Surface Instability of Extremely Soft Solids 'Flowing' Through Confinement*

A wide range of problems in physics, engineering, and biology involve flows of elastic materials. We studied extrusion-like flows of extremely soft elastic solids (with shear modulus G ~ O(0.1 - 10) Pa) through confinement and identified conditions leading to instability on the extrusion front (gel-air free interface). The observed instability features the formation of spontaneous buckles in the direction transverse to the flow, resulting in a furrow-like morphology that deepens over time. The ‘furrowing’ instability features distinctive characteristics from known elastic surface instabilities and appears to be a new kind of instability.

First, we review the rheological characteristics of these extremely soft solids that enable motion inside narrow gaps by letting the bulk material continuously evert out, without fracturing. Next, we contrast the characteristics of the furrowing instability from those of other elastic surface instabilities using experiments and theory. Our study may provide insights into various processes involving the motion and stability of soft solids, including biomaterials, inside confined geometries.