Investigations on the flow behaviour in microfluidic device due to surface roughness: a computational fluid dynamics simulation

In the field of micro-fluidics device, as the cross section of micro-channel comes down to the scale of few tens of micro-meters, surface area to volume ratio increases significantly, and due to this, surface dependent phenomenon dominates during flow of the fluid. This surface dependent phenomenon is mainly governed by surface roughness as an important parameter which directly influences on flow and results in the loss of pressure head due to the building of localised pressure as well as eddy flow. To understand this mechanism, a computational fluid dynamics (CFD) simulation is carried out. In the present CFD simulation, fluid and solid interactions are modelled in two different types. The first is modelled as pure slip between them so that the effect of roughness can be investigated as a main source of friction factor. The second model consists the effect of the pure adhesion by maintain zero relative velocity on the surface of micro-channel. Behaviour of fluid flow and increase in pressure-drop are observed differently in the both types of model. It is observed that the rise in pressure-drop occurs exponentially as size of a channel reduces from 300 to 100 µm. This phenomenon reveals the science of the size effect on micro-channels. The surface roughness of micro-channel is simulated and it is also observed that the surface finish up to few tens of nanometers does not affect the fluid flow. However, the flow resistance increases as the surface roughness increases up to few hundreds of nanometers, and the pressure-drops along the channel length. In the present case, an elevated temperature of fluid mitigates the effect of surface roughness up to some extent for the efficient flow of fluid in a micro-fluidic device. Hence, micro-fluidic device with nano-finished micro-channel and elevated temperature of fluid is recommended for economic and efficient utilisation of the device.