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An analytical theory is developed to describe the dynamics of a closed lipid bilayer membrane (vesicle) freely suspended in a general linear flow. Considering a nearly spherical shape, the solution to the creeping-flow equations is obtained as a regular perturbation expansion in the excess area. The analysis takes into account the membrane fluidity, incompressibility and resistance to bending. The constraint for a fixed total area leads to a non-linear shape evolution equation at leading order. As a result two regimes of vesicle behavior, tank-treading and tumbling, are predicted depending on the viscosity contrast between interior and exterior fluid. Below a critical viscosity contrast, which depends on the excess area, the vesicle deforms into a tank-treading ellipsoid, whose orientation angle with respect to the flow direction is independent of the membrane bending rigidity. In the tumbling regime, the vesicle exhibits periodic shape deformations with a frequency that increases with the viscosity contrast. Non-Newtonian rheology such as normal stresses is predicted for a dilute suspension of vesicles. The theory is in good agreement with published experimental data for vesicle behavior in simple shear flow.

The movements of beads pulled by several kinesin-1 (conventional kinesin) motors are studied both theoretically and experimentally. While the velocity is approximately independent of the number of motors pulling the beads, the walking distance or run-length is strongly increased when more motors are involved. Run-length distributions are measured for a wide range of motor concentrations and matched to theoretically calculated distributions using only two global fit parameters. In this way, the maximal number of motors pulling the beads is estimated to vary between two and seven motors for total kinesin concentrations between 0.1 and 2.5 μg/ml or between 0.27 and 6.7 nM. In the same concentration regime, the average number of pulling motors is found to lie between 1.1 and 3.2 motors.

We develop an analytical theory to explain the experimentally observed morphological transitions of quasispherical giant vesicles induced by alternating electric fields. The model treats the inner and suspending media as lossy dielectrics, and the membrane as an impermeable flexible incompressible-fluid sheet. The vesicle shape is obtained by balancing electric, hydrodynamic, bending, and tension stresses exerted on the membrane. Our approach, which is based on force balance, also allows us to describe the time evolution of the vesicle deformation, in contrast to earlier works based on energy minimization, which are able to predict only stationary shapes. Our theoretical predictions for vesicle deformation are consistent with experiment. If the inner fluid is more conducting than the suspending medium, the vesicle always adopts a prolate shape. In the opposite case, the vesicle undergoes a transition from a prolate to oblate ellipsoid at a critical frequency, which the theory identifies with the inverse membrane charging time. At frequencies higher than the inverse Maxwell-Wagner polarization time, the electrohydrodynamic stresses become too small to alter the vesicle's quasispherical rest shape. The model can be used to rationalize the transient and steady deformation of biological cells in electric fields.

We propose a novel, to our knowledge, method for the determination of tie lines in a phase diagram of ternary lipid mixtures. The method was applied to a system consisting of dioleoylphosphatidylcholine (DOPC), egg sphingomyelin (eSM), and cholesterol (Chol). The approach is based on electrofusion of single- or two-component homogeneous giant vesicles in the fluid phase and analyses of the domain areas of the fused vesicle. The electrofusion approach enables us to create three-component vesicles with precisely controlled composition, in contrast to conventional methods for giant vesicle formation. The tie lines determined in the two-liquid-phase coexistence region are found to be not parallel, suggesting that the dominant mechanism of lipid phase separation in this region changes with the membrane composition. We provide a phase diagram of the DOPC/eSM/Chol mixture and predict the location of the critical point. Finally, we evaluate the Gibbs free energy of transfer of individual lipid components from one phase to the other.

Electric pulses applied to fluid phospholipid vesicles deform them and can induce the formation of pores, which reseal after the end of the pulse. The mechanical and rheological properties of membranes in the gel phase differ significantly from those of fluid membranes, thus a difference in the vesicle behavior in electric fields is expected. However, studies addressing this problem are scarce. Here, we investigate the response of giant gel-phase vesicles to electric pulses and resolve the dynamics of deformation with microsecond resolution. We find that the critical transmembrane potential leading to poration is several times higher as compared to that of fluid membranes. In addition, the resealing of the pores is arrested. Interestingly, the vesicle shapes change from ellipsoidal to spherocylindrical during the electric pulse and the membrane becomes periodically wrinkled with ridges aligned with the field direction and wavelengths in the micrometre range. Such membrane wrinkling has not been reported previously. The corrugations comply with universal laws of wrinkling of surfaces with lengthscale dimensions from nanometres to metres.

We calibrate coarse-grained interaction potentials suitable for screening large data sets in top-down fashion. Three new algorithms are introduced: (i) automated decomposition of molecules into coarse-grained units (fragmentation); (ii) Coarse-Grained Reference Interaction Site Model-Hypernetted Chain (CG RISM-HNC) as an intermediate proxy for dissipative particle dynamics (DPD); and (iii) a simple top-down coarse-grained interaction potential/model based on activity coefficient theories from engineering (using COSMO-RS). We find that the fragment distribution follows Zipf and Heaps scaling laws. The accuracy in Gibbs energy of mixing calculations is a few tenths of a kilocalorie per mole. As a final proof of principle, we use full coarse-grained sampling through DPD thermodynamics integration to calculate log P for 4627 compounds with an average error of 0.84 log unit. The computational speeds per calculation are a few seconds for CG RISM-HNC and a few minutes for DPD thermodynamic integration.

We present a comprehensive evaluation of the recently developed pair configuration to molecular activity coefficient (PAC−MAC) method. PAC−MAC is a force-fieldbased quasichemical method for rapid calculation of binary phase diagrams. The accuracy of the method is tested by comparing the calculated excess mixing free energy with experimental data for 1092 binary mixtures. The root mean squared error (RMSE) is shown to be 0.15 k B T. Furthermore, a comparison with UNIFAC and molecular simulations is performed. UNIFAC shows a significantly higher accuracy (RMSE: 0.07 k B T), whereas molecular simulations lead to comparable results. The accuracy of both molecular simulations as well as PAC−MAC depends highly on the used force field. The binary parameters of UNIFAC are optimized using experimental miscibility data, whereas the force field parameters are not. Therefore, a better performance of UNIFAC is expected. A concise data set shows the capacity of PAC− MAC in predicting results obtained using Monte Carlo simulations.