Slip Flow in Hydrophilic Nanopores of Silica Colloidal Crystals

Slip flow, a fluid flow enhanced in comparison to that calculated using continuum equations, has been reported for many nanopores, mostly those with hydrophobic surfaces. We investigated the flow of water, hexane, and methanol through hydrophilic nanopores in silica colloidal crystals. Three silica sphere sizes were used to prepare the crystals: 150 ± 30, 500 ± 40, and 1500 ± 100 nm. The spheres were pressure-packed in a fused silica capillary with an inner diameter of 75 μm. The resulting colloidal crystals had an average pore radius of 18 ± 4, 66 ± 6, and 215 ± 14 nm for the three silica sphere sizes used. The colloidal crystals were demonstrated to possess almost perfect packing. The fluids were flown through the colloidal crystals, and the pressure drop was measured using a pressure transducer. The flow rates varied from 10 to 80 nL/min. Water showed no-slip Hagen-Poiseuille flow with no enhancement for all of the pore sizes. Hexane showed a 20-fold flow enhancement for the smallest pore size, and the enhancement diminished for the medium pore size and was absent for the largest pore size. Methanol also showed a 20-fold flow enhancement for the smallest pores, about a 15-fold enhancement for the medium pores, and no enhancement for the largest pore size. The reduction in flow enhancement was significantly steeper for hexane than for methanol with an increasing pore size. These results demonstrate a significant slip flow in small (15 nm) hydrophilic nanopores for non-wetting fluids, which is size- and fluid-property-dependent. These observations are important for understanding fluid dynamics in liquid chromatography and naturally occurring nanoporous media.