Session E15: Bio: Vesicles and Micelles

Branching, Superdiffusion and Stress Relaxation in Surfactant Micelles

We investigate the mechanism of branch formation and its effects on the dynamics and rheology of a model cationic micellar fluid using molecular dynamics (MD) simulations. Branched structures are formed upon increasing counter ion density. A sharp decrease in the solution viscosity with increasing salinity has long been attributed to the sliding motion of micellar branches along the main chain. Simulations not only provide firm evidence of branch sliding in real time, but also show enhanced diffusion of surfactants by virtue of such motion. Insights into the mechanism of stress relaxation associated with branch sliding will be discussed. Specifically, an externally imposed stress damps out more quickly in a branched system compared to that in an unbranched one. References: Dhakal and Sureshkumar, J. Chem. Phys. 143, 024905~(2015); ACS Macro Letters, 5, 108-11 (2016).   

Brownian dynamics simulations of lipid bilayer membrane with hydrodynamic interactions in LAMMPS

Lipid bilayer membranes have been extensively studied by coarse-grained molecular dynamics simulations. Numerical efficiencies have been reported in the cases of aggressive coarse-graining, where several lipids are coarse-grained into a particle of size $4\sim6$ nm so that there is only one particle in the thickness direction. Yuan {\it et al.} proposed a pair-potential between these one-particle-thick coarse-grained lipid particles to capture the mechanical properties of a lipid bilayer membrane (such as gel-fluid-gas phase transitions of lipids, diffusion, and bending rigidity). In this work we implement such interaction potential in LAMMPS to simulate large-scale lipid systems such as vesicles and red blood cells (RBCs). We also consider the effect of cytoskeleton on the lipid membrane dynamics as a model for red blood cell (RBC) dynamics, and incorporate coarse-grained water molecules to account for hydrodynamic interactions. The interaction between the coarse-grained water molecules (explicit solvent molecules) is modeled as a Lennard-Jones (L-J) potential. We focus on two sets of LAMMPS simulations: 1. Vesicle shape transitions with varying enclosed volume; 2. RBC shape transitions with different enclosed volume.   

Lamellar ordering, droplet formation and phase inversion in exotic active emulsions

We present the results of numerical simulations of the behaviour of a mixture of a passive isotropic fluid and an active polar nematic gel, in presence of surfactant favouring emulsification. The active stress appearing in the Navier-Stokes equation depends on the polarization field and on an activity parameter whose sign determines the contractile or extensile character of the gel. Focussing on cases for which the underlying free energy favours the lamellar phase in the passive limit, we show that the interplay between nonequilibrium and thermodynamic forces creates a range of multifarious exotic emulsions. When the active component is contractile (e.g., an actomyosin solution), moderate activity greatly enhances the efficiency of lamellar ordering, whereas strong activity favours the creation of passive droplets within an active matrix. For extensile activity (e.g., materials based on bacterial suspensions), instead, we observe an emulsion of spontaneously rotating droplets. By tuning the overall composition, we can also create high internal phase emulsions, which undergo catastrophic phase inversion when switching off the activity. Therefore, we find that activity may provide a single control parameter to design composite materials with a rich range of morphologies.   

Stable low-resolution simulations of two-dimensional vesicle suspensions

Vesicles, which resist bending and are locally inextensible, serve as experimental and numerical proxies for red blood cells. Vesicle flows, which are governed by hydrodynamic and elastic forces, refer to flow of vesicles that are filled with and suspended in a Stokesian fluid. In this work we present algorithms for stable and accurate low-resolution simulations of the vesicle flows in two-dimensions. We use an integral equation formulation of the Stokes equation coupled to the interface mass continuity and force balance. The problem poses numerical difficulties such as long-range hydrodynamic interactions, strong nonlinearities and stiff governing equations. These difficulties make simulations with long time horizons challenging, especially at low resolutions. We develop algorithms to control aliasing errors, correct errors in vesicle's area and arc-length, and avoid collision of vesicles. Additionally, we discuss several error measures to study the accuracy of the simulations. Then we closely look at how accurate the low-resolution simulations can capture true physics of the vesicle flows.   

Inducing morphological changes in lipid bilayer membranes with microfabricated substrates

Lateral organization of lipids and proteins into distinct domains and anchoring to a cytoskeleton are two important strategies employed by biological membranes to carry out many cellular functions. However, these interactions are difficult to emulate with model systems. Here we use the physical architecture of substrates consisting of arrays of micropillars to systematically control the behavior of supported lipid bilayers -- an important step in engineering model lipid membrane systems with well-defined functionalities. Competition between attractive interactions of supported lipid bilayers with the underlying substrate versus the energy cost associated with membrane bending at pillar edges can be systematically investigated as functions of pillar height and pitch, chemical functionalization of the microstructured substrate, and the type of unilamellar vesicles used for assembling the supported bilayer. Confocal fluorescent imaging and AFM measurements highlight correlations that exist between topological and mechanical properties of lipid bilayers and lateral lipid mobility in these confined environments. This study provides a baseline for future investigations into lipid domain reorganization on structured solid surfaces and scaffolds for cell growth.   

Equilibrium Shapes of Compound Vesicles

Many biological structures have a fine internal structure in which a membrane is geometrically confined by another membrane. Here we investigate how the equilibrium shape of a double membrane system changes as the length of the internal membrane is increased. A repulsive pressure is introduced between the membranes to prevent the membranes from intersecting. Large repulsive pressures yield complex response diagrams with bifurcation points where modal identities may changes. Regions in parameter space where such behavior occurs are then mapped.