Two Types Of Microscale Concentration Gradient Driven Flow Problem In Non-equilibrium State

In the micro-scale flow,due to the dominant role of the viscous force,it is difficult to drive the fluid to form an effective flow using the volume force or pressure difference force.The driving forces,such as a chemical potential or an electric field,are introduced necessarily.For microfluidic problems driven by chemical potential gradients(such as the difference in concentration of non-electrolyte solutions),when using the N-S equation under the continuum framework to describe it,the traditional non-slip boundary conditions is no longer applied.Slip boundaries are introduced to modify it,while the coupling of chemical potentials and flows makes the problem more complicated.In this paper,we study the flow in the pores of the forward osmosis(FO)membrane(internal flow problem)and the self-driven micromotor(external flow problems).In the FO problem,the concentration gradient of the solute is usually given prior to simulation on both sides of the membrane,and the solvent(water molecules)spontaneously flows in the pore to the reverse concentration gradient direction,that is,from the low solute concentration to the high concentration.Based on the physics model of slip surface,the Fluid-Structure Interaction(FSI)problem caused by the FO flow is studied.However,due to the conflict between the slip boundary and the traditional FSI module of the non-slip FSI interface,the FSI bidirectional can't be performed.Therefore,this paper uses the relationship between force and displacement in solid mechanics to calculate the deformation of the interface,and then takes advantage of the moving mesh of the deformed geometry.The shape of the FSI interface(watershed)changes during the iteration,and then a new pressure load is generated.Finally,iterative calculations were performed several times to achieve the purpose of bidirectional coupling,and the pore deformation of the elastic wall was obtained.The results show that the elastic wall deformation caused by the concentration gradient is mainly along the radial direction,and there is no obvious deviation in the axial direction.This is completely different from the deformation of membrane pores driven by hydraulic differential pressure at the macroscopic scale,which is dominated by axial deformation.The results are qualitatively consistent with the macroscopic experiments of the FO membrane,which verifies the correctness of the model.The Janus micro-motor based on the diffusion phoresis mechanism is a kind of active artificial particle that can convert chemical energy into mechanical energy and sense the external environment with high sensitivity.The application needs to consider external complex conditions,such as the Janus particle near the wall surface,which affected by the confinement effect.At present,some scholars have observed through experiments that when the Janus particles are confined between two vertical walls,the Janus particles will perform a uniform linear motion along the intersection of the two walls in equilibrium.For two situations with or without gravity field,the posture parameters(active surface orientation,deflection angle)and distance from the wall are studied when the Janus particles confined to two vertical walls are in equilibrium.The results show that under both conditions,the combined effects of diffusion phoretic force,external force,and near-wall drag force make the Janus particle behave completely different in equilibrium.When gravity is not considered,the near-wall drag force is away from the bottom wall surface,so the active surface of the Janus particles must deviate from the bottom wall surface to generate a diffusional force toward the bottom wall surface and maintain the vertical equilibrium.When the gravity of the micro-motor can't be ignored,the active surface of the particles should tend toward the bottom wall side in order to obtain a diffusional phoretic force that balances with gravity.In the normal direction of the side wall,the Janus particle parameters in the equilibrium state is the same as which in the absence of gravity for both cases with and without gravity.This study will not only deepen the understanding of the relevant micro-scale flow mechanism,but will also contribute to the promotion of applied research in related fields,such as optimizing the structure and efficiency of micro-motors,and improving water treatment devices based on FO membranes.