Electroosmotic flow(EOF) is a promising way for driving and mixing fluids in microfluidics.For the parallel-plate microchannel with the hydrophobic surface, this paper solved the governing equations using the finite element method(FEM), and the effects of microchannel height, electric strength and ionic concentration on EOF were thus investigated.The simulation indicates that the transient characteristics of EOF are similar in hydrophobic and hydrophilic microchannels, the steady time of EOF is proportional to the square of microchannel height, and the scale is microsecond.EOF velocity is proportional to the electric strength and independent of the channel height, and decreases slowly with the ionic concentration, which is lower than that in hydrophilic microchannel due to the presence of slip length in hydrophobic microchannel.The results can provide valuable insights into the optimal design of microchannel surfaces to achieve accurate EOF control in hydrophobic microchannel.
Electroosmotic flow(EOF) is widely used to transport and mix fluids in microfluidic chips.Aiming at the parallel-plate microchannel with surface roughness,governing equations describing the EOF were established and the influence of surface roughness and electrical double layer(EDL) on EOF was investigated by using finite element method(FEM).The simulation results indicated that,when the EDL thickness was comparable to 0.3 times of the height of surface roughness,the presence of surface roughness could alter EDL near the surface,and reduce the EOF significantly.When the EDL thickness was much smaller or much greater than the height of surface roughness,EOF was less affected by surface roughness.The relationship between the EOF speed in bulk flow region and the relative EDL thickness was "V-shaped".
A microtribometer is used to measure and compare pull-off forces and friction forces exerted on （a） micro-dimpled silicon surfaces, （b） bare silicon surfaces, and （c） octadecyltrichlorosilane （OTS） treated silicon surfaces at different relative humidity （RH） levels separately. It is found that above a critical RH level, the capillary pull-off force increases abruptly and that the micro-dimple textured surface has a lower critical RH value as well as a higher pull-off force value than the other two surfaces. A micro topography parameter, namely sidewall area ratio, is found to play a major role in controlling the capillary pull-off force. Furthermore, micro-dimpled silicon surface is also proved to be not sensitive to variation in RH level, and can realize a stable and decreased friction coefficient compared with un-textured silicon surfaces. The reservoir-like function of micro dimples is considered to weaken or avoid the breakage effect of liquid bridges at different RH levels, thereby maintaining a stable frictional behaviour.
