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  4. Air mediates the impact of a compliant hemisphere on a rigid smooth surface
 
research article

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

Zheng, Siqi
•
Dillavou, Sam
•
Kolinski, John M.  
April 14, 2021
Soft Matter

Fleeting contact between solids immersed in a fluid medium governs the response of critically important materials, from coffee to soil. Rapid impact of soft solids occurs in systems as diverse as car tires, soft robotic locomotion and suspensions, including soil and coffee. In each of these systems, the dynamics are fundamentally altered by the presence of a fluid layer mediating solid contact. However, observing this class of interactions directly is challenging, as the relevant time and length scales are extremely small. 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 and spatial resolution using the Virtual Frame Technique (VFT). In each experiment, a pocket of air is trapped in a dimple between the impactor and the substrate, preventing direct solid-solid contact at the apex of the hemisphere. Thus, unlike the quasi-static Hertzian solution where contact forms in an ellipse, in each rapid air-mediated impact, contact forms in an annular region which rapidly grows both inward toward the impact axis, and rapidly outward away from the impact axis. We find that the radius of initial contact varies non-monotonically with V, indicating a transition between elastically dominated dynamics to inertially dominated dynamics. Furthermore, we find that for slower impact speeds, where the outer contact front cannot outpace the Rayleigh velocity, contact expands in a patchy manner, indicating an elasto-lubricative instability. These behaviors, observed using the VFT, occur in regimes relevant to a wide variety of soft systems, and might modulate frictional properties during contact. The size of the air pocket varies with V and impactor stiffness. Our measurements reveal an unanticipated, sudden transition of the air pocket's size as V increases beyond 1 m s(-1) and multiple modes of air entrainment at the advancing solid-solid contact front that depend on the front's velocity.

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Type
research article
DOI
10.1039/d0sm02163f
Web of Science ID

WOS:000640321200006

Author(s)
Zheng, Siqi
Dillavou, Sam
Kolinski, John M.  
Date Issued

2021-04-14

Published in
Soft Matter
Volume

17

Issue

14

Start page

3813

End page

3819

Subjects

Chemistry, Physical

•

Materials Science, Multidisciplinary

•

Physics, Multidisciplinary

•

Polymer Science

•

Chemistry

•

Materials Science

•

Physics

•

colloidal gels

•

spinodal decomposition

•

rheology

•

dynamics

•

gelation

•

behavior

•

stress

•

flow

Editorial or Peer reviewed

REVIEWED

Written at

EPFL

EPFL units
EMSI  
Available on Infoscience
June 5, 2021
Use this identifier to reference this record
https://infoscience.epfl.ch/handle/20.500.14299/178549
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