Mechanism governing separation in microfluidic pinched flow fractionation devices

Abstract

We present a computational investigation of the mechanism governing size-based particle separation in microfluidic pinched flow fractionation. We study the behavior of particles moving through a pinching gap (i.e., a constriction in the aperture of a channel) in the Stokes regime (negligible fluid and particle inertia) as a function of particle size. The constriction aperture is created by a plane wall and spherical obstacle, and emulates the pinching segment in pinched flow fractionation devices. The simulation results show that the distance of closest approach between the particle and obstacle surfaces (along a trajectory) decreases with increasing particle size. We then use the distance of closest approach to investigate the effect of short-range repulsive non-hydrodynamic interactions (e.g., solid-solid contact due to surface roughness, electrostatic or steric repulsion, etc.). We define a critical trajectory as the one in which the minimum particle-obstacle separation is equal to the range of the non-hydrodynamic interactions. The results further show that the initial offset of the critical trajectory (defined as the critical offset) increases with particle size. We interpret the variation of the critical offset with particle size as the basis for size-based microfluidic separation in pinched flow fractionation. We also compare the effect of different driving fields on the particle trajectories;we simulate a constant force driving the particles in a quiescent fluid as well as a freely suspended particles in a pressure-driven flow. We observe that the particles driven by a constant force approach closer to the obstacle than those suspended in a flow (for the same initial offset). On the other hand, the increment in the critical offset (as a function of particle size) is larger in the pressure-driven case than in the force-driven case. Thus, pressure-driven particle separation using pinched flow fractionation would prove more effective than its force-driven counterpart (e.g., particles settling under gravity through a pinching gap).

Inertia-Enhanced Pinched Flow Fractionation

“…At the smallest flow rate with Re = 0.8, where the inertial effects are still minimal and the separation may be viewed as purely PFF, the center position of the 9.9 μm particle stream is found to locate at about 225 μm in the expansion region with a large dispersion (see Figure ). This deflection is approximately 25% greater than the estimated value of 180 μm from the streamline analysis, which is due primarily to the neglected corner effect on particle positions. − …”

“…Note that the first term on the right hand side of eq gives only the particle center-to-center distance in the channel expansion. This equation is obtained with the assumption that particles strictly follow streamlines, which, as explained in previous numerical studies, − may cause inaccuracy due to the corner effect.…”

“…For the second approach, the particle dispersion in PFF can be dramatically reduced by adding a tilted sidewall as well as vertical focusing channels to the pinched segment to precisely align particles of different sizes . In addition, the particle separation in PFF has been numerically investigated using, for example, boundary-integral method , and lattice Boltzmann method, , which provides guidance for the optimal design of future PFF microdevices.…”

Deterministic separation of suspended particles in a reconfigurable obstacle array

“…Active methods include dielectrophoresis [1], magnetophoresis [2], acoustophoresis [3], various optical methods [4], [5] and a family of field flow fractionation methods with different fields driving the separative displacement [6]- [9]. Passive methods are generally based on hydrodynamics and particle-solid interactions between the species and the stationary phase in the fluidic system [10], [11], including hydrodynamic filtration [12], pinched flow fraction [13]- [16] and separation by inertial and Dean flows [17]. Deterministic lateral displacement (DLD) is a separation method that can be implemented in both active and passive modes.…”

Particle size effect on sorting with optical lattice

“…Examples include the study of the transport of colloidal particles through arrays of micro-scale potential landscapes in the HOT generated potential wells [1][2][3][4][5][6] , investigation of transport and separation of overdamped particles in a microfluidic system 7 , sorting of chiral particles exploiting lattice potentials 8 , dynamic ordering and locking states of colloidal monolayers on a decagonal quasiperiodic surface 9 , sortings of particles within a microfluidic chip using a dual-channel line optical tweezers with a 'Y' shape channel 10 and, experimental investigation for the transport of 100 and 500 nm plasmonic nanoparticles in a two-dimensional optical lattice 11 . An array of obstacles on a surface is another technique for separating mesoscale particles [12][13][14][15][16][17][18][19][20][21][22][23] . In both cases, sorting occurs based on the fact that because of the potential wells (obstacles), particles' path deviates from the direction of the external force (fluid flow).…”