Dynamic elastocapillary coalescence of fish gill lamellae.

Aquatic animals like fishes and larval amphibians have flexible gills with a large surface area for gas exchange. When exposed to air, gills typically collapse and coalesce due to the elastocapillary effect, reducing gas exchange and potentially causing death. To resist these effects, some amphibians are hypothesized to have evolved stiffened gills, but these elastocapillarity effects have not been investigated empirically or theoretically. Here, we examine the deformations of artificial elastomeric gill lamellae under quasi-static and dynamic liquid crossing scenarios, inspired by conditions faced by amphibious animals when leaving water. First, we discovered multiple equilibrium states when the liquid interface is pinned to the lamellae tips, where lamellae either coalesce or remain separated depending on the liquid volume constraints. Moreover, we observe a unidirectional collapse pattern, termed the 'dominos pattern', under spatially variant drainage rate. A reduced-order dynamic model provides quantitative insights into these equilibria based on the lamellae properties, liquid volumes and drainage conditions leading to dominos patterns. These results inspire novel hypotheses about how elastocapillary may influence the evolution of gill structure in amphibious species, and also provide bioinspiration for engineering applications such as polymorphic display devices using flexible lamellae.