MechE Colloquium: Air mediates the impact of a compliant hemisphere on a rigid smooth surface

Fleeting contact between solids immersed in a fluid medium governs the response of critically important materials. Indeed, the fluid layer mediating solid contact fundamentally alters the interaction between grains of soil or coffee, directly modifying the constitutive properties of suspensions; typically this interface is obscured, making direct study of its kinematics very challenging.

Here we directly image the interface between a soft elastic hemisphere and a flat rigid substrate during rapid impact over a wide range of impact velocities $V$ at high temporal resolution using the Virtual Frame Technique (VFT). In each experiment, a pocket of air is trapped between the impactor and the substrate, preventing direct solid-solid contact at the apex of the hemisphere, and altering the area of contact. The size of the air pocket varies non-monotonically with V and impactor stiffness, initially increasing in a regime where elastic stresses balance lubrication stresses. At sufficiently large V, the inertial stress dominates the elastic stress, and the air pocket size decreases as V continues to increase. Our measurements reveal an unanticipated, sudden transition of the air pocket's size as V increases beyond ~ 1.5 m/s. Several modalities of contact front advancement emerge, and these modalities appear to correlate with the ratio of the outward velocity of the front to the Rayleigh velocity c_R of the elastic impactor. When v_out/c_R > 1, the material ahead of the advancing contact front cannot deform, and little air is entrained; however, when v_{out}/c_R < 1 the material ahead of the contact front deforms and entrains air, leading to the emergence of a patchy contact texture arising from an elasto-lubricative instability. Using the unique capabilities of the VFT, we identify several V-dependent transitions of fluid-mediated soft contact that can inform engineering design in systems as diverse as car tires, soft robotic locomotion and suspensions such as soil and coffee.

Kolinski studied both engineering mechanics and mathematics at the University of Illinois at Urbana–Champaign and graduated with Bachelor's degrees in both subjects in 2008, before earning a Master's degree in applied mathematics (Sc.M.) and a Ph.D. in applied physics from Harvard University, in 2010 and 2013, respectively. His Ph.D. thesis on "The role of air in droplet impact on a smooth, solid surface" was supervised by Lakshminarayanan Mahadevan and Shmuel Rubinstein.[3][4][5][6][7] Supported by a Fulbright-Israel post-doctoral fellowship, he moved in 2014 to Israel to work with Eran Sharon and Jay Fineberg at the Racah Institute of Physics at the Hebrew University of Jerusalem. There he studied the inter-facial instabilities in fluid and solid systems such as water bells and the fracture of hydrogels.[8][9][10]
Since May 2017, Kolinski has been a Tenure Track Assistant Professor at EPFL and the head of the Laboratory of Engineering Mechanics of Soft Interfaces (EMSI) at EPFL's School of Engineering.

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MechE Colloquium: Air mediates the impact of a compliant hemisphere on a rigid smooth surface