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Register a new userGeneration of Motion of Drops with Interfacial Contact
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A liquid drop moves on a solid surface if it is subjected to a gradient of wettability or temperature. However, the pinning defects on the surface manifested in terms of a wetting hysteresis, or first-order nonlinear friction, limit the motion in the sense that a critical size has to be exceeded for a drop to move. The effect of hysteresis can, however, be mitigated by an external vibration that can be either structured or stochastic, thereby creating a directed motion of the drop. Many of the well-known features of rectification, amplification, and switching that are generic to electronics can be engineered with such types of movements. A specific case of interest is the random coalescence of drops on a surface that gives rise to self-generated noise. This noise overcomes the pinning potential, thereby generating a random motion of the coalesced drops. Randomly moving coalesced drops themselves exhibit a directed diffusive flux when a boundary is present to eliminate them by absorption. With the presence of a bias, the coalesced drops execute a diffusive drift motion that can have useful applications in various water and thermal management technologies.
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A new mechanism for the passive removal of drop on a horizontal surface is described that does not require pre-fabrication of a surface energy gradient. The method relies upon the preparation of alternate hydrophilic/hydrophobic stripes on a surface. When one side of this surface is exposed to steam, with its other surface convectively cooled with cold water, steam condenses as a continuous film on the hydrophilic stripes but as droplets on the hydrophobic stripes. Coalescence leads to a random motion of the center of mass of the fused drops on the surface, which are readily removed as they reach near the boundary of the hydrophobic and hydrophilic zones thus resulting in a net diffusive flux of the coalesced drops from the hydrophobic to the hydrophilic stripes of the surface. Although an in-situ produced thermal gradient due to differential heat transfer coefficients of the hydrophilic and hydrophobic stripes could provide additional driving force for such a motion, it is, however, not a necessary condition for motion to occur. This method of creating directed motion of drops does not require a pre-existing wettability gradient and may have useful applications in thermal management devices.
In forced wetting, a rapidly moving surface drags with it a thin layer of trailing fluid as it is plunged into a second fluid bath. Using high-speed interferometry, we find characteristic structure in the thickness of this layer with multiple thin flat triangular structures separated by much thicker regions. These features, depending on liquid viscosity and penetration velocity, are robust and occur in both wetting and de-wetting geometries. Their presence clearly shows the inadequacy of theoretical analysis that ignores the instability in the transverse direction.
We report on a new mode of self-propulsion exhibited by compact drops of active liquids on a substrate which, remarkably, is tractionless, i.e., which imparts no mechanical stress locally on the surface. We show, both analytically and by numerical simulation, that the equations of motion for an active nematic drop possess a simple self-propelling solution, with no traction on the solid surface and in which the direction of motion is controlled by the winding of the nematic director field across the drop height. The physics underlying this mode of motion has the same origins as that giving rise to the zero viscosity observed in bacterial suspensions. This topologically protected tractionless self-propusion provides a robust physical mechanism for efficient cell migration in crowded environments like tissues.
Sessile drops of soft hydrogels were vibrated vertically by subjecting them to a mechanically induced Gaussian white noise. Power spectra of the surface fluctuation of the gel allowed identification of its resonant frequency that decreases with their mass, but increases with its shear modulus. The principal resonant frequencies of the spheroidal modes of the gel of shear moduli ranging from 55 Pa to 290 Pa were closest to the lowest Rayleigh mode of vibration of a drop of pure water. These observations coupled with the fact that the resonance frequency varies inversely as the square root of the mass in all cases suggest that they primarily correspond to the capillary (or a pseudo-capillary) mode of drop vibration. The contact angles of the gel drops also increase with the modulus of the gel. When the resonance frequencies are corrected for the wetting angles, and plotted against the fundamental frequency scale (gamma/mu)^0.5, all the data collapse nicely on a single plot provided that the latter is shifted by a shear modulus dependent factor (1+mu.L/gamma). A length scale L, independent of both the modulus and the mass of the drop emerges from such a fit.
The sliding of non-Newtonian drops down planar surfaces results in a complex, entangled balance between interfacial forces and non linear viscous dissipation, which has been scarcely inspected. In particular, a detailed understanding of the role played by the polymer flexibility and the resulting elasticity of the polymer solution is still lacking. To this aim, we have considered polyacrylamide (PAA) solutions of different molecular weights, suspended either in water or glycerol/water mixtures. In contrast to drops with stiff polymers, drops with flexible polymers exhibit a remarkable elongation in steady sliding. This difference is most likely attributed to different viscous bending as a consequence of different shear thinning. Moreover, an optimal elasticity of the polymer seems to be required for this drop elongation to be visible. We have complemented experimental results with numerical simulations of a viscoelastic FENE-P drop. This has been a decisive step to unravel how a change of the elastic parameters (e.g. polymer relaxation time, maximum extensibility) affects the dimensionless sliding velocity.
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