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SUMMARY:MechE Colloquium: Air mediates the impact of a compliant hemispher
 e on a rigid smooth surface
DTSTART:20210525T121500
DTEND:20210525T131500
DTSTAMP:20260916T051433Z
UID:46efa2dda4165febea35bfee6d0ae8d6720aa678b9eed7302539bc92
CATEGORIES:Conferences - Seminars
DESCRIPTION:Prof. John Kolinski\, Engineering Mechanics of Soft Interfaces
  Laboratory (EMSI)\, EPFL School of Engineering (STI)\, Institute of Mecha
 nical Engineering (IGM)\nAbstract:\nFleeting contact between solids immers
 ed in a fluid medium governs the response of critically important material
 s. Indeed\, the fluid layer mediating solid contact fundamentally alters t
 he interaction between grains of soil or coffee\, directly modifying the c
 onstitutive properties of suspensions\; typically this interface is obscur
 ed\, making direct study of its kinematics very challenging.\n\nHere we di
 rectly image the interface between a soft elastic hemisphere and a flat ri
 gid 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 hemi
 sphere\, and altering the area of contact. The size of the air pocket vari
 es non-monotonically with V and impactor stiffness\, initially increasing 
 in a regime where elastic stresses balance lubrication stresses. At suffic
 iently large V\, the inertial stress dominates the elastic stress\, and th
 e air pocket size decreases as V continues to increase. Our measurements r
 eveal an unanticipated\, sudden transition of the air pocket's size as V i
 ncreases beyond ~ 1.5 m/s. Several modalities of contact front advancement
  emerge\, and these modalities appear to correlate with the ratio of the o
 utward velocity of the front to the Rayleigh velocity c_R of the elastic i
 mpactor. 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 ela
 sto-lubricative instability. Using the unique capabilities of the VFT\, we
  identify several V-dependent transitions of fluid-mediated soft contact t
 hat can inform engineering design in systems as diverse as car tires\, sof
 t robotic locomotion and suspensions such as soil and coffee.\n\nBio:\nKol
 inski studied both engineering mechanics and mathematics at the Unive
 rsity of Illinois at Urbana–Champaign and graduated with Bachelor's deg
 rees in both subjects in 2008\, before earning a Master's degree in appli
 ed mathematics (Sc.M.) and a Ph.D. in applied physics from Harvard Uni
 versity\, in 2010 and 2013\, respectively. His Ph.D. thesis on "The role o
 f air in droplet impact on a smooth\, solid surface" was supervised by La
 kshminarayanan Mahadevan and Shmuel Rubinstein.[3][4][5][6][7] Supported
  by a Fulbright-Israel post-doctoral fellowship\, he moved in 2014 to Isra
 el to work with Eran Sharon and Jay Fineberg at the Racah Institute of Phy
 sics at the Hebrew University of Jerusalem. There he studied the inter-fa
 cial instabilities in fluid and solid systems such as water bells and the 
 fracture of hydrogels.[8][9][10]\nSince May 2017\, Kolinski has been a Ten
 ure Track Assistant Professor at EPFL and the head of the Laboratory of 
 Engineering Mechanics of Soft Interfaces (EMSI) at EPFL's School of Engine
 ering.
LOCATION:https://epfl.zoom.us/j/82207717593
STATUS:CONFIRMED
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