Department of Physics, University of Ottawa, MacDonald Hall, K1N 6N5 Ottawa, Canada.
Anal Chem. 2013 Jun 18;85(12):5981-8. doi: 10.1021/ac400802g. Epub 2013 May 29.
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 Faxén-mode field-flow fractionation. We experimentally demonstrate, for the first time, the existence of the Faxén-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 Faxén-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.
我们展示了通过 18μm 微通道洗脱的胶体的重力场流分级和流体动力学色谱。使用视频显微镜和介观模拟，我们研究了具有较大比重和中性浮力的胶体的平均保留率。我们考虑了胶体尺寸的整个范围，包括几乎刚好适合微通道的颗粒和纳米颗粒。理想理论预测了四种操作模式，从流体动力学色谱到 Faxén 模式场流分级。我们首次实验证明了 Faxén 模式场流分级的存在以及从流体动力学色谱到正常模式场流分级的转变。此外，视频显微镜和模拟表明，在空间反转点之上，保留率会大大降低，由于阻力增加，空间和 Faxén 模式区域的保留率变化受到抑制。我们证明，如果将与微通道壁的流体动力学相互作用（壁阻力）添加到理想理论中，理论可以准确预测保留率。这些影响并没有限制适用性，而是允许调整微流道尺寸以确保高选择性。我们的发现表明，在确定高度约束系统中的流速时，颗粒速度测量方法必须考虑壁引起的滞后。
