The dynamics of a droplet on an inclined plane containing a chemical step, implying a heterogeneity in the wettability, have been widely studied because of their relevance to many applications. However, the modeling of such dynamics remains inaccurate due to the lack of implementation of contact angle hysteresis. In this work, we implement a chemical potential wetting boundary condition that includes hysteresis in a well-balanced lattice Boltzmann simulation to address that specific shortcoming. We investigate the behavior of droplet dynamics including this hysteresis force, and subsequently, also probe the effects of chemical step strength, inclination angle, and droplet volume on the droplet dynamics. We observe that the dynamics at the leading and the trailing edges of the droplet are significantly impacted by hysteresis effects and the chemical step strength. In addition, we conclude that for varying inclination angles, the hysteresis contribution is comparable to other contributing forces in the precise manipulation of the droplet.

We propose a capacitive multi-osmosis (CMO) system that can operate both in Forward Osmosis (FO) and Reverse Osmosis (RO) modes, depending on the electric potential difference across the membrane. We employ a hybrid-scale modeling approach that integrates pore-scale and membrane-scale models to evaluate system performance. The two models have been unified and validated via numerical investigations and physical experiments on a generic FO system. Based on insights from our models, we identify three competing drivers of osmosis under an electric field—pressure gradient, electroosmosis, and an anomalous drag. In conventional electric-assisted FO systems, the electrodes participate in superfluous electrochemical reactions due to contact with the feed and draw solutions. In the CMO system, the electrodes are placed outside the system and exert their electric field without physical interaction with the solutions or any system components, thereby preventing any reactions. The non-contact electric fields generated when the system is subjected to controlled electric potential differences can significantly enhance the permeate recovery in an FO system, paving the way for FO scale-up.

We investigated the evaporative crystallization of aqueous glycine sessile droplets on hydrophilic glass, hydrophobic Teflon surfaces, and hydrophobic Teflon surfaces, where the contact angle is manipulated dynamically with electrowetting. Microscopy experiments and analytical characterization revealed that the size, morphology, and polymorphic form (α, β, and γ) of the glycine crystals are influenced by the surface wettability as well as the amplitude and frequency of electrowetting. On a hydrophilic glass surface, a coffee-stain-shaped residue composed of a mixture of bipyramidal α and needle-like β crystals was observed. On a hydrophobic Teflon surface, the droplets evaporated with minimum contact line pinning, producing hemispherical residue shapes, and bipyramidal α crystals smaller than 100 μm were formed. On a Teflon surface with electrowetting, glycine could be manipulated to crystallize into distinct polymorphic forms (β and γ) and residue shapes not observed on hydrophilic glass and hydrophobic Teflon surfaces. The frequency and amplitude of electrowetting were optimized to produce single large crystals. We observed the highest chance of producing single-millimeter-scale crystals at a frequency of 1 kHz and a voltage amplitude of 80 Vrms. We attribute this observation to a combination of nucleation at lower bulk supersaturation compared to the experiment on Teflon surfaces and electrowetting-induced mixing most prominent at 1 kHz. Our results highlight the opportunities arising from the dynamic manipulation of surface wettability

Concentrated emulsions flowing through channels of varying widths are omnipresent in daily life, from dispensing mayonnaise in our kitchens to large-scale industrial processing of food, pharmaceuticals, etc. Local changes in channel geometry affect the stability of emulsions over length scales far beyond the droplet magnitude, for example through propagation of coalescence events called a coalescence avalanche. The underlying mechanisms are not well understood. In this work, we investigated the stability of concentrated emulsions flowing through microchannels featuring a constriction. We found that in this model geometry, the acceleration of the droplets induced near the entrance of the constriction triggers a coalescence event between the leading and the trailing droplet, but only above a critical droplet velocity. This separation-induced coalescence event, in turn, was found to trigger a coalescence avalanche in the upstream direction. Analysis of the flow behavior through particle image velocimetry and particle tracking velocimetry revealed that the propagation also follows a separation-induced coalescence mechanism, due to the retraction of the interface of the trailing droplet upon coalescence and the corresponding acceleration of the liquid inside the coalesced fluid thread. The constriction ratio was found to enhance the coalescence occurrence but did not affect the speed of coalescence propagation.

As part of the final projects of our introductory lab course, students conceived experiments related to the umbrella topic of 'Physics of toys and sports' and carried out the experiments at their homes. This paper revisits two of these experiments described by student teams and illustrates how self-conceived experiments provide opportunities to truly engage students in doing science.

Emulsion stability in a flow field is an extremely important issue relevant for many daily-life applications such as separation processes, food manufacturing, oil recovery etc. Microfluidic studies can provide micro-scale insight of the emulsion behavior but have primarily focussed on droplet breakup rather than on droplet coalescence. The crucial impact of certain conditions such as increased pressure or elevated temperature frequently used in industrial processes is completely overlooked in such micro-scale studies. In this work, we investigate droplet coalescence in flowing oil-in-water emulsions subjected to higher than room temperatures namely between 20 to 70 ^∘C. We use a specifically designed lab-on-a-chip application for this purpose. Coalescence frequency is observed to increase with increasing temperature. We associate with this observation the change in viscosity at higher temperatures triggering a stronger perturbation in the thin aqueous film separating the droplets. Using the scaling law for rupture time of such a thin film, we establish a mechanism leading to a higher coalescence frequency at elevated temperatures.