External force attached binding focus of particles and its application

The particle focus in channel flow means a randomly initialized particle will finally run at an equilibrium position at the channel cross-section. The binding focus is a particle focus phenomenon containing two adjacent particles (one rigid and the other soft) where the particles can form and share a new equilibrium position. In this paper, we found that migrating the rigid particle laterally can generate a passive migration of the soft. This phenomenon is called external force attached binding focus (EFABF). This paper modeled the EFABF in three-dimensional using the immersed boundary-lattice Boltzmann method. To validate the model, the inertial focus of a single particle and the binding focus of two particles were confirmed numerically. To further study the conditions of EFABF, we mainly discussed the migrating conditions of the soft particle. Two patterns to migrate the soft particle, rigid particle ahead (RPA) and soft particle ahead (SPA), were observed. The Reynolds number of ten is proposed as suitable for EFABF to obtain a remarkable migration scope. Moreover, we further analyzed the mechanism of EFABF for a deeper understanding. Finally, as its application, we designed a label-free single-cell separation architecture by replacing the soft particle with a spherical circulating tumor cell and setting the rigid particle to be magnetically manipulated. Numerical results indicate that the soft particle (cell) can be successfully driven to stride over streamlines and form a new equilibrium position by migrating the rigid particle, making the separation pathway well-controlled.