network, the two-phase flow is analogized as quasi-static imbibition, and water vapor transport is described as the diffusion process. Based on the developed model, parameter studies are performed to elucidate the effects on evaporation of the pore size distribution (PSD) and the aspect ratio (i.e. throat-to-pore ratio). In the wide PSD network, the drying induced two-phase displacement is a random process, and the impact of the aspect ratio is not significant. Two-phase displacement in the narrow PSD network is also a random process when the aspect ratio is high; but for the lower aspect ratio, the drying fronts are stably receded. In the case of the high aspect ratio, the evaporation rate with respect to network liquid saturation can be characterized by four periods in both the narrow and wide PSD networks: i.e. an initial drying period, a constant rate period, a falling rate period, and a receding front period, similar to that in the hydrophilic case. [Copyright &y& Elsevier]

• An electro-solidification model is established and the deviation between experiments and simulations is within 15%. • The bottom filament is reproduced in the simulation by considering the influence of the electric field on water freezing. • The effects of the Ca E , We, θ T and Sa number are analyzed. • The value of α ad varies from 0 to 0.2 and the dimensionless correlation of the freezing fraction α ad was obtained. The process of ice formation on outdoor electrical equipment is determined by the impact of supercooled droplets on the cold surface under an electric field. Clarifying the impact-freezing of a supercooled droplet under an electric field is important for designing anti-icing surfaces in the power industry. Here, we developed a coupled electro-solidification model by combining the volume of fluid (VOF) method, a dynamic contact angle model, the two-phase electrohydrodynamic (EHD) model, the modified enthalpy solidification method and the nucleation effect. Then, the impact dynamics of a water droplet on a cold superhydrophobic surface under an electric field were studied numerically. The effects of the electric field intensity, Weber number, initial droplet temperature and conductivity on the spreading factor β m and the freezing fraction α ad are analyzed. By specifically considering the effect of the electric field enhancing the water to freeze, the bottom filament is successfully recreated in the simulation. This phenomenon was first reproduced in our simulation, and it would significantly increase the probability of icing on the cold surface under the electric field, even on the superhydrophobic surface. Finally, based on the numerical results and published experimental data, we fit the freezing fraction relation of the supercooled water droplet impinging on cold superhydrophobic surface under the electric field, which can be used to predict the icing growth and prevention of icing on transmission lines. The numerical model developed here can also be directly used to simulate ice growth and guide the design of anti-icing surfaces in power transmission systems. [ABSTRACT FROM AUTHOR]

• Effect of surface inclination on impacting droplet dynamic behaviors is explored. • Superhydrophobic surface with an inclination of 30° has the best anti-icing performance. • Lubricant infused surface is more suitable for extremely cold environment. Water droplet freezing on the surface structures has a gigantic impact on the aircraft, power transmission lines and industrial facilities. Thereby, it is essential and meaningful to clearly understand the droplet impacting and icing behaviors on the supercooled superhydrophobic surface at an angle for the prevention of ice accretion. In the present research, the dynamic behaviors of a water droplet impacting on the superhydrophobic surface with different inclination and supercooling degree were visually studied by a high-speed camera. Moreover, the droplet dynamic behaviors were analyzed in terms of the contact time, freezing onset time, maximum spreading factor, rebounding energy, average sliding velocity and adhesion morphology on the surface. The results prove that the droplet rebounding process can be promoted by increasing the surface inclination. Moreover, with the gradually decreasing surface temperature the droplet successively undergoes full rebound, partial rebound and no rebound on the surface with an inclination of 30°, and the boundary temperatures of these three modes are −31.25 and −33.75 °C, respectively. Furthermore, the governing factor of the superhydrophobic surface anti-icing performance turns from the droplet rebounding energy into the average sliding velocity with an increase in the surface inclination, and the surface with an inclination of 30° has the best anti-icing performance. It is worth noting that reducing the surface de-icing difficulty rather than improving the anti-icing performance is more suitable for the extremely cold environment. [ABSTRACT FROM AUTHOR]

• A multiscale CFD model based on DDPM is developed to study the FB-ALD process. • Dynamic mesh method is adopted to introduce the ultrasonic field to the FB reactor. • Ultrasonic vibration enhances the gas-solid contact efficiency and deposition rate. • Ultrasonic vibration leads to a 25% reduction in purge time in the half-ALD cycle. • The maximum precursor utilization for uniform coating is quantitatively predicted. Atomic layer deposition (ALD) is a unique surface modification technology by providing conformal ultrathin films on particulate materials. Fluidized bed ALD (FB-ALD) shows excellent scale-up potential for the mass production of particle coating, where different external fields have been developed to enhance the fluidization quality of cohesive micro-nanoparticles. Ultrasonic vibration is a promising assisting method for it can promote effective agglomeration breakage as well as enhance the heat and mass transfer rates in the reactor. In this paper, a multiscale computational fluid dynamics (CFD) simulation based on the dense discrete phase method (DDPM) is developed to study the ultrasonic vibration-assisted FB-ALD process. Dynamic mesh method and discrete element method are adopted to introduce the ultrasonic field and compute the particle movement with an accurate description of particle-particle and particle-vibrating wall interactions. Results show that ultrasonic vibration effectively promotes the contact efficiency between uncoated particles and precursor molecules in the precursor pulsing process, leading to higher deposition rates and coating uniformity. During the purge process, ultrasonic vibration also increases the removal rate of ALD byproducts, thus shortening the purge time by 25%. Through the CFD-DDPM model, the maximum precursor utilization rate for uniform coating can be predicted quantitatively under a given precursor mass fraction and a ratio of the pulse time to the theoretical saturation time. The multiscale numerical model developed in this work can be adapted to optimize the process conditions for the scale-up of FB-ALD reactors. [ABSTRACT FROM AUTHOR]

Water droplet icing on the supercooled surface is ubiquitous and presents gigantic harmfulness on the power transmission lines, aircraft and outdoor equipment. Therefore, a good understanding of droplet impact and freezing process on the supercooled superhydrophobic surface is crucial. In this paper, aluminium sheet was adopted as the substrate to prepare the superhydrophobic (SH) surface. A high-speed camera was applied to explore the dynamic behaviors of a water droplet impacting on the SH surface with various supercooling degrees visually. Moreover, the droplet spreading factor, dimensionless height, characteristics time, rebounding energy and mass fraction of the secondary droplets were adopted to reflect the dynamic behaviors. The results indicate that the dynamic behaviors of a droplet (with an initial temperature of 15 °C, equivalent diameter of 2.94 mm and impacting velocity of 0.99 m·s −1 ) impacting on the SH surface (160° in static contact angle) with a lower supercooling degree is consisted of three regimes: spreading, retraction and rebounding. Whereas, the retraction and rebounding regime are limited at −17 °C and −24 °C, respectively. Moreover, in the retraction regime, the governing factor of the droplet frozen onset time transforms from the interface heat transfer ability into incubation time at the surface temperature below −34.4 °C. Furthermore, the droplet separating and rebounding are prevented once the remaining energy at the end of the retraction regime is lower than 35% of the droplet initial energy. [ABSTRACT FROM AUTHOR]

• A three-dimensional extended enthalpy-porosity solidification model is established and the deviation between experiments and simulations is within 10%. • Five impact-freezing modes are reported on cold superhydrophobic surface. • The contact time of partial coalescence rebound mode is shorter than that of the single-droplet impact at room-temperature. • The interaction between the droplets increases the likelihood of ice adhesion. The real physical process of ice formation involves multiple droplets rather than a single isolated one. Clarifying the mechanism of multi-droplet impact-freezing is important for designing functional surfaces for anti-icing. The rebound behavior of double droplets simultaneously impinging on a cold superhydrophobic surface (SHS) was investigated experimentally and numerically, which is a classical simplified case in multi-droplet impingement. We found five impacting-freezing modes, including no-coalescence rebound (NCR), partial-coalescence rebound (PCR), complete-coalescence rebound (CCR), no-coalescence adhesion (NCA) and coalescence adhesion (CA). Moreover, the effects of horizontal spacing, impact velocity and surface temperature on the spreading factor and uprising sheet height during the impacting-freezing process were also analyzed. The contact time of PCR mode is even shorter than that of a single droplet impact at room temperature, which is verified and explained in our results. However, as the surface temperature decrease, the behavior of the coalescing droplet transitions from PCR to CCR and CA mode. The increase in Weber number could cause the adhesion of double droplets. Moreover, the horizontal spacing strongly affects the coalescing process. Multiple droplet impact-freezing will lead to severe icing on the substrate due to multiple droplets interaction involved. This study is beneficial for anti-icing and de-icing technologies. [ABSTRACT FROM AUTHOR]

A 3D lattice Boltzmann model is adopted to simulate the flow and mass transfer in a packed bed bioreactor for photo biohydrogen production. In the simulation, the non-equilibrium extrapolation strategy is extended to the 3D curved boundary treatment for velocity and concentration. The effects of accumulation mode of particles, influent velocity and illumination intensity are investigated on flow and concentration distributions in the photobioreactor. Additionally, the hydrogen production performance of the photobioreactor is evaluated based on the mean values of substrate consumption efficiency and hydrogen yield. The simulation results indicate that the product concentration and the hydrogen production performance peak at an illumination intensity of 6000 lx, whereas both of them decrease with increasing influent velocity. The accumulation mode of body-centered cubic structure leads to a higher hydrogen production performance and a lower average drag coefficient; this provides a guidance to the design of photobioreactor for efficient hydrogen production. [ABSTRACT FROM AUTHOR]

Abstract: The dynamic bubbling behaviors in a bubble column with a micro-orifice of 54μm in diameter are studied by visual experiments. The instant images are obtained by high-speed photographic technique. With the help of image processing, a real-time tracing is applied to the bubbles formation and motion, then the evolution of the bubbling features, including bubble size, shape and velocity are discussed. The bubbles steadily grow on the micro-orifice and the linear variations of the bubble radius, aspect ratio and velocity with the height are exhibited at a low gas flow rate. With the increase in the height, the bubble velocity reduces firstly and then turns to increase, while the bubble radius and aspect ratio appears a bifurcation structure. The effect of gas flow rate on bubbling behaviors is investigated and the fast Fourier transform is adopted to analyze the periodicity of the bubbling system. A critical gas flow rate (5.4ml/min) is found, at which the bubbling status turns from single bubbling to non-interval periodic bubbling. [Copyright &y& Elsevier]

• The effect of microgroove structure on the fluid dynamics and film conformality in spatial atomic layer deposition is investigated. • Microgroove structure hinders the precursor inflow and purging gas outflow, leading to non-uniformity of precursor distribution. • Both growth per cycle and film conformality decrease as substrate moves faster, but increase slightly with higher carrier gas flow rate. • By optimizing carrier gas flow rate and substrate velocity, the precursor utilization is increased by over two times to obtain a 100% conformal film. Spatial atomic layer deposition (ALD) is a promising high-throughput technique capable of producing ultrathin films on large substrates. Compared to flat wafers, deposition on substrates with microstructures has a wider range of applications such as photovoltaic cells, electronics, flexible displays, etc. However, spatial ALD on microstructure substrates is a complex and strong-coupled process of fluid flow, heat and mass transfer, as well as chemical reactions. The fluid dynamics and the precursor distribution in spatial ALD are of great importance to obtain conformal growth with high throughput. In this study, a two-dimensional model coupling computational fluid dynamics with chemical kinetics is established to quantitatively explore the effect of microgroove structures on the fluid dynamics, precursor distribution and film conformality in an atmospheric spatial ALD system. Slip boundary condition is adopted to model the slip flow regime in the micro-gap between the injector and the substrate surface, and dynamic layering method is implemented to simulate an entire ALD cycle with the in-line movement of the substrate. Results show that while the flow field is very smooth with a flat substrate, vortices always exist in the micro-gap with microgroove substrates. Due to the angle differences between the flow direction and the vertical microstructure surface, the inflow and outflow of precursors and purging gas are prevented at the microgroove corners. A relatively lower moving speed of the substrate is beneficial for the saturated film growth and better film conformality. Increasing the carrier gas flow rate can effectively reduce the non-uniformity and non-conformality of the film, but the precursor utilization also decreases. Compared with the nonoptimal conditions, within a moderate range of carrier gas flow rate and substrate speed, the precursor utilization and film conformality have been improved. Focusing on the effect of the micro-scale features of the substrate, this study is a valuable step in extending spatial ALD for ultrathin films on miniature devices. [Display omitted] [ABSTRACT FROM AUTHOR]