Active elastocapillarity in soft solids with negative surface tension

Active solids consume energy to allow for actuation, shape change, and wave propagation not possible in equilibrium. Whereas active interfaces have been realized across many experimental systems, control of three-dimensional (3D) bulk materials remains a challenge. Here, we develop continuum theory and microscopic simulations that describe a 3D soft solid whose boundary experiences active surface stresses. The competition between active boundary and elastic bulk yields a broad range of previously unexplored phenomena, which are demonstrations of so-called active elastocapillarity. In contrast to thin shells and vesicles, we discover that bulk 3D elasticity controls snap-through transitions between different anisotropic shapes. These transitions meet at a critical point, allowing a universal classification via Landau theory. In addition, the active surface modifies elastic wave propagation to allow zero, or even negative, group velocities. These phenomena offer robust principles for programming shape change and functionality into active solids, from robotic metamaterials down to shape-shifting nanoparticles.

Automated summary

Researchers have developed continuum theory and microscopic simulations to describe a 3D soft solid with an active surface boundary, which controls shape change and wave propagation. Unlike thin shells and vesicles, they discovered that the 3D elasticity of the bulk material plays a crucial role in snap-through transitions between different shapes, which can be universally classified using Landau theory. Additionally, the active surface modifies elastic wave propagation by allowing zero or negative group velocities.