Numerical Simulation Study On The Factors Affecting The Energy Conversion Efficiency Of Pt-Au Bimetallic Nanomotors

Micro/nanomotor are functional micro/nanodevices inspired by biological motors that can convert other forms of energy in the surrounding environment into its own kinetic energy.Compared with biomotors,micro/nanomotors can perform various complex tasks in the micro-environment through the pre-conceived way,which greatly improves the ability to manipulate things in microscale.With its unique autonomous motion performance,man-made micro/nanomotors have been applied in various fields,which has attracted great attention from researchers.As a type of micro-machines,the energy conversion efficiency of micro/nanomotors has a decisive effect on its ability for applications,but the currently developed motors have low energy conversion efficiency,which seriously hinders their practical application.Therefore,so it is very important to explore the influencing factors and improvement measures of micro-nano motor energy conversion efficiency.In view of this,the article takes the earliest studied Pt-Au bimetallic nanomotor driven by the self-electrophoresis mechanism as an example,and does the following work through the COMSOL Multiphysics finite element simulation software based on multiphysics coupling:First,the Computational Fluid Dynamics(CFD)and finite element methods are briefly introduced and a two-dimensional axisymmetric geometric model is established.The concept and structure of the electric double layer(EDL)are also explained.A one-dimensional numerical simulation study was conducted on the EDL to further analyze its physical properties.The results show that:the motor surface in the fluid medium will form a diffuse electric double layer structure,which is composed of the Stern layer and the diffusion layer,and the negatively charged motor surface potential is equal to the difference between the Zeta potential and the voltage of the Stern layer.The concentration of counter-ions in the EDL is higher than that of the co-ions and there is a potential drop inside the Stern layer,and the finite thickness of the EDL is about an integer Debye length?_D,the absolute value of the Stern layer voltage is positively related to the absolute value of the potential of the particle surface.The approximation models of finite thickness EDL and thin EDL have their own applicable conditions and in good agreement with the theoretical formula.Secondly,geometric parameters and numerical models are set for Pt-Au bimetallic motors with different shapes and compositions.Considering the flux changes caused by catalytic electrochemical reactions and the degree of it,the influence of shapes and compositions on energy conversion efficiency is studied.The results show that:for different shapes and sizes of axisymmetric Pt-Au bimetallic motors with a certain volume and length,its speed and energy conversion efficiency increase with the decrease of the ratio of the difference between the central radius R_C and the radius R_Eat both ends to the R_C.If the ratio is the same,its speed and efficiency increase with the decrease of Rc.Changing the proportion of Pt segment in bimetallic motor,when L_Pt/L_Pt-Au is between 0.27?0.8,the speed and efficiency increase with the increase of the proportion;when L_Pt/L_Pt-Au is between 0.8?0.875,the speed and efficiency reaches the maximum;when L_Pt/L_Pt-Au>0.875 When the efficiency drops sharply.Finally,considering the motor may encounter a complex fluid medium in actual situations,the numerical model and boundary conditions are further revised to study the effect of solution conductivity and viscosity on the energy conversion efficiency of Pt-Au bimetallic motor.The results indicate that:the voltage of the Stern layer is not evenly distributed on the surface of the motor,resulting in a significant difference in the Zeta potential between the Pt and Au surfaces,and this difference decreases with the increase in conductivity;the motor metal surface potential and the Stern layer voltage both increase with the increase of conductivity,and the speed and energy conversion efficiency both decrease with the increase of conductivity;which show that increasing the difference between the Zeta potentials of the two metal surfaces of Pt and Au,or reducing the overall Zeta potential of the motor surface by surface modification is expected to improve the movement behavior of the Pt-Au bimetallic motor in high conductivity solutions.The speed and energy conversion efficiency of the Pt-Au bimetallic motor both increase as the viscosity of the solution decreases,and the rate of increase becomes larger and larger;which shows that in the actual application of the motor,the viscosity of the surrounding solution can be reduced by appropriately increasing the local temperature,thereby promoting the motor to perform the task more efficiently.The results of this paper help to understand the physical mechanism of self-electrophoresis propulsion in a deeper level,which can not only provide new ideas for the design and preparation of Pt-Au bimetallic nanomotors,but also have a guiding significance for promoting the efficient application of such motors in complex environments.