Randy Ewoldt

University of Illinois at Urbana-Champaign, Mechanical Science and Engineering




CEB 218, 810 S. Clinton Street


Yield-stress fluids, including gels and pastes, are effectively fluid at high stress and solid at low stress. In liquid-solid impacts, yield-stress fluids can stick and accumulate where they impact, motivating several applications of these rheologically-complex materials (including fire suppression and spray coating). Here we experimentally study yield-stress fluids impacting three types of surfaces where they may (or may not!) stick: pre-coated surfaces, hot surfaces, and permeable surfaces. Using high-speed video and quantitative analysis, we report various regimes of splashing, Leidenfrost effects, and flow-through. Existing dimensionless groups do not adequately characterize all these regimes. Incorporating relevant lengthscales, we demonstrate successful dimensionless groups that organize the dynamics into a lower-dimensional space. This provides insight into the physics of droplet impact problems. Moreover, it potentially allows for fluid design and extrapolation of these results to dynamically and geometrically similar situations beyond the explicit material and parameter values explored here.

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