Simple scaling laws for the evaporation of droplets pinned on pillars: Transfer-rate- and diffusion-limited regimes

Reaction-limited evaporation for the color-gradient lattice Boltzmann model

We propose a reaction-limited evaporation model within the color-gradient lattice Boltzmann (LB) multicomponent framework to address the lack of intrinsic evaporation mechanisms. Unlike diffusion-driven approaches, our method directly enforces mass removal at the fluid interface in a reaction-limited manner while maintaining numerical stability. Using the inherent color-gradient magnitude and a single adjustable parameter, evaporation sites are chosen in a computationally efficient way with seamless mass exchange between the components, with no change to the core algorithm. Extensive validation across diverse interface geometries and evaporation flux magnitudes demonstrates high accuracy, with errors below 5% for unit density ratios. For density contrasts, the method remains robust in the limit of smaller evaporation flux magnitudes and density ratios. Our approach extends the applicability of the color-gradient LB model to scenarios involving reaction-limited evaporation, such as droplet evaporation on heated substrates, vacuum evaporation of molten metals, and drying processes in porous media.

Force of adhesion between droplets and super-hydrophobic surfaces: Closed-form analytical expressions

Super-hydrophobic and liquid-repellent surfaces can be characterized experimentally in tensile adhesion experiments with the force (FAdh) required to detach a droplet from such surfaces, but analytical expressions that relate FAdh to the surface energy, w, are still missing. In this work, we derive analytical expressions for FAdh between droplets or radius r and super-hydrophobic surfaces on which the contact angle is greater than 150°. By applying the general condition for the onset of instability in different mechanical configurations, we find FAdh = -πwr and FAdh ≈ -(4/5)πwr, for the fixed-force and fixed-grip configurations, respectively, as well as other expressions that depend on the ratio of the spring constant of a generic force measuring apparatus to the surface tension of the liquid composing the droplet. These expressions open the possibility of retrieving w or, equivalently, the receding contact angle, by measuring FAdh on these systems.

Millifluidic valves and pumps made of tape and plastic

There is growing interest in producing micro- and milli-fluidic technologies made of thermoplastic with integrated fluidic control elements that are easy to assemble and suitable for mass production. Here, we developed millifluidic valves and pumps made of acrylic layers bonded with double-sided tape that are simple and fast to assemble. We demonstrate that a layer of pressure-sensitive adhesive (PSA) is flexible enough to be deformed at relatively low pressures. A chemical treatment deposited on specific regions of the PSA prevents it from sticking to the thermoplastic, which enabled us to create three different types of valves in normally open or closed configurations. We characterized different aspects of their performance, their operating pressures, the cut-off pressure values to open or close the valves (for different configurations and sizes), and the flow rate and volume pumped by seven different micropumps. As an application, we implemented a glucose assay with integrated pumps and valves, automatically generating glucose dilutions and reagent mixing. The ability to create polymeric microfluidic control units made with tape paves the way for their mass manufacturing.

Effect of PTFE Particle Size on Superhydrophobic Coating for Supercooled Water Prevention

Polytetrafluoroethylene (PTFE) chemically repels water droplets due to the nature of fluorine substituents. This paper presents an experimental study on the impact of PTFE particle size and temperature on the hydrophobicity of a surface. The present study analyzes hydrophobicity due to both the chemical properties of PTFE and the microstructure created by PTFE particles. Herein, studies of the contact angle and the sliding angle of these surfaces are described in supercooled-water conditions ranging from −10 to 0 °C. From the equations governing the surface tension and sliding angle of a droplet on a superhydrophobic surface, it is found that particle size has a much greater effect on hydrophobicity than temperature. An increase in the PTFE particle size greatly reduces the sliding angle, which indicates a lower amount of energy required to remove the droplet from the surface.