自扩散泳微观转动马达的介观模拟

Mesoscale simulation of self-diffusiophoretic microrotor

转动的微尺度马达是一类重要的微流器件.近年来,因为其重要的应用及理论价值引起了学术界的广泛关注.本文提出了一种新型的自扩散泳驱动的微观转动马达模型.通过介观动力学模拟,验证了该模型的有效性.模拟结果表明,该自扩散泳微观转动马达可以单向地自驱动转动,并且转动速度和马达表面发生的催化化学反应速率(即自产生的浓度梯度场强弱)、以及液体分子与马达之间的相互作用有关.此外,研究了两个转动马达共存时的运动行为,重点考察了马达之间的流体力学相互作用和浓度梯度场效应对马达转动的影响.该自扩散泳微观转动马达为设计实用的微流器件提供了新的思路和参考,也为研究活性胶体系统的集体行为提供了理想模型.

Artificial micro-scale or nano-scale machines that are capable of converting energy to mechanical work, have long been pursued by science and engineering communities for their potential applications in microfluidics, biology and medicine. From a physics point of view, they are also ideal models to investigate fundamental statistical phenomena in non-equilibrium active matters. Inspired by bio-machines and bio-motors like ATP synthase and flagellum motors,we propose a simple design of rotary motors based on pure self-diffusiophoresis effects. The basic design of the rotor consists of three colloidal beads with different surface properties, which leads to different interactions between the beads and solvent molecules. Chemical reactions are imposed on the surface of one of the beads, which creates a source of one of the two solvent molecules and generates a local concentration gradient. The other two beads connected to the catalytic bead have different affinities to the solvent molecules, which leads to asymmetric diffusiophoretic forces on the two non-catalytic beads. A net torque is thus obtained from difference of the diffusiophoretic forces between the two non-catalytic beads. In our simulation, we employ hybrid molecular dynamics（MD） simulations and multiparticle collision dynamics（MPC） to investigate the motion of microrotors. The binary fluid is composed with A-type and B-type solvent particle whose interactions are described by multi-particle collision dynamics while beads-particle interactions are modeled by molecular dynamics. In MPC, all fluid particles execute alternating streaming and collision steps. During streaming steps, the solvents move ballistically. During collision steps, particles are sorted into square cells and only interact with particles in the same cell under a specific stochastic rotation rule. MPC algorithm locally conserves mass, linear momentum, angular momentum and energy, and properly captures thermal fluctuation, mass diffusion, dissipation and hydrodynamic interactions. In our simulation, standard MPC parameters are employed which correspond to a liquid-like behavior of fluid. In MD, beads-solvent interactions are described by Lennard-Jones potential with different parameter combinations and the equations of motion is integrated by velocity-Verlet algorithm. To perform hybrid molecular dynamic simulations with multi-particle collision dynamics, between two MPC collision steps, 50 MD steps are implemented for the solvent particles that are in the interaction range of colloidal beads. We first investigate the solvent concentration distribution around static microrotor, and confirm that the catalytic bead generates a steadystate local concentration gradient. Net angular displacements are obtained when the rotor is allowed to rotate freely.The rotational direction and speed of the micorotor are determined by bead-solvent interactions, the rotor geometry,the solvent viscosity and the catalytic reaction ratio. We also study the scenario in which two rotors are placed in close vicinity to each other. We find that the coupling between the concentration fields around the rotors reduces the rotational speed of both rotors.