Investigating Cell-Induced Mixing Dynamics in Microfluidic Droplets Using the Lattice Boltzmann Method

This study investigates the impact of cell dynamics on mixing efficiency within a microfluidic droplet, emphasizing the relationship between cell motion, deformability, and resultant asymmetry in velocity and concentration fields. Simulations were conducted for droplets containing encapsulated cells at varying Peclet numbers (Pe = 100-800) and coupling constants (k = 0.0025, 0.005, 0.0075). The mixing index was significantly enhanced by the presence of the encapsulated cell, particularly at high Peclet numbers, where cell-induced disturbances in the velocity field disrupted symmetrical flow patterns, improving mixing. An asymmetry index quantified deviations in velocity and concentration fields caused by cell motion. Results revealed a complex interplay between cell deformability and fluid-cell interactions. Lower coupling constants corresponded to weaker velocity field asymmetry but, paradoxically, higher mixing indices. This counterintuitive finding was examined by analyzing the asymmetry in the x-component of the velocity field, aligned with the primary concentration gradient. Disturbances in this direction enhanced convective transport across the diffusion interface, crucial for efficient mixing. The study also examined cell trajectory and membrane deformation during droplet generation. Cells with moderate deformability exhibited greater off-center movement, leading to increased rotational dynamics and chaotic flow patterns conducive to enhanced mixing. In contrast, cells with higher or lower deformability followed more constrained paths, resulting in less effective mixing. These findings suggest optimal mixing within microfluidic droplets occurs when cells exhibit moderate deformability, balancing fluid-cell coupling and the ability to induce flow field asymmetry. These insights could inform the design of microfluidic systems for applications requiring precise mixing control, such as biomedical diagnostics and chemical synthesis.