Leidenfrost-induced active matter dynamics of spherical hydrogels

Dropping a spherical hydrogel onto a heated surface triggers rapid vapor formation and vigorous bouncing governed by the Leidenfrost effect. The vapor trapped by the local deformation of the hydrogel is suddenly released, leading to additional kinetic energy that propels the hydrogel. When the hydrogel is confined within a heated enclosure where both the bottom and walls are heated, it experiences simultaneous Leidenfrost effects from multiple surfaces, resulting in explosive and multidirectional motion. The motion of the hydrogel strongly depends on the surface temperature, exhibiting distinct dynamic regimes. At low temperatures, it fails to acquire sufficient momentum from the heated walls, resulting in limited movement. Within an optimal temperature range, however, the hydrogel exhibits explosive motion driven by continuous energy input, effectively behaving as an active matter system powered by thermal gradients. To quantitatively investigate the Leidenfrost-driven active matter behavior, we track the motion of the hydrogel using image processing techniques and extract its trajectory and horizontal velocity. We further analyze temperature-dependent collision frequency and develop the theoretical model the motion duration, comparing the predictions with experimental observations. We anticipate that this study will shed light on the fundamental physics of active matter, particularly autonomous motion driven by energy-supplying boundaries.