Real-time micromanipulation through 3D stress-free flows*

Modern micromanipulation techniques, such as optical tweezers, have been established as essential approaches to examining many crucial biophysical phenomena. These approaches, however, often stem from the concept of trapping particles through certain stress fields, which restricts their applications to stress-sensitive phenomena. By employing polyhedral symmetries in a multi-channel microfluidic design, we show that the tasks of displacing and trapping a particle can be indeed separated into two distinct sets of flow operations, characterized by their unique groups of symmetries. Combining all "displacing" modes, targeted particles are entrained by uniform flows in all possible directions, giving rise to stress-free micromanipulation in 3D, without any need for traps. To examine the capacity of such trap-free micromanipulation, we engineered complex microscale paths of manipulated particles by programming the flow within each channel, as controlled in real-time via pressure regulators. In contrast to those trap-based manipulations, all particles in the stress-free flows follow the desired path, without the need to supervise any traps based on the particles' locations. Additionally, we optimized the actual paths followed by the manipulated particles by incorporating curvature-dependent flow speed, using, for instance, a conformal-mapping strategy.