Abstract- Whereas the governing equations of fluids and solids as single phases were derived over a century ago, the physics at the interface between these continua can be surprisingly rich and complex - instabilities emerge, interfacial forces become dominant, and mechanical fields vary across scales, from the molecular- to the sample-scale. Our lab probes these phenomena with impacting droplets and cracks in hydrogels, where we directly image the kinematics of obscured interfaces using 3D microscopy. In this talk, I will discuss two vignettes: first, the emergence of an interfacial instability beneath an impacting droplet of alcohol on an atomically smooth mica substrate, and second, the geometry and stability of complex cracks. These seemingly disparate systems are connected on a variety of levels, from their sensitivity to defects, to the duality of contact formation and bond rupture, which implicate mechanical fields across scales. A discussion of some open questions and future perspectives will conclude the talk.

Bio- Dr. Kolinski studied Applied Mathematics (Sc.M.) and Applied Physics (Ph.D.) at Harvard University, completing a PhD under the supervision of L. Mahadevan and Shmuel Rubinstein on the role of air in droplet impact. John did his post-doc at the Hebrew University of Jerusalem in Israel supported by the Fulbright post-doctoral fellowship. At HUJI, he worked on interfacial instabilities in soft matter in the labs of Eran Sharon and Jay Fineberg. Since 2017, he is a tenure-track assistant professor in the Institute of Mechanical Engineering at EPFL, where his group studies propagating singularities in the form of cracks and contact lines.