Capillary kinetics of ferrofluid in hydrophilic microscope slide nanochannels

The capillary filling speed of ferrofluid in hydrophilic microscope slide nanofluidic channels is investigated under various temperature and magnetic field conditions. Nanochannels with depths ranging from 50 to 150 nm and widths ranging from 30 to 200 μm are fabricated on a slide substrate of borosilicate glass with a thickness of 1.1 mm using buffered oxide wet etching and glass-glass fusion bonding techniques. The capillary filling speed of the ferrofluid is measured experimentally and compared with the theoretical prediction. It is found that the obtained experimental capillary filling speed is significantly lower than that predicted by the classical Washburn equation, and is attributed to the erroneous assumption of a constant contact angle irrespective of flow rate and other flow obstructions. The experimental results show that filling speed reduces with a reducing channel depth, a lower operating temperature and an increased filling length, but is insensitive to channel width. By contrast, the filling speed increases under the effects of an external magnetic field.