Proc. Natl Acad. Sci. 113, 3976–3981 (2016)

Credit: © NAS

Single-particle trapping methods typically rely on one or more forces holding a specific type of particle in place. Confinement via hydrodynamic forces, however, has the advantage of not posing any restrictions on a particle's physicochemical properties.

Anish Shenoy and colleagues have now developed a microfluidic hydrodynamic trap capable of simultaneously confining two micrometre-sized polystyrene beads. A six-branched cross, consisting of three ingoing and three outgoing microfluidic channels, was the 'catchment area'; by clever control of the inlet and outlet flows using pressure regulators, the authors created a flow-field featuring two stagnation points (pictured).

The experimental set-up was dubbed a Stokes trap by Shenoy et al., as the use of a high-viscosity glycerol–water solution justified a Stokes-regime description. The device uses a model-predictive control algorithm, which enables the streamline topology to be adjusted and the two particles steered. Admittedly, it requires active feedback control because the trapping loci correspond to unstable equilibrium positions, which is a slight disadvantage compared with passive trapping methods. On the other hand, expanding the Stokes trap with more branches may add even more controllability.