Numerical Simulation Of Breaking Scallop Theorem Using Bubble Dynamics

The self-driving experiments of Janus particles reveal that there is a strong interaction between the bubbles and the adjacent Janus particles at the microscale,and they can drive the rapid movement of the Janus particles.However,due to the limitation of the speed of the high-speed camera,the experiment cannot provide the direct mechanical information for their interaction.In order to study this problem,the numerical simulation method is used to simulate the expansion and contraction of the virtual wall compared to the growth and collapse of the microbubbles.With the COMSOL Multiphysics multiphysics coupled simulation platform,the finite element method is used to perform the bubble driving process of the Janus particles.First,on the basis of the Janus particle bubble driving experiment,the model is reasonably simplified,a Janus particle bubble driven numerical model is established,and the scallop theorem is used to verify the model.Scallop's theorem refers to the fact that when the object moves at a low Reynolds number in an incompressible Newtonian fluid,the net displacement must be zero.From its content,it can be seen that the simulation of reversibility and single-degree-of-freedom boundary motion does not make Janus particles produce effective motion.Through the analysis of the boundary flow in the model,it is concluded that during a period of bubble growth and collapse,the net flow at the upper and lower boundaries of the fluid domain is approximately zero,and consistent with the scallop theorem,thus verifying the correctness of the model and the feasibility of use the finite element method simulating bubble actuation.Secondly,from Janus particle bubble-driven experiments,it is known that the microbubbles grow slowly and collapse rapidly,and do not have the reversibility of the micro-scale flow problem.Therefore,this paper uses the validated model to further simulate the non-reversible and single-degree-of-freedom boundary motion.Through the analysis of the boundary flow,it can be concluded that there is a net flow at both the upper and lower boundary of the fluid field during a bubble growth collapse period,and there is a net displacement of the Janus particles,which indicates that the bubble dynamics can break the theoretical constraints of the scallop theorem.The reason for this is that the collapse of bubbles at a high speed causes a significant increase in the Re number,and the introduction of inertial force to drive the Janus particles a net displacement.Finally,the influence of the Janus particle size,bubble size,the characteristic velocity of the bubble collapse stage,and the slip length on the bubble driving process was investigated.It was concluded that the increase in the size of the Janus particle hinders the bubble-driven process,while the bubble size and the characteristic velocity of the collapse stage can promote the bubble driving process,and the slip length has little effect on the bubble driving process.