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What is active wetting?

Uwe Thiele

TL;DR

This paper addresses the ambiguous use of 'active wetting' across biology and active matter. It proposes a coarse taxonomy—equilibrium, relaxational, driven, and reactive wetting—to organize static and dynamic wetting phenomena and surveys examples from biomolecular condensates, cell layers, and active particle systems. A tentative, operational definition of active wetting is offered, grounded in microscopic chemo-mechanical coupling within active liquids, while highlighting caveats and overlaps with the four classical categories. The framework connects wetting behavior to classical relations like the spreading coefficient $S=\gamma_{\mathrm{lg}}(\cos \theta_{\mathrm{eq}}-1)$ and the dynamic law $U \sim \theta_{\mathrm{dyn}}^{3}-\theta_{\mathrm{eq}}^{3}$, and outlines future work to derive general, cross-system principles.

Abstract

In recent years the term \textit{active wetting} has gained some traction in works describing, analyzing and modeling a wide variety of wetting phenomena, for instance, in the contexts of biomolecular condensates, of cell layers or cell aggregates, and of active Brownian particles. The present perspective proposes a coarse classification of wetting phenomena including a tentative definition of active wetting. First, different categories of static and dynamic wetting of passive liquids are briefly discussed, in particular, distinguishing equilibrium wetting, relaxational wetting, driven wetting, and reactive wetting. Second, an overview is given of the various phenomena recently described as active wetting. We conclude by discussing a possible definition of active wetting together with a number of caveats one might want to keep in mind when using such classifications.

What is active wetting?

TL;DR

This paper addresses the ambiguous use of 'active wetting' across biology and active matter. It proposes a coarse taxonomy—equilibrium, relaxational, driven, and reactive wetting—to organize static and dynamic wetting phenomena and surveys examples from biomolecular condensates, cell layers, and active particle systems. A tentative, operational definition of active wetting is offered, grounded in microscopic chemo-mechanical coupling within active liquids, while highlighting caveats and overlaps with the four classical categories. The framework connects wetting behavior to classical relations like the spreading coefficient and the dynamic law , and outlines future work to derive general, cross-system principles.

Abstract

In recent years the term \textit{active wetting} has gained some traction in works describing, analyzing and modeling a wide variety of wetting phenomena, for instance, in the contexts of biomolecular condensates, of cell layers or cell aggregates, and of active Brownian particles. The present perspective proposes a coarse classification of wetting phenomena including a tentative definition of active wetting. First, different categories of static and dynamic wetting of passive liquids are briefly discussed, in particular, distinguishing equilibrium wetting, relaxational wetting, driven wetting, and reactive wetting. Second, an overview is given of the various phenomena recently described as active wetting. We conclude by discussing a possible definition of active wetting together with a number of caveats one might want to keep in mind when using such classifications.
Paper Structure (7 sections, 5 figures)

This paper contains 7 sections, 5 figures.

Table of Contents

  1. Introduction
  2. Equilibrium wetting
  3. Relaxational wetting
  4. Driven wetting
  5. Reactive wetting
  6. Active wetting
  7. Conclusion and outlook

Figures (5)

  • Figure 1: Sketches of equilibrium wetting: (a) complete wetting, (b) partial wetting, and (c) non-wetting for a liquid on a rigid smooth solid substrate; (d) equilibrated capillary rise, (e) drop of partially wetting liquid on a soft solid substrate, (f) surfactant-laden drop in an equilibrated state with steady respectively uniform surfactant concentrations in liquid bulk and at all interfaces.
  • Figure 2: Sketches of relaxational wetting: (a) spreading drop (b) dewetting liquid film via nucleation of a hole, (c) the process of capillary rise, (d) coupled drop spreading and surfactant adsorption dynamics.
  • Figure 3: Sketches of driven wetting: (a) drop sliding down an incline, (b) foot (or film) profile drawn out of a liquid bath by a moving plate (Landau-Levich geometry).
  • Figure 4: Sketches of reactive wetting: (a) moving drop self-propelled by an adsorption reaction that renders the liquid-solid interface less wettable, (b) evaporative dewetting front for a drop or film of a suspension/solution leaving behind a pattern of solute deposition.
  • Figure 5: Sketches of active wetting: (a) static active wetting layer at membrane, (b) resting cell aggregate and (c) cell monolayer on solid substrate, (d) individual crawling cell with protrusion, (e) moving and (f) splitting drop of active polar liquid.