Bouncing off the walls : the influence of gas-kinetic and van der Waals effects in drop impact

A model is developed for liquid drop impact on a solid surface that captures the thin film gas flow beneath the drop, even when the film’s thickness is below the mean free path in the gas so that gas kinetic effects (GKE) are important. Simulation results agree with experiments, with the impact speed threshold between bouncing and wetting reproduced to within 5 least 50 mapped and provides experimentally verifiable predictions. There are two principal modes of contact leading to wetting and both are associated with a van der Waals driven instability of the film

Colloidal deposits from evaporating sessile droplets:Coffee ring versus surface capture

Suppression of the coffee ring effect is desirable in many industrial applications which utilize colloidal deposition from an evaporating liquid. Here we focus on the role of particle arrest at the liquid-air interface (surface capture) which occurs at high evaporation rates. It is known experimentally that this phenomenon inhibits particles from reaching the contact line, leading to a deposit which is closer to uniform. We are able to describe this effect using a simple 1D modeling framework and, utilizing asymptotic theory, parametrize our model by the ratio of the vertical advection and diffusion timescales. We show that our model is consistent with existing frameworks for small values of this parameter, but also predicts the surface layer formation seen experimentally at high evaporation rates. The formation of a surface layer leads to a deposit morphology which mimics the evaporative flux density and so is closest to uniform when evaporation has a constant strength across the liquid-air interface.</p

Colloidal deposits from evaporating sessile droplets:A computationally efficient framework for predicting the final deposit shape

A modeling framework for the complete evaporation of particle-laden droplets, including touchdown events where the air-liquid interface approaches the substrate, is presented. The inclusion of particle jamming into the model, creating a transition from free advection to an immobile porous plug, is essential to making realistic predictions for the deposit's dimensions. By removing the need to track jammed particle fronts explicitly, as often considered, we are able to run simulations until solute has jammed everywhere in the drop. This allows for valid comparisons with experimental findings on the dried deposit's topography. Our model can also be easily applied to general contact line geometries, allowing us to explore the influence of contact line curvature on the local deposit profile. Published by the American Physical Society 202

Emergence of dissipation and hysteresis from interactions among reversible, non-dissipative units: The case of fluid-fluid interfaces

We examine the nonequilibrium nature of two-phase fluid displacements in a quasi-two-dimensional medium (a model open fracture), in the presence of localized constrictions ("defects"), from a theoretical and numerical standpoint. Our analysis predicts the capillary energy dissipated in abrupt interfacial displacements (jumps) across defects, and relates it to the corresponding hysteresis cycle, e.g. in pressure-saturation. We distinguish between "weak" (reversible interface displacement, exhibiting no hysteresis and dissipation) and "strong" (irreversible) defects. We expose the emergence of dissipation and irreversibility caused by spatial interactions, mediated by interfacial tension, among otherwise weak defects. We exemplify this cooperative behavior for a pair of weak defects and establish a critical separation distance, analytically and numerically, verified by a proof-of-concept experiment

Emergence of dissipation and hysteresis from interactions among reversible, non-dissipative units:The case of fluid-fluid interfaces

We examine the nonequilibrium nature of two-phase fluid displacements in a quasi-two-dimensional medium (a model open fracture), in the presence of localized constrictions ("defects"), from a theoretical and numerical standpoint. Our analysis predicts the capillary energy dissipated in abrupt interfacial displacements (jumps) across defects, and relates it to the corresponding hysteresis cycle, e.g. in pressure-saturation. We distinguish between "weak" (reversible interface displacement, exhibiting no hysteresis and dissipation) and "strong" (irreversible) defects. We expose the emergence of dissipation and irreversibility caused by spatial interactions, mediated by interfacial tension, among otherwise weak defects. We exemplify this cooperative behavior for a pair of weak defects and establish a critical separation distance, analytically and numerically, verified by a proof-of-concept experiment