Swim Pressure and Phase Separation of Active Fluids and Capillary-Driven Self-Propulsion(1/3)

An active fluid is a dispersion of active particles in fluid media. Active particles like E. coli perform self-propelled motions by converting energy to work. The artificial active particles have also been synthesized and their motions are achieved by spatially-asymmetric catalytic reactions. Active fluids have been demonstrated out of equilibrium and exhibited unique behaviors. Because active fluids are essential for the development of new generation devices, they have been studied extensively in recent years. When a system is at thermodynamic equilibrium, the pressure is uniform everywhere and independent of the properties of confining walls. Consequently, the pressure is a state function. However, because the active fluid is a nonequilibrium system, whether the equation of state exists or depends on the wall-particle interactions is still unclear. It has been reported recently that the pressure for active fluids made of non-spherical active particles varied with wall-particle interactions, although the pressure is a state function for active spheres. Additionally, whether the hydrodynamic interactions can destroy the state properties of active fluids is an open question. On the other hand, when the concentration of active particles is high, the active fluid was found to phase separated into a dense phase and a dilute phase. If the pressure of active fluids isn’t a state function, the thermodynamic criteria for phase separation cannot be applicable to active fluids. Thus, in this three-year project, fundamental properties of active fluids will be explored by “dissipative particle dynamics simulations.” In the 1st ~2nd years, both bulk and surface pressures will be calculated in unbounded and confined systems. The criteria associated with the onset of phase separation will be explored. In the 2nd~3rd years, as the two phases coexist in the system, the interfacial behaviors between the two phases and the criteria of phase coexistence will be investigated. Our goal is to develop a theoretical model that can explain the occurrence of phase separation. In addition to theoretical simulations, the capillary-driven self-propelled motion will be studied. The precise generation and control of drop motion are essential for a variety of applications such as microfluidic systems. The drop motion driven by external forces is usually hindered by contact line pinning caused by contact angle hysteresis (CAH). In order to overcome this resistance, the surface has to be modified to have low/negligible CAH. On such surfaces, the self-propulsion of a liquid droplet can be driven by the Maragoni effect as non-volatile solutes are contained in the drop. This project aims at fabrication of surfaces with low/negligible hysteresis and to achieve the self-propulsion of liquid drops. Two typical approaches to acquire CAH-free surfaces are total wetting surfaces and liquid-infused porous surfaces. The resultant surface must have long-term stability and withstand against various solvents. For the applications in droplet-based microfluidics, both mini-sized and micro-sized drops will be considered. On the basis of those experiments, the detailed mechanism responsible for self-propelled aqueous/non-aqueous liquid drops (soluto-capillary flow) will be proposed.