Surface instability during the extrusion of viscoelastic gels*

The surfaces of soft solids are known to spontaneously deform and manifest various types of morphologies when subject to compressive stress. Different surface instabilities emerge depending on two main factors: the shear modulus of the material and the applied compressive strain. There have been many investigations of the mechanics of these phenomena and researchers have found countless engineering uses of soft deformable surfaces with these tunable surface morphologies.

In this presentation, we will present a new mechanism that can create surface instabilities during extrusion-type flows. In particular, we found that when an ultra-soft viscoelastic gel (G = O(1-10) Pa) is extruded in a cylindrical annular channel, the front of the fluid spontaneously deforms and develops a periodic surface structure at some critical distance along the channel. During the extrusion of the material, the free surface manifests a cascade of different surface instabilities. Initially, a wrinkle-like instability occurs, then fingers with cusped edges start to grow. This crease-like structure then undergoes stress relaxation behavior as the material flows in the channel. We will present the experimental setup, introduce important parameters in the system, and discuss how different parameters affect this cascade of surface instabilities during the gel flow in an annular channel. Lastly, we will also discuss the potential cause and mechanism of this instability.