Motility of Microtubules on the Inner Surface of Water-in-Oil Emulsion Droplets

Steering Active Emulsions with Liquid Crystals

Colloids dispersed in liquid crystals (LCs) diffuse preferentially along the LC director because this direction of displacement generates the lowest hydrodynamic drag. In this article, we report on the active transport of micrometer-sized nematic droplets of 4'-pentyl-4-biphenylcarbonitrile (5CB) propelled through a continuous LC phase formed from aqueous solutions of disodium cromoglycate (DSCG) by Marangoni stresses (generated through the addition of sodium dodecyl sulfate (SDS)). We observe the nematic droplets to exhibit motion guided by the continuous LC phase, but in contrast to passive diffusion, the LC droplets move preferentially in a direction perpendicular to the continuous-phase LC director. Our results suggest that the LC droplets, with internal symmetry broken by the Marangoni flow, interact through orientation-dependent van der Waals forces with the LC continuous phase, biasing the orientation of the droplets and the direction of propulsion orthogonal to the far-field director of the continuous LC phase. This proposal is supported by measurements of the orientations of droplets of 5CB and 4-ethoxy-4'-(6-acryloyloxyhexyloxy) azobenzene (RM257) polymerized in a preradial director configuration, which reveal the polymerized droplets to adopt orientations that are biased toward the perpendicular of the far-field DSCG director. Additionally, we demonstrate that preferential motion parallel to the continuous-phase LC director is recovered when using self-propelled isotropic oil droplets. We also observe periodic changes in the instantaneous velocities of LC droplets. We show the changes to correlate with the formation and detachment of satellite droplets, consistent with the solubilization of the nematic oil into surfactant assemblies near the trailing edge of the droplets and their accumulation near a stagnation region downstream of the droplet. Overall, our results provide fundamental insights into ways in which LC ordering can change the dynamics of active colloidal systems and hint at principles by which the motion of active colloids can be steered.

Encapsulation of the cytoskeleton: towards mimicking the mechanics of a cell

The cytoskeleton of a cell controls all the aspects of cell shape changes and motility from its physiological functions for survival to reproduction to death. The structure and dynamics of the cytoskeletal components: actin, microtubules, intermediate filaments, and septins - recently regarded as the fourth member of the cytoskeleton family - are conserved during evolution. Such conserved and effective control over the mechanics of the cell makes the cytoskeletal components great candidates for in vitro reconstitution and bottom-up synthetic biology studies. Here, we review the recent efforts in reconstitution of the cytoskeleton in and on membrane-enclosed biomimetic systems and argue that co-reconstitution and synergistic interplay between cytoskeletal filaments might be indispensable for efficient mechanical functionality of active minimal cells. Further, mechanical equilibrium in adherent eukaryotic cells is achieved by the formation of integrin-based focal contacts with extracellular matrix (ECM) and the transmission of stresses generated by actomyosin contraction to ECM. Therefore, a minimal mimic of such balance of forces and quasi-static kinetics of the cell by bottom-up reconstitution requires a careful construction of contractile machineries and their link with adhesive contacts. In this review, in addition to cytoskeletal crosstalk, we provide a perspective on reconstruction of cell mechanical equilibrium by reconstitution of cortical actomyosin networks in lipid membrane vesicles adhered on compliant substrates and also discuss future perspectives of this active research area.

Adsorption Mechanism of Oil-in-Water on a TiO2/Al2O3-Polyvinylidene Fluoride (PVDF) Ultrafiltration Membrane

For the sake of gaining a clear idea of the adsorption mechanism involved with an oil emulsion-membrane system, Daqing crude oil emulsion and two types of polyvinylidene fluoride (PVDF) ultrafiltration membranes made in our laboratory were used as the objects to pursue the adsorption characteristics in this system. Several isotherm and kinetics models were used here to simulate the adsorption process; the effect of variables such as time, initial concentration, temperature, as well as scanning electron microscopy (SEM) and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra were investigated to assist in understanding the mechanism. The results show that the Redlich-Peterson model and the pseudo-first-order kinetic model are the best fitting models, with all of the models exhibiting correlation coefficient ( R²) values of >0.98, suggesting an endothermic adsorption process that involves a combination of chemical and physical mechanisms. Moreover, the thermodynamic parameters, such as Δ_r G_m^θ, Δ_r H_m^θ, and Δ_r S_m^θ were also calculated from the temperature dependence, indicating a nonspontaneous process, and increases in temperature had a negative effect on the oil-in-water (o/w) adsorption. Ultimately, further evidence is obtained from the microstructural and infrared spectral analyses.