Free-surface Interfacial Instability of Squeezed Elastic Liquid Droplets

Various methods to control, suppress, or amplify interfacial instabilities are discussed to harness engineering uses of soft materials, and to advance our understanding of the soft matter physics. Researchers are naturally drawn to physical systems that exhibit instabilities in situations outside the bounds outlined by classical theories. For example, a few systems demonstrate `inverse' viscous fingers - protrusions growing at interfaces where a more viscous/elastic material displaces a less viscous/elastic material - which otherwise would not be present in physical systems that obey Saffman-Taylor mechanisms.

Surprisingly, and to the best of our knowledge, no attention has been given to the stability of the moving air/gel interface of a viscoelastic droplet with a fixed volume, as the droplet is squeezed between parallel surfaces, and its free interface is pushed radially outward. Available literature on this scenario does not report the appearance of any interfacial instability, while we experimentally observed formation of an unusual wrinkle forming at the radially advancing free interface; we are not aware of any previous reports of this phenomenon. We characterize the problem for a wide range of material viscoelasticity, discuss the critical conditions leading to the formation of the instability, and explain the role of elasticity in this unusual interfacial instability.