What happens when a foam interacts with a sliding surface? That’s the question at the heart of this study, which finds three major regimes of foam-surface interaction. On smooth surfaces (Image 1), foams will simply slide against the wall without sticking or deforming. When surface roughness is about as large as the foam’s wall thickness (Image 2), the foam will stick to individual asperities, then slip to the next rough spot as the wall moves. But when the surface roughness is large compared to the foam wall (Image 3), the foam will remain anchored to the surface and all the shear from the wall’s movement goes into deforming the bulk of the foam.
Researchers thus found they could change foam’s behavior by changing the surface roughness. They also looked at the reverse situation: a surface with fixed roughness — like, say, a human tongue — and how tuning the size of foam bubbles might alter perception and ease of swallowing. That’s what we’re looking at in the last image, where a spoon slides a foam along a surface with roughness similar to the human tongue. (Image and research credit: M. Marchand et al.)