66 research outputs found
Generation of Motion of Drops with Interfacial Contact
A liquid drop moves on a solid surface if it is subjected to a gradient of
wettability or temperature. However, the pinning defects on the surface
manifested in terms of a wetting hysteresis, or first-order nonlinear friction,
limit the motion in the sense that a critical size has to be exceeded for a
drop to move. The effect of hysteresis can, however, be mitigated by an
external vibration that can be either structured or stochastic, thereby
creating a directed motion of the drop. Many of the well-known features of
rectification, amplification, and switching that are generic to electronics can
be engineered with such types of movements. A specific case of interest is the
random coalescence of drops on a surface that gives rise to self-generated
noise. This noise overcomes the pinning potential, thereby generating a random
motion of the coalesced drops. Randomly moving coalesced drops themselves
exhibit a directed diffusive flux when a boundary is present to eliminate them
by absorption. With the presence of a bias, the coalesced drops execute a
diffusive drift motion that can have useful applications in various water and
thermal management technologies
Elastic Cheerios effect: self-assembly of cylinders on a soft solid
A rigid cylinder placed on a soft gel deforms its surface. When multiple
cylinders are placed on the surface, they interact with each other via the
topography of the deformed gel which serves as an energy landscape; as they
move, the landscape changes which in turn changes their interaction. We use a
combination of experiments, simple scaling estimates and numerical simulations
to study the self-assembly of cylinders in this elastic analog of the Cheerios
effect for capillary interactions on a fluid interface. Our results show that
the effective two body interaction can be well described by an exponential
attraction potential as a result of which the dynamics also show an exponential
behavior with respect to the separation distance. When many cylinders are
placed on the gel, the cylinders cluster together if they are not too far
apart; otherwise their motion gets elastically arrested
Elasto-buoyant heavy spheres: a unique way to test non-linear elasticity
Extra-large deformations in ultra-soft elastic materials are ubiquitous, yet
systematic studies and methods to understand the mechanics of such huge strains
are lacking. Here we investigate this complex problem systematically with a
simple experiment: by introducing a heavy bead of radius in an
incompressible ultra-soft elastic medium. We find a scaling law for the
penetration depth () of the bead inside the softest gels as . While this result is inconsistent with an ideal neo-Hookean
model of elastic deformation, according to which the displacement fields must
diverge, it is vindicated by an original asymptotic analytic model developed in
this article. This model demonstrates that the observed relationship is
precisely at the demarcating boundary of what would be required for the field
variables to either diverge or converge. This correspondence between a unique
mathematical prediction and the experimental observation ushers in new insights
into the behavior of the deformations of strongly non-linear materials
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