Riding on Elastocapillary Rails

Partially wetting droplets naturally deform soft substrates by surface tension. These deformations are usually localized to a narrow region near the contact line, forming a so-called 'elastocapillary ridge'. In this study, we explore the dynamics of a droplet sliding along such a substrate, examining three variables simultaneously via interferometry: the macroscopic shape of the droplet, the microscopic formation of the elastocapillary ridge, and the associated energy dissipation. Our observations reveal a fascinating interplay between these factors. At a higher droplet velocity, the droplet becomes bullet-like, the dissipation levels off, and we observe a pair of parallel ridges that slowly fade away behind the droplet (which we call "elastocapillary rails"). This formation of the elastocapillary rails highlights that the droplet can reshape itself to minimize dissipation.