Design and mesoscopic description of self-propelled nanomotor in complex environment

Abstract

Self-propulsion of micron and nanoscale objects in the low Reynolds number regime are commonly observed in biological systems. Micron and nanoscale synthetic self-propelled objects have been fabricated recently and the mechanisms that underlie their operation have been described. Researches about such motors are the interesting topic because of their potential applications as vehicles for drug delivery, cargo transport, motion-based bio-sensing, nanoscale assembly, targeted synthesis, nano- and microfluidics, etc. One type of such motors based on phoretic self-propulsion exhibit various dynamical phenomena that have attracted increasing attentions in the front interdiscipline fields of soft condensed matter, statistical physics, and nanotechnology. Studies on designing interesting nano-motor that can execute special tasks and exploration of their dynamics behavior in complex active matter have been more attractive recently. In this review, we introduce the mesoscopic dynamical scheme that is based on a coarse-grain description of molecular collisions-multiple particles collision dynamics (MPC). There are several attractive features of such a mesoscopic description. Due to simple dynamics, it is both easy and efficient to simulate. The equations of motion are easily written and the techniques of nonequilibriun statistical mechanics can be used to derive macroscopic laws and correlation function expressions for the transport properties. One can derive accurate analytical expressions for the transport coefficient. Especially, the mesoscopic description can be combined with full molecular dynamics in order to describe the properties of solute species, such as motor or colloids, in solution. Since all of the physical conservation laws are satisfied, hydrodynamic interactions, which play an important role in the dynamical properties of such systems, are automatically taken into account without additional assumption. This method can be combined with full molecular dynamics (MD) to construct a hybrid MPC-MD method. Furthermore, the reaction in solution can be taken into account, which extends the method as reactive multiple particles dynamics (RMPC). Therefore, the mesoscopic method can be utilized to simulate complex systems. In these examples, the motors move by selfphoresis, where the gradient of some field across the motor, which is generated by asymmetrical activity. It induces fluid flow in the surrounding medium resulting in propulsion. The phoretic propulsion mechanisms include self-electrophoresis, self-diffusiophoresis and selfthermophoresis. Here, we describe the basic theory for phoretic propulsion mechanisms. Then, we briefly review the process of simulation on self-propelled motor in recent years. Especially, we introduce the results in designing motor with different dynamics by means of MPC. For example, an asymmetric gear with homogeneous surface properties is presented as a prototype to fabricate catalytic microrotors. Lastly, we introduce the dynamical properties of sphere dimer motors, composed of linked catalytic and non-catalytic monomers, in active media. Synthetic chemically powered nanomotors often rely on the environment for their fuel supply. The propulsion properties of such motors can be altered if the environment they move is chemically active. Two examples are presented. The chemotactic properties of a sphere dimer motor are studied in a gradient field of fuel. We also consider how a sphere dimer motor moves in a chemically active medium and interacts with a chemical wave. It is found that a chemical wave can reflect a dimer motor, which suggests that the effect can provide a possible mechanism for the control of nanomotor motion in a patterned chemical system. Our investigation provides an introduction to the broader issue of self-propulsion in active media and suggests the possibility of a new class of applications and control scenarios.