74 research outputs found
The mechanical equilibrium of soft solids with surface elasticity
Recent experiments have shown that surface stresses in soft materials can
have a significant strain-dependence. Here we explore the implications of this
surface elasticity to show how, and when, we expect it to arise. We develop the
appropriate boundary condition, showing that it simplifies significantly in
certain cases. We show that surface elasticity's main role is to effectively
stiffen a solid surface's response to in-plane tractions, in particular at
length-scales smaller than a characteristic elastocapillary length. We also
investigate how surface elasticity effects the Green's-function problem of a
line force on a flat, linear-elastic substrate. There are significant changes
to this solution, especially in that the well-known displacement singularity is
regularised. This raises interesting implications for soft phenomena like
wetting contact lines, adhesion and friction. Finally, we discuss open
questions, future directions, and close ties with existing fields of research
Strain-dependent solid surface stress and the stiffness of soft contacts
Surface stresses have recently emerged as a key player in the mechanics of
highly compliant solids. The classic theories of contact mechanics describe
adhesion with a compliant substrate as a competition between surface energies
driving deformation to establish contact and bulk elasticity resisting this.
However, it has recently been shown that surface stresses provide an additional
restoring force that can compete with and even dominate over elasticity in
highly compliant materials, especially when length scales are small compared to
the ratio of the surface stress to the elastic modulus, . Here, we
investigate experimentally the contribution of surface stresses to the force of
adhesion. We find that the elastic and capillary contributions to the adhesive
force are of similar magnitude, and that both are required to account for
measured adhesive forces between rigid silica spheres and compliant, silicone
gels. Notably, the strain-dependence of the solid surface stress contributes
significantly to the stiffness of soft solid contacts.Comment: 6 pages, 3 figure
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