Pressure barrier in an axisymmetric capillary microchannel with sudden expansion

In this paper, we present an analytical and experimental study of the pressure barrier developed in an axisymmetric microchannel with sudden expansion working as a capillary valve for regulating liquid flow for microfluidics. The analysis is based on the variation in the free energy of the liquid-solid-gas interfaces with liquid volume. The pressure barrier developed at the meniscus is obtained in a rather simple form valid for both hydrophilic and hydrophobic channel walls. The maximum pressure barrier, termed the burst pressure, for a capillary valve necessary to stop liquid flow is found to be a function of the channel geometry and liquid-channel interfacial properties. The analytical results are confirmed by flow visualization experiments in which the burst pressure and the growth of the meniscus are measured. The measurements were carried out for microchannels made of hydrophilic and hydrophobic materials with different diameters and expansion angles in combination with liquids having different interfacial properties.