Search

Hysteresis of wetting, like the Coulombic friction at solid/solid interface, impedes the motion of a liquid drop on a surface when subjected to an external field. Here, we present a counterintuitive example, where some amount of hysteresis enables a drop to move on a surface when it is subjected to a periodic but asymmetric vibration. Experiments show that a surface either with a negligible or high hysteresis is not conducive to any drop motion. Some finite hysteresis of contact angle is needed to break the periodic symmetry of the forcing function for the drift to occur. These experimental results are consistent with simulations, in which a drop is approximated as a linear harmonic oscillator. The experiment also sheds light on the effect of the drop size on flow reversal, where drops of different sizes move in opposite directions due to the difference in the phase of the oscillation of their center of mass. [ABSTRACT FROM AUTHOR]

A liquid drop placed over a solid surface with a wettability gradient self-propels to minimize its surface energy. It can also climb an inclined plane if the applied gradient strength is high enough. We investigate the motion of liquid drops over an inclined gradient surface using a unique 3D computational technique. The technique combines diffuse interface in a smoothed particle hydrodynamics simulation to study the internal fluid structure and the contact line dynamics. Simulation results reveal that drop motion is dependent on its volume, surface inclination, and the strength of the wettability gradient. It has been demonstrated that, depending on these parameters, a drop can experience upward or downward motion or can remain stationary on the inclined plane. Finally, drop mobility maps which give an idea about the regimes of uphill and downhill movement of a drop over gradient surfaces have been proposed. [ABSTRACT FROM AUTHOR]

We study anisotropic wetting in systems governed by Lennard−Jones interactions. Molecular simulation is used to obtain the macroscopic contact angle a fluid adopts on face-centered-, body-centered-, and simple-cubic lattices with the (100), (110), or (111) face in contact with the fluid. Several amorphous substrates are also examined. Substrates are modeled as a static collection of particles. For a given set of calculations, the atomistic density of the substrate and the particle−particle interactions (surface−fluid and fluid−fluid) remain fixed. These constraints enable us to focus on the extent to which substrate structure influences the contact angle. Three substrate−fluid interaction strengths are considered, which provide wetting conditions that span from near-dry to near-wet. Our results indicate that the manner in which particles are organized within the substrate significantly influences the contact angle. For strong substrates (near-wet case), a change in the substrate structure can change the cosine of the contact angle by as much as 0.5. We also examine how well certain structural and energetic features of the substrate−fluid system serve as suitable metrics for predicting the variation of the contact angle with substrate topography. Three parameters are considered: the density of atoms within the crystalline plane closest to the fluid, a measure of the effective strength of the substrate−fluid interaction, and the roughness of the solid−liquid interface. The effective strength of the substrate potential shows the strongest correlation with the contact angle. This energy-based parameter is defined in a general manner and therefore could serve as a useful tool for describing the anisotropic wetting of solids. In contrast, the metrics based on planar density and interface roughness are found to correlate with contact angle data relatively weakly. [ABSTRACT FROM AUTHOR]

We present an extensive molecular simulation study of the behavior of a flexible hydrophobic 25-mer polymer at interfaces presenting a range of chemistries from hydrophobic (−CH3) to hydrophilic (−CONH2). We quantify the free energy of adsorption, conformational equilibria, and translational and conformational dynamics of the polymer at these diverse interfaces. Water-mediated interactions drive the polymer to adsorb strongly at a hydrophobic interface and repel it from hydrophilic ones. At hydrophilic surfaces, van der Waals interactions between the polymer and the surface mitigate this water-mediated repulsion, leading to weak adsorption of the polymer. Although the polymer is strongly adsorbed to hydrophobic surfaces, it is also most dynamic there. Translational diffusion and conformational dynamics are faster at hydrophobic surfaces compared to those at hydrophilic ones. In bulk water, the polymer collapses into compact globular shapes, whereas the thermodynamic stability of folded polymers is significantly lowered at hydrophobic surfaces. The polymer spreads into pancake-like 2D conformations at hydrophobic surfaces and gradually beads up into globular shapes as the surface is made more hydrophilic. Interestingly, the binding thermodynamics and dynamics correlate with macroscopic droplet contact angles that characterize the wetting properties of the different interfaces. [ABSTRACT FROM AUTHOR]