Abstract
We investigate the effects of surface stiffness on the air cushioning at the bottom of a liquid drop impacting onto a soft solid and the resulting entrapment of a central bubble. This was achieved using ultra-high-speed interferometry at 5 million frames per second and spatial resolution of 1.05 μm per pixel. The soft solid delays the effects of gas compressibility resulting in much larger air discs than corresponding impacts onto rigid surfaces. Using an effective impact velocity equal to half of the actual impact velocity brings the soft solid scaling behavior better in line with rigid substrate scaling. We also observe extended gliding of the drop as it initially avoids contact with the surface spreading over a thin layer of air and investigate the threshold velocity for the transition from gliding to ring contact. Such extended gliding layers have previously been seen for high-viscosity drop impacts, but not for low-viscosity liquids at the impact velocities used herein.
Highlights
We investigate the effects of surface stiffness on the air cushioning at the bottom of a liquid drop impacting onto a soft solid and the resulting entrapment of a central bubble
The soft solid delays the effects of gas compressibility resulting in much larger air discs than corresponding impacts onto rigid surfaces
Droplet impacts are important in a variety of natural and industrial processes, from aircraft icing and inkjet printing, to spray cooling and beyond.[1]
Summary
Droplet impacts are important in a variety of natural and industrial processes, from aircraft icing and inkjet printing, to spray cooling and beyond.[1]. Li and Thoroddsen[9] using time-resolved interferometery with only 200 ns between frames observed the formation of the dimple in the drop and the rapid expansion after the initial ring contact They provided empirical corrections to the adiabatic compression theory of Mandre et al for H* shown in eqn (2). For their highest impact velocities, they observed compression ratios as high as 14 when comparing the final volume of the central bubble to the initial volume of the air disc. Our focus is on the formation of a dimple in the droplet, the size of the entrapped air bubble, the dynamics of contact and the subsequent wetting of the substrate
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