Abstract
We show that foams and emulsions can display a fundamentally different normal response to a simple shear deformation. While foams dilate or push outwards on the shearing surfaces, known as a positive Poynting effect, in emulsions the Poynting effect can have either sign and can be tuned by changing the emulsion properties. We relate the sign of the Poynting effect to the presence of a compressible contact network supported by adhesive contacts. When the concentration of surfactant in the continuous phase is low, the emulsions are nonadhesive and push outward on their shearing surfaces, as do the foams. When the surfactant concentration is increased, the emulsions become adhesive due to depletion interactions, and the Poynting effect changes sign. We argue that the adhesive contact network develops a shear modulus that stiffens in response to dilation, which leads to the negative Poynting effect.
Highlights
When an elastic solid is sheared, it can expand or contract in the direction perpendicular to the shear plane
We argue that the adhesive contact network develops a shear modulus that stiffens in response to dilation, which leads to the negative Poynting effect
After some algebra, we find χ = − ∂G, (2) ∂ξY 0 where G(ξY ) is the differential shear modulus evaluated at γ = 0 and arbitrary dilation
Summary
When an elastic solid is sheared, it can expand or contract in the direction perpendicular to the shear plane. If the gap between the shearing surfaces is held fixed, the solid instead pushes or pulls on the surfaces. Both of these phenomena are manifestations of the Poynting effect [1]. While there is no mechanical bound fixing the sign of the effect, most materials expand or develop a compressive stress when sheared. It has become clear that some materials, such as filamentous biopolymer networks, display a negative Poynting effect—they contract or develop a tensile normal stress [2,3]
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