EPJ E Colloquium - What is active wetting?

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Published on 06 July 2026

Wetting describes how liquids spread, recede, or form droplets on various substrates. While classical wetting theory was developed largely for passive liquids, the term “active wetting” has recently been used in diverse nonequilibrium settings, including biomolecular condensates, cell layers and aggregates, and suspensions of self-propelled particles. This growing use makes it timely to ask what, if anything, the term should mean across such different systems.

In a new Colloquium, published in EPJ E, Uwe Thiele (University of Münster, Germany), discusses a tentative classification of wetting phenomena distinguishing equilibrium wetting, where interfacial energies determine a final static state; relaxational wetting, where a system evolves toward such a state; driven wetting, where external forcing maintains motion or deformation; reactive wetting, where chemical or material changes modify the involved interfaces; and active wetting, where internal energy-consuming processes such as motility, growth, or active stresses affect wetting behavior.

It is then pointed out that the boundaries between these categories are not sharp. Whether a process is called active, driven, reactive, or relaxational can depend on the level of description: a microscopic model, a continuum theory, or a phenomenological approximation may suggest different classifications of the same system. The article therefore argues that it should always be explicitly stated what is meant by “active wetting” rather than treating it as a self-evident label.

More broadly, the work frames classification itself as a useful scientific tool, not a final verdict. Ambiguities and exceptions may reveal how different modeling approaches capture different aspects of living and synthetic active matter. Future comparative studies across systems should refine these taxonomies and clarify which mechanisms genuinely unite active wetting phenomena.