62 research outputs found
Spontaneous phase separation of ternary fluid mixtures
We computationally study the spontaneous phase separation of ternary fluid mixtures using the lattice Boltzmann method both when all the surface tensions are equal and when they have different values. To rationalise the phase diagram of possible phase separation mechanisms, previous theoretical works typically rely on analysing the sign of the eigenvalues resulting from a simple linear stability analysis, but we find this does not explain the observed simulation results. Here, we classify the possible separation pathways into four basic mechanisms, and develop a phenomenological model that captures the composition regimes where each mechanism is prevalent. We further highlight that the dominant mechanism in ternary phase separation involves enrichment and instability of the minor component at the fluid-fluid interface, which is absent in the case of binary fluid mixtures
Design principles for Bernal spirals and helices with tunable pitch
Using the framework of potential energy landscape theory, we describe two in
silico designs for self-assembling helical colloidal superstructures based upon
dipolar dumbbells and Janus-type building blocks, respectively. Helical
superstructures with controllable pitch length are obtained using external
magnetic field driven assembly of asymmetric dumbbells involving screened
electrostatic as well as magnetic dipolar interactions. The pitch of the helix
is tuned by modulating the Debye screening length over an experimentally
accessible range. The second design is based on building blocks composed of
rigidly linked spheres with short-range anisotropic interactions, which are
predicted to self-assemble into Bernal spirals. These spirals are quite
flexible, and longer helices undergo rearrangements via cooperative, hinge-like
moves, in agreement with experiment
Edge Fracture in Complex Fluids
We study theoretically the edge fracture instability in sheared complex fluids, by means of linear stability analysis and direct nonlinear simulations. We derive an exact analytical expression for the onset of edge fracture in terms of the shear-rate derivative of the fluid’s second normal stress difference, the shear-rate derivative of the shear stress, the jump in shear stress across the interface between the fluid and the outside medium (usually air), the surface tension of that interface, and the rheometer gap size. We provide a full mechanistic understanding of the edge fracture instability, carefully validated against our simulations. These findings, which are robust with respect to choice of rheological constitutive model, also suggest a possible route to mitigating edge fracture, potentially allowing experimentalists to achieve and accurately measure flows stronger than hitherto possible
Control of Superselectivity by Crowding in Three-Dimensional Hosts
Motivated by the fine compositional control observed in membraneless droplet organelles in cells, we investigate how a sharp binding-unbinding transition can occur between multivalent client molecules and receptors embedded in a porous three-dimensional structure. In contrast to similar superselective binding previously observed at surfaces, we have identified that a key effect in a three-dimensional environment is that the presence of inert crowding agents can significantly enhance or even introduce superselectivity. In essence, molecular crowding initially suppresses binding via an entropic penalty, but the clients can then more easily form many bonds simultaneously. We demonstrate the robustness of the superselective behavior with respect to client valency, linker length, and binding interactions in Monte Carlo simulations of an archetypal lattice polymer model
Multicomponent flow on curved surfaces : A vielbein lattice Boltzmann approach
We develop and implement a novel finite difference lattice Boltzmann scheme to study multicomponent flows on curved surfaces, coupling the continuity and Navier-Stokes equations with the
Cahn-Hilliard equation to track the evolution of the binary fluid interfaces. The standard lattice
Boltzmann method relies on regular Cartesian grids, which makes it generally unsuitable to study
flow problems on curved surfaces. To alleviate this limitation, we use a vielbein formalism to write
down the Boltzmann equation on an arbitrary geometry, and solve the evolution of the fluid distribution functions using a finite difference method. Focussing on the torus geometry as an example of
a curved surface, we demonstrate drift motions of fluid droplets and stripes embedded on the surface
of such geometries. Interestingly, they migrate in opposite directions: fluid droplets to the outer
side while fluid stripes to the inner side of the torus. For the latter we demonstrate that the global
minimum configuration is unique for small stripe widths, but it becomes bistable for large stripe
widths. Our simulations are also in agreement with analytical predictions for the Laplace pressure of
the fluid stripes, and their damped oscillatory motion as they approach equilibrium configurations,
capturing the corresponding decay timescale and oscillation frequency. Finally, we simulate the
coarsening dynamics of phase separating binary fluids in the hydrodynamics and diffusive regimes
for tori of various shapes, and compare the results against those for a flat two-dimensional surface. Our finite difference lattice Boltzmann scheme can be extended to other surfaces and coupled
to other dynamical equations, opening up a vast range of applications involving complex flows on
curved geometries
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