Field-Flow Fractionation and Hydrodynamic Chromatography on a Microfluidic Chip Tyler N. Shendruk, ‡ Radin Tahvildari, ‡ Nicolas M. Catafard, Lukasz Andrzejewski, Christian Gigault, Andrew Todd, Laurent Gagne-Dumais, Gary W. Slater, and Michel Godin* Department of Physics, University of Ottawa, MacDonald Hall, K1N 6N5 Ottawa, Canada ABSTRACT: We present gravitational field-flow fractionation and hydrodynamic chromatography of colloids eluting through 18 μm microchannels. Using video microscopy and mesoscopic simulations, we investigate the average retention ratio of colloids with both a large specific weight and neutral buoyancy. We consider the entire range of colloid sizes, including particles that barely fit in the microchannel and nanoscopic particles. Ideal theory predicts four operational modes, from hydrodynamic chromatography to Faxe ́ n-mode field-flow fractionation. We experimentally demonstrate, for the first time, the existence of the Faxe ́ n-mode field-flow fractionation and the transition from hydrodynamic chromatography to normal-mode field-flow fractionation. Furthermore, video microscopy and simulations show that the retention ratios are largely reduced above the steric-inversion point, causing the variation of the retention ratio in the steric- and Faxe ́ n-mode regimes to be suppressed due to increased drag. We demonstrate that theory can accurately predict retention ratios if hydrodynamic interactions with the microchannel walls (wall drag) are added to the ideal theory. Rather than limiting the applicability, these effects allow the microfluidic channel size to be tuned to ensure high selectivity. Our findings indicate that particle velocimetry methods must account for the wall-induced lag when determining flow rates in highly confining systems. F ield-flow fractionation (FFF), 1,2 a broad class of separation techniques, is achieved by imposing a transverse force f across a channel of height h to a solution of solutes such as colloids, 3 macromolecules, 4 or cells. 5 A concentration gradient is established in response to the competition between the thermal energy, k B T, and the potential energy drop across the channel, fh. The system can thus be described in terms of the dimensionless retention parameter, λ = k B T/fh. This solute distribution is carried through a channel by a Poiseuille flow with a characteristic retention ratio (average colloid velocity normalized by average fluid velocity, R = ⟨=⟩/⟨v⟩). Colloids of different sizes r have different concentration distributions and therefore different retention ratios, R. This simple system possesses surprisingly rich elution behavior. Previously, we considered the ideal retention theory (ignoring complications such as nonparabolic flow, 6 nondilute concentration effects, 7 slip, 8 or hydrodynamic/lift interac- tions 9 ), and predicted the existence of four distinct operational modes. 10 The transitions between them were mapped; however, only three of the four modes and only a single transition had ever been previously observed experimentally. For large λ, thermal energy dominates and solutes diffuse across the entire channel. Only steric interactions with the walls limit their distribution, as shown in Figure 1. This is the hydrodynamic chromatography limit of FFF (HC), 11 where larger particles elute before smaller particles. Ideally, density- matched tracer particles used for micro-particle image velocimetry 12 elute in HC, in the absence of any net external body forces. If the potential energy dominates over the thermal energy, the concentration profile across the channel height becomes exponential. In this normal-mode field-flow fractionation (nFFF), ensembles of small solutes can loosely be thought of as point particles subject to an external force that increases with particle size r (nFFF in Figure 1). Therefore, larger particles stay close to the accumulation wall where the lower flow velocity causes them to move more slowly than smaller particles. 1 Experimental observation of the transition between HC and nFFF has not previously been reported. However, nFFF can only continue to exist for a certain size range before the size dependence changes once again. Larger (heavier) particles are pushed against the wall and steric interactions exclude the particles from sampling the slow Received: March 17, 2013 Accepted: May 7, 2013 Published: May 7, 2013 Figure 1. Schematic of ideal field-flow fractionation operational modes. Article pubs.acs.org/ac © 2013 American Chemical Society 5981 dx.doi.org/10.1021/ac400802g | Anal. Chem. 2013, 85, 5981−5988