Physicochemical Hydrodynamics of Droplets out of Equilibrium: A Perspective Review

Abstract

Droplets abound in nature and technology. In general, they are multicomponent, and, when out of equilibrium, with gradients in concentration, implying flow and mass transport. Moreover, phase transitions can occur, with either evaporation, solidification, dissolution, or nucleation of a new phase. The droplets and their surrounding liquid can be binary, ternary, or contain even more components, and with several even in different phases. In the last two decades the rapid advances in experimental and numerical fluid dynamical techniques has enabled major progress in our understanding of the physicochemical hydrodynamics of such droplets, further narrowing the gap from fluid dynamics to chemical engineering and colloid & interfacial science and arriving at a quantitative understanding of multicomponent and multiphase droplet systems far from equilibrium, and aiming towards a one-to-one comparison between experiments and theory or numerics. This review will discuss various examples of the physicochemical hydrodynamics of droplet systems far from equilibrium and our present understanding of them. These include immiscible droplets in a concentration gradient, coalescence of droplets of different liquids, droplets in concentration gradients emerging from chemical reactions (including droplets as microswimmers) and phase transitions such as evaporation, solidification, dissolution, or nucleation, and droplets in ternary liquids, including solvent exchange, nano-precipitation, and the so-called ouzo effect. We will also discuss the relevance of the physicochemical hydrodynamics of such droplet systems for many important applications, including in chemical analysis and diagnostics, microanalysis, pharmaceutics, synthetic chemistry and biology, chemical and environmental engineering, the oil and remediation industries, inkjet-printing, for micro-and nano-materials, and in nanotechnolgoy.

The Fluid Dynamics of Disease Transmission

“…This effect can be caused by differences in temperature between bulk fluid and bubble cap apex by cap evaporative cooling, and by chemical or biological compound gradients. It can also be exacerbated by volatile compounds such as alcohol (Lohse & Zhang 2020) and salts, with important implications for the ocean-atmosphere coupling. All of these cases lead to thicker bubbles that continue, however, to thin as t −2/3 ).…”

Designing morphology of separated phases in multicomponent liquid mixtures

“…Note that there are only N − 1 independent volume fractions and N − 1 independent chemical potentials. Note also that the interaction parameters {χ ij } need to satisfy the condition N i,j=1 a i χ ij a j < 0 for any {a i } with N i=1 a i = 0 to ensure the stability of interfaces [1].…”

“…Phase separation and multi-phase coexistence are ubiquitous ranging from the simple demixing of water and oil to more sophisticated industrial processes related to medicine, food, cosmetics, energy, environment, etc. [1] Phase separation and multi-phase coexistence also occur in nature, where they give rise to structural colors in birds [2][3][4] and produce a plethora of intracelullar condensates [5][6][7][8].…”

“…While the minimization of the bulk free energy determines the number of coexisting phases, their compositions and volume fractions, the minimization of surface energies determines how these phases arrange in space. (Here, we neglect buoyancy effects, hydrodynamics, and chemical reactions, which can also affect morphology [1,7,26]. )…”