Sessile drops of surfactant solutions on nonhorizontal solid surfaces

On the effect of static and dynamic contact angles on humid air condensation heat transfer

Adaptation of application of drop area method to characterize hydrophobicity of materials

Contact angle, as a representative measure of surface wettability, is often employed to interpret contact lens surface properties. The literature is often contradictory and can lead to confusion. This literature review is part of a series regarding the analysis of hydrogel contact lenses using contact angle techniques. Here we present an overview of contact angle terminology, methodology, and analysis. Having discussed this background material, subsequent parts of the series will discuss the analysis of contact lens contact angles and evaluate differences in published laboratory results. The concepts of contact angle, wettability and wetting are presented as an introduction. Contact angle hysteresis is outlined and highlights the advantages in using dynamic analytical techniques over static methods. The surface free energy of a material illustrates how contact angle analysis is capable of providing supplementary surface characterization. Although single values are able to distinguish individual material differences, surface free energy and dynamic methods provide an improved understanding of material behavior. The frequently used sessile drop, captive bubble, and Wilhelmy plate techniques are discussed. Their use as both dynamic and static methods, along with the advantages and disadvantages of each technique, is explained. No single contact angle technique fully characterizes the wettability of a material surface, and the application of complimenting methods allows increased characterization. At present, there is not an ISO standard method designed for soft materials. It is important that each contact angle technique has a standard protocol, as small protocol differences between laboratories often contribute to a variety of published data that are not easily comparable.

The present study evaluated the contact angles (CAs) of four denture base materials subjected to different surface treatments using deionized water and saliva substitutes. A total of 32 rectangular specimens were manufactured using four different denture base materials: heat-cured compression molded Lucitone 199 (C), milled Lucitone 199 (M), Formlabs Denture Base RP (P), and SR Ivocap High Impact (I). The CA of the surface of the unaltered, mechanically polished, and sandblasted surface specimens was evaluated after the application of five saliva substitutes: Biotene, VEGA, Spry, Moi-Stir, Dentilube, and ionized water. Ten droplet measurements were obtained for each group, with each droplet analyzed for advancing contact angle (ACA), receding contact angle (RCA), and the contact angle hysteresis (CAH) was calculated. The data of the experiment was analyzed using 2-way ANOVA, (α=0.05) with Tukey's test. CAH was demonstrated to have statistically significant differences among the denture bases (p<0.05), with unaltered 3D printed exhibiting the largest CAH, followed by unaltered milled. The unaltered denture bases exhibited higher CAH than the polished, and there were no significant differences in CAH among the polished denture bases (p>0.05). Sandblasting increased the ACA of the milled and conventional bases. The saliva substitutes exhibited differences in ACA, with Spry and VEGA having the highest ACA, and Biotene had the lowest CA of all the saliva substitutes evaluated. The manufacturing methods of denture bases influences the CAH, while the chemical composition of the denture base specimens does not appear to affect CAH. Sandblasting increases the ACA for the milled and conventional groups. Saliva substitutes do impact the ACA. Drawing from previous research, it is hypothesized that a 3D-printed denture base or sandblasting a milled denture base may offer greater resistance to dislodgement.

The Effect of Drop (Bubble) Size on Advancing and Receding Contact Angles for Heterogeneous and Rough Solid Surfaces as Observed with Sessile-Drop and Captive-Bubble Techniques

The effect of liquid properties to contact angle hysteresis

Review: 214 refs.

The debate on the dependence of apparent contact angles on drop contact area or three-phase contact line: A review

Drying of porous media is governed by a combination of evaporation and movement of the liquid phase within the porous structure. Contact angle hysteresis induced by surface roughness is shown to influence multi-phase flows, such as contact line motion of droplet, phase distribution during drainage and coffee ring formed after droplet drying in constant contact radius mode. However, the influence of contact angle hysteresis on liquid drying in porous media is still an unanswered question. Lattice Boltzmann model (LBM) is an advanced numerical approach increasingly used to study phase change problems including drying. In this paper, based on a geometric formulation scheme to prescribe contact angle, we implement a contact angle hysteresis model within the framework of a two-phase pseudopotential LBM. The capability and accuracy of prescribing and automatically measuring contact angles over a large range are tested and validated by simulating droplets sitting on flat and curved surfaces. Afterward, the proposed contact angle hysteresis model is validated by modeling droplet drying on flat and curved surfaces. Then, drying of two connected capillary tubes is studied, considering the influence of different contact angle hysteresis ranges on drying dynamics. Finally, the model is applied to study drying of a dual-porosity porous medium, where phase distribution and drying rate are compared with and without contact angle hysteresis. The proposed model is shown to be capable of dealing with different contact angle hysteresis ranges accurately and of capturing the physical mechanisms during drying in different porous media including flat and curved geometries.

The hydrostatic shape, transient deformation, and asymptotic shape of a small liquid drop with uniform surface tension adhering to a planar wall subject to an overpassing simple shear flow are studied under conditions of Stokes flow. The effects of gravity are considered to be negligible, and the contact line is assumed to have a stationary circular or elliptical shape. In the absence of shear flow, the drop assumes a hydrostatic shape with constant mean curvature. Families of hydrostatic shapes, parameterized by the drop volume and aspect ratio of the contact line, are computed using an iterative finite-difference method. The results illustrate the effect of the shape of the contact line on the distribution of the contact angle around the base, and are discussed with reference to contact-angle hysteresis and stability of stationary shapes. The transient deformation of a drop whose viscosity is equal to that of the ambient fluid, subject to a suddenly applied simple shear flow, is computed for a range of capillary numbers using a boundary-integral method that incorporates global parameterization of the interface and interfacial regriding at large deformations. Critical capillary numbers above which the drop exhibits continued deformation, or the contact angle increases beyond or decreases below the limits tolerated by contact angle hysteresis are established. It is shown that the geometry of the contact line plays an important role in the transient and asymptotic behaviour at long times, quantified in terms of the critical capillary numbers for continued elongation. Drops with elliptical contact lines are likely to dislodge or break off before drops with circular contact lines. The numerical results validate the assumptions of lubrication theory for flat drops, even in cases where the height of the drop is equal to one fifth the radius of the contact line.

Arbitrary Lagrangian-Eulerian finite element method for liquid sloshing with contact angle hysteresis in microgravity

Understanding fluid behavior at the liquid-solid interface is essential for advancing micro/nanoscale technologies, yet current models struggle to capture the complexity introduced by surface heterogeneity. In this study, we investigate the interplay between nanoscale surface roughness, wettability, and hydrodynamic slip by using molecular dynamics simulations on nanopatterned silica surfaces. By systematically varying the droplet size and surface geometry, we characterize the influence of nanoscale pinning on contact angle hysteresis and determine the macroscopic equilibrium contact angles for six distinct rough surfaces. Parallel simulations of force-driven water flow through nanochannels reveal the local and average slip lengths associated with each surface. For smooth surfaces, slip length correlates well with contact angle, aligning with predictions from well-known theoretical models by Huang et al. and Sendner et al. However, on nanopatterned surfaces, significant deviations arise due to contact line pinning, which disrupts the expected correlation between wettability and slip. We observe a clear decoupling between hydrophobicity and interfacial mobility, which can be exemplified by rose petal-like behavior where high contact angles are accompanied by strong pinning and minimal slip. These findings underscore the limitations of existing theoretical models when applied to heterogeneous interfaces and emphasize the need for refined frameworks that account for pinning effects. This study offers such a framework, providing new insight into interfacial transport and guiding the design of functionally structured surfaces.

The liquid drop captured at the capillary end, which is observed in capillary valve and pendant drop technique, is investigated theoretically and experimentally. Because of contact line pinning of the lower meniscus, the lower contact angle is able to rise from the intrinsic contact angle (θ*) so that the external force acting on the drop can be balanced by the capillary force. In the absence of contact angle hysteresis (CAH), the upper contact angle remains at θ*. However, in the presence of CAH, the upper contact angle can descend to provide more capillary force. The coupling between the lower and upper contact angles determines the equilibrium shape of the captured drop. In a capillary valve, the pinned contact line can move across the edge as the pressure difference exceeds the valving pressure, which depends on the geometrical characteristic and wetting property of the valve opening. When CAH is considered, the valving pressure is elevated because the capillary force is enhanced by the receding contact angle. For a pendant drop under gravity, the maximal capillary force is achieved as the lower contact angle reaches 180° in the absence of CAH. However, in the presence of CAH, four regimes can be identified by three critical drop volumes. The lower contact angle can exceed 180°, and therefore the drop takes on the shape of a light bulb, which does not exist in the absence of CAH. The comparisons between Surface Evolver simulations and experiments are quite well.

Adhesive and marine biofouling release properties of coatings containing surface-oriented perfluoroalkyl groups were investigated. These coatings were prepared by cross-linking a copolymer of 1H,1H,2H,2H-heptadecafluorodecyl acrylate and acrylic acid with a copolymer of poly(2-isopropenyl-2-oxazoline) and methyl methacrylate at different molar ratios. The relationships between contact angle, contact angle hysteresis, adhesion, and marine biofouling were studied. Adhesion was determined by peel tests using pressure-sensitive adhesives. The chemical nature of the surfaces was studied by using X-ray photoelectron spectroscopy. Resistance to marine biofouling of an optimized coating was studied by immersion in seawater and compared to previous, less optimized coatings. The adhesive release properties of the coatings did not correlate well with the surface energies of the coatings estimated from the static and advancing contact angles nor with the amount of fluorine present on the surface. The adhesive properties of the surfaces, however, show a correlation with water receding contact angles and contact angle hysteresis (or wetting hysteresis) resulting from surface penetration and surface reconstruction. Coatings having the best release properties had both the highest cross-link density and the lowest contact angle hysteresis. An optimized coating exhibited unprecedented resistance to marine biofouling. Water contact angle hysteresis appears to correlate with marine biofouling resistance.

Contact angle hysteresis on fluoropolymer surfaces