Gradient induced droplet motion over soft solids*

On a sufficiently-soft substrate, a resting fluid droplet will cause significant deformation of the substrate.  For such solids, it has been found experimentally and theoretically that the elastic energy resulting from this deformation causes the fluid contact angle to deviate from that given by Young's Law for rigid surfaces.  By introducing a bulk or surface gradient in stiffness or surface energy respectively, the contact angle becomes asymmetric causing a force imbalance that may cause the droplet to migrate to a region of smaller total energy, i.e. a stiffer region of the substrate.  In contrast to droplet migration over rigid surfaces, it is known that the dominant resistance to motion over soft solids is viscoelastic dissipation within the deformed solid as opposed to fluid dissipation.  In this talk we will present our analytic model to describe the droplet response to a substrate heterogeneity.  Finally, we will discuss the consequences for droplet motion over soft solids.