2 research outputs found
Elastocapillary Interaction of Particles on the Surfaces of Ultrasoft Gels: A Novel Route To Study Self-Assembly and Soft Lubrication
We
study the interaction of small hydrophobic particles on the
surface of an ultrasoft elastic gel, in which a small amount of elasticity
of the medium balances the weights of the particles. The excess energy
of the surface of the deformed gel causes them to attract as is the
case with the generic capillary interactions of particles on a liquid
surface. The variation of the gravitational potential energies of
the particles resulting from their descents in the gel coupled with
the superposition principle of Nicolson allow a fair estimation of
the distance dependent attractive energy of the particles. This energy
follows a modified Bessel function of the second kind with a characteristic
elastocapillary decay length that decreases with the elasticity of
the medium. An interesting finding of this study is that the particles
on the gel move toward each other as if the system possesses a negative
diffusivity that is inversely proportional to friction. This study
illustrates how the capillary interaction of particles is modified
by the elasticity of the medium, which is expected to have important
implications in the surface force driven self-assembly of particles.
In particular, this study points out that the range and the strength
of the capillary interaction can be tuned in by appropriate choices
of the elasticity of the support and the interfacial tension of the
surrounding medium. Manipulation of the particle interactions is exemplified
in such fascinating mimicry of the biological processes as the tubulation
and phagocytic engulfment and in the assembly of particles that can
be used to study nucleation and clustering phenomena in well-controlled
settings
Surface Folding-Induced Attraction and Motion of Particles in a Soft Elastic Gel: Cooperative Effects of Surface Tension, Elasticity, and Gravity
We report some experimental observations
regarding a new type of
long-range interaction between rigid particles that prevails when
they are suspended in an ultrasoft elastic gel. A denser particle
submerges itself to a considerable depth inside the gel and becomes
elasto-buoyant by balancing its weight against the elastic force exerted
by the surrounding medium. By virtue of a large elasto-capillary length,
the surface of the gel wraps around the particle and closes to create
a line singularity connecting the particle to the free surface of
the gel. A substantial amount of tensile strain is thus developed
in the gel network parallel to the free surface that penetrates to
a significant depth inside the gel. The field of this tensile strain
is rather long-range because of a large gravito-elastic correlation
length and sufficiently strong to pull two submerged particles into
contact. The particles move toward each other with an effective force
following an inverse linear distance law. When more monomers or dimers
of the particles are released inside the gel, they orient rather freely
inside the capsules where they are located and attract each other
to form closely packed clusters. Eventually, these clusters themselves
interact and coalesce. This is an emergent phenomenon in which gravity,
capillarity, and elasticity work in tandem to create a long-range
interaction. We also present the results of a related experiment,
in which a particle suspended inside a thickness-graded gel moves
accompanied by the continuous folding and the relaxation of the gel’s
surface