Search

• The CA relaxation profile was used to determine the advancing CA of sessile drop. • Sessile drop of surfactant solution was formed from an equilibrated pendant drop. • Two advancing contact angles were detected for TX-100 and C 12 E 6 solution drops. • Only one advancing CA was detected for the sessile drop of SDS solution. • Pre-equilibration of the drop before deposition affected the CA relaxation profile. This study investigated the uniqueness of advancing contact angle (CA) by examining the evaporation process of a sessile drop of surfactant solution [sodium dodecyl sulfate (SDS), hexaethylene glycol monododecyl ether (C 12 E 6) and Triton X-100 (TX-100)] on parafilm. The sessile drop was formed from a pendant drop that had reached its equilibrium surface tension in an atmosphere with ∼100% humidity. The relaxation of the drop profile and wetting diameter were recorded from the moment the drop touched the parafilm, and the data were then analyzed to determine the advancing CA. The results showed that for SDS solution drops, only one advancing CA was observed at each SDS concentration. However, the experimental data of TX-100 and C 12 E 6 solution drops indicated the presence of two distinct regions that showed a near-constant CA with a quasi-equilibrium advance of the triple line; hence matching the definition of an advancing CA. The possibility of the existence of two advancing CA was further explored and the presence of two advancing CA for the sessile drops of TX-100 and C 12 E 6 solution was verified by closely examining the advance of the triple line. [ABSTRACT FROM AUTHOR]

The dynamics of a receding contact line of water drops on polymeric substrates (poly(methyl methacrylate), PMMA, and polycarbonate, PC) was studied using the sessile drop method. For smoother substrates, multiple stages were observed: the constant contact radius (CCR) mode, the constant contact angle (CCA) mode, and the mixed mode. In the CCA mode, the contact angle (CA) remained nearly constant and the receding dynamic CA could be identified. The relation between the receding dynamic CA and the withdrawal rate was experimentally determined, and the receding dynamic CA was found to decrease with increasing liquid withdrawal rate. A decrease of 5–10° was found for PMMA substrates, whereas a decrease of 10–20° was observed for PC substrates for commonly applied liquid withdrawal rates. Our results quantitatively agree with those obtained by the capillary tube method and the plate method, and we demonstrated that the sessile drop method can serve as an alternative method for studying contact line dynamics. In addition, for the rough PC substrates, the CCA mode was absent and the receding dynamic CA could not be identified. Therefore, we demonstrated that on rough polymeric substrates, the sessile drop method must be used with caution, and complete CA relaxation profiles and liquid withdrawal rates should be reported. [ABSTRACT FROM AUTHOR]

Abstract: The drop shape (sessile or pendant) method is widely used to measure the surface tension of glass melts. The accuracy of surface tension measurement has been improved by applying the technologies of video-image digitization and best-fitting algorithms. An accurate surface tension can be determined using hundreds of edge points from drop profiles and the best-fit with the drop profiles from the Young–Laplace equation. In this study, we examine the accuracy of measuring the surface tension of glass melts using the sessile or pendant drop methods. Experimental data show that the accuracy of surface tension measurement using the pendant drop method (±3mN/m) is better than that obtained by using the sessile drop method (±9 to ±20mN/m, depending on the contact angle). [Copyright &y& Elsevier]

The θ/2 method, a widely used technique on measuring the contact angle of a sessile drop, assumes that the drop profile is part of a sphere. However, the shape profile of a sessile drop is governed by the Young–Laplace equation and is different from a sphere, especially for drops with a large bound number (e.g. large volume or small surface tension). The spherical assumption, therefore, causes errors on evaluating the contact angles. The deviation of contact angle from the θ/2 method is evaluated from a theoretical calculation in this work. A simple means is given for correcting the measurement error. The corrected angle results from the drop volume, surface tension, liquid density and the contact angle from θ/2 method. An algorithm for finding the correct contact angle without knowing the density and surface tension is also given. At the end, two examples of pendant drops are given for the illustration. [Copyright &y& Elsevier]

Focuses on the measurement of dynamic/advancing/receding contact angle by video-enhanced sessile drop tensiometry. Use of a technique to obtain the values of capillary constant and the radius of curvature at apex from the edge coordinates and the theoretical curve obtained from the Laplace equation.

Describes a video-enhanced plate method for simultaneous measurements of surface tension and contact angle. Error caused by the assumption of zero constant angle for the Wiljelmy plate; Capture of the profile of the air-water interface adjacent to a flat plate; Agreement between the curve of the air-water interface.

[Display omitted] • Early stages of the dynamic wetting of a water drop on a smooth wood was studied. • Relaxations of contact angle and wetting diameter were monitored from 0.003 to 70 s. • Two regions, where the drop was in a near-quasi equilibrium state, were observed. • Values of CA at these two regions were noticeably different: ∼66° and ∼ 50°. • The oscillatory behavior of a water drop at these two regions was examined as well. An advancing contact angle (CA) can be measured by numerous different techniques that monitor different time intervals of a dynamic wetting process. This does raise the question: would the same value of advancing CA be obtained at different time intervals of the wetting process? This study monitored the relaxations of CA and wetting diameter of a water drop on a Diospyros crassiflora substrate (R q = 0.41 μm) from 0.001 to 0.3 s and from 0.1 to 70 s by utilizing the same sessile drop contact angle analyzer with two different acquisition rates of 6770 and 30 fps, respectively. Subsequent analysis of the data showed that when the relaxations of the CA and wetting diameter were monitored from the moment a water drop came in contact with wood the substrate, two regions having a near-constant CA with a near quasi-equilibrium advance of the triple line were observed, and both these regions matched well with the definition of an advancing CA. For a water drop, the average CA at these two regions were found to be significantly different; ∼66° and ∼50°. The damped oscillatory behavior exhibited by the water drop during these two regions was discussed in detail as well. [ABSTRACT FROM AUTHOR]

The breakpoints of the profiles of the advancing (θ a) and receding (θ r) contact angles can be used to determine the critical micelle concentration of surfactant solution. Although the breakpoints of θ a and θ r matched with that of surface tension for some surfactant solutions (i.e. SDS), the physical meaning behind these breakpoints is yet to be ascertained. This study attempts to interpret these breakpoints by examining the evaporation process of the sessile drops of SDS, Triton X-100 and C 12 E 6 solutions on parafilm. The surface area perturbance, during the period θ a and θ r were detected, resulted in a noticeable deviation of the L-G and L-S interfacial tensions (γ lg and γ ls) from their respective equilibrium states. As θ a and θ r were detected during the L-G and L-S area perturbations, these deviations of γ lg and γ ls played a crucial role in the relaxation profiles of θ a and θ r. Furthermore, the relaxations of θ a and θ r were affected by the extent up to which the γ lg and γ sl re-equilibrated with the drop concentration. This extent of re-equilibration, in turn, was dependent on the dynamic surface tension of a perturbed interface. Subsequently, it was found out that the breakpoints of θ a and θ r indicated the lowest bulk concentrations at which the γ lg and γ sl re-equilibrated when either θ a or θ r was detected using the evaporation method. • Breakpoints of θ a and θ r for SDS, TX-100 & C 12 E 6 solution drops were interpreted. • Detection of θ a & θ r coincided with changes in surface area of the solution drops. • These changes resulted in the deviation of surface concentrations from equilibrium. • The extent of re-equilibration depended on the surfactant's mass transport rate. • Breakpoints indicated the concentration where re-equilibration occurred promptly. [ABSTRACT FROM AUTHOR]

The critical micelle concentration (cmc) is an important parameter in colloidal and surface chemistry and a wide range of techniques have been devised for cmc determination. In this study, the determination of the cmc of surfactant solutions from the unique advancing and receding contact angle (CA) data was investigated. The sessile drop was formed from a pendant drop which had reached its equilibrium surface tension (ST) in a saturated atmosphere (~100% humidity). The CA was determined from the best-fit between the edge locations of drop and the Young-Laplace theoretical profile. The advancing and receding CAs were determined from the CA relaxation profiles of the sessile drops. It was observed that the breakpoints of the advancing and receding CAs of SDS solution drop coincided with the cmc determined from the ST data. However, the comparatively slower adsorption and bulk diffusion rates of TX-100 and C 12 E 6 made the CA breakpoints deviate from that of ST. Thus, it was concluded that the approach of using the unique CA data for the determination of cmc could be applied only to those surfactants with a relatively high mass transport rate, such as SDS. Unlabelled Image • The unique advancing and receding CA of SDS, TX-100 and C 12 E 6 solution drops were determined from the CA relaxation profiles. • The sessile drop was formed from a pendant drop which had reached its equilibrium surface tension in a saturated atmosphere. • The values θ a and θ r decreased with an increasing bulk concentration of the surfactant solutions at C < cmc. • Breakpoints of θ a and θ r matched well with the cmc from surface tension for SDS solution. • Breakpoints of θ a and θ r did not coincide with the cmc for TX-100 and C 12 E 6 solutions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Feasibility of determining the cmc of partially dissociated ionic surfactant DTAB using the contact angle data was studied. • Relaxations of contact angle (CA), drop volume, and wetting diameter of a solution drop were monitored during evaporation. • The advancing (θ a) and receding (θ r) CA were determined from the aforementioned relaxation profiles. • Breakpoints of θ a and θ r matched well with the cmc from surface tension data; thereby, verifying the applicability. • Movement of a solution drop's triple line during evaporation was noted to be mainly dependent on local surface morphology. The contact angle (CA) data has been utilized for determining the critical micelle concentration (cmc) of fully dissociated surfactant SDS, but not for nonionic surfactants Triton X-100 and C 12 E 6 [Thu et al., J.Mol.Liq (2019) 111582]. Could this approach work for a partially dissociated ionic surfactant such as dodecyltrimethylammonium bromide (DTAB)? To verify this, this work studied the evaporation process of DTAB (aq) solution drops on parafilm. The relaxations of CA, wetting diameter, and drop volume were recorded during evaporation to determine the advancing/receding (θ a and θ r) CA's. The data revealed that the breakpoints of θ a and θ r matched well with the cmc from the ST data; thereby, confirming that the aforementioned approach is viable for a partially dissociated ionic surfactant. Additional examination of the relaxation data and drop profiles revealed that the movement of a solution drop's triple line (pinning-depinning behavior) during the evaporation process is primarily dependent on the localized surface morphology. [ABSTRACT FROM AUTHOR]

Could a uniquerecedingcontact angle be indicatedfor describing the wetting properties of a real gas–liquid–solidsystem? Could a receding contact angle be defined if the triple lineof a sessile drop is not moving at all during the whole measurementprocess? To what extent is the receding contact angle influenced bythe intrinsic properties of the system or the measurement procedures?In order to answer these questions, a systematic investigation wasconducted in this study on the effects of substrate roughness andrelative humidity on the behavior of pure water drops spreading andevaporating on polycarbonate (PC) surfaces characterized by differentmorphologies. Dynamic, advancing, and receding contact angles werefound to be strongly affected by substrate roughness. Specifically,a receding contact angle could not be measured at all for drops evaporatingon the more rugged PC surfaces, since the drops were observed stronglypinning to the substrate almost until their complete disappearance.Substrate roughness and system relative humidity were also found responsiblefor drastic changes in the depinning time (from ∼10 to ∼60min). Thus, for measurement observations not sufficiently long, nomovement of the triple line could be noted, with, again, the failureto find a receding contact angle. Therefore, to keep using conceptssuch as the receding contact angle as meaningful specifications ofa given gas–liquid–solid system, the imperative to carefullyinvestigate and report the inner characteristics of the system (substrateroughness, topography, impurities, defects, chemical properties, etc.)is pointed out in this study. The necessity of establishing methodologicalstandards (drop size, measurement method, system history, observationinterval, relative humidity, etc.) is also suggested. [ABSTRACT FROM AUTHOR]

Highlights: [•] Study on triple line dynamics of evaporating drops on PC, ABS, PVC and PMMA. [•] Pinning/depinning behavior was observed for the drops evaporation on PVC and PMMA. [•] Pinning behavior was observed for the drops evaporation on PC and ABS. [•] Relationship between triple line dynamics and surface topographies was detailed. [•] θ, wetting diameter, height, volume and shape of the drop were deeply analyzed. [Copyright &y& Elsevier]

Abstract: A notable universal relationship has been proposed in the literature for the evolution of dimensionless droplet height and wetting diameter during the initial spreading stage of droplet impingement. In this study, this universal relationship was investigated by employing three sets of measurements. Sequential images were recorded, and the whole droplet profile ensembles were plotted to facilitate this study. These sets of experiments were designed by changing impact velocity, surface hydrophobicity, or solution property. The experimental results illustrate that the importance of parameters causing the data variation is in the order of surface hydrophobicity>initial impact velocity>surfactant on wetting diameter, and surface hydrophobicity≈initial impact velocity>surfactant on droplet height. No universal relationship was observed for dimensionless droplet height and wetting diameter. [Copyright &y& Elsevier]

Abstract: A water drop was released from 13.6mm above a glass or paraffin surface. The images were recorded sequentially (the silhouette and 58° top view) from the moment that the droplet impacted solid surface until it reached equilibrium. The full water droplet profile during spreading was obtained from the digitized images. Digitized droplets were then used to detail the dynamic impact and spreading behaviors, including the droplet morphology and the relaxations of wetting diameter, droplet height, contact angle, and spreading velocity. The dynamic wetting behaviors of a water droplet impinging on a hydrophilic and a hydrophobic surface were investigated in this study. Digitized droplet profiles allow one to clarify the independent motion of two related but independent parts, central region and rim, of an impinging droplet. An interesting plateau region on the relaxation of droplet height was observed in the first cycle for both surfaces. For paraffin surface, three particular growth modes in droplet height were detected in each oscillation at early spreading stages. It takes only three oscillation cycles (0.085s) for the water droplet on glass substrate to dissipate its energy and reach its equilibrium state. On the other hand, the oscillation of the droplet on paraffin substrate lasts 28 cycles (0.266s). Moreover, several other new phenomena were also observed and discussed in this work. [Copyright &y& Elsevier]