Optimal control of particle separation in inertial microfluids
At intermediate Reynolds numbers, particles in a micro fluidic
channel assemble at fixed distances from the channel axis and
bounding walls, an effect first discovered by Segré and Silberberg
. This behavior is very different from particle movement at low
Reynolds numbers, where due to kinematic reversibility rigid particles
do not cross streamlines. The Segré-Silberberg effect can be
described in terms of an effective lift force acting on the
Devices utilizing inertial lift forces for the separation of bacteria and red blood cells have recently been demonstrated . The separation is most efficient for large size differences since the inertial lift force scales with the third power of the particle radius. Here, we show that one can use optimal control theory to determine external control forces generated, for example, by optical tweezers to steer particles and to separate them when they are similar in size.
We study the system by mesoscopic simulations of the fluid using multi-particle collision dynamics (MPCD) . We determine lift forces and single-particle probability distributions in steady state and analyze their dependence on particle radius and Reynolds number. We show that the Boltzmann distribution for the potential connected to the lift force reproduces the observed distribution functions.
We use the lift force determined by MPCD to set up a Smoluchowski equation which describes the particle motion in lateral channel direction. We then employ the formalism of optimal control  to determine proles of the external force, which help to steer particles and thereby maximize particle separation. We verify that the calculated force profiles are indeed able to separate particles of similar size by performing independent simulations of the corresponding Langevin equations.
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 A. J. Mach and D. Di Carlo, Biotechnol. Bioeng., 107, 302 (2010).
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 F. Tröltzsch, Optimal Control of Partial Dierential Equations, American Mathematical Society, first edition (2010).