Gravitation effect on concentration of ions near ion-selective microparticle

Developing of laboratories on a chip or μ TAS (micro-total analysis systems) is a great challenge of the modern microfluidics. With the help of such devices, it will be possible to conduct many chemical analyses of human liquids for the preliminary diagnosis of various diseases. Despite the great importance of this task and certain successes in the field of experimental study of the behavior of the electrolyte in micro-devices, there are many undescribed effects, which opens up the possibility for a theoretical study of these processes. Usually, the flat geometry of devices is used in design of laboratories on a chip, which is explained by the simplicity of manufacturing, but our research shows that using the spherical geometry of the device allows to design a universal device that can work as a micro-pump, micromixer and micro-concentrator. The paper presents a theoretical analysis of the concentration effects of ions in a micro-device with presence of the pressure driven flow. The device presents a spherical chamber in the center of which an ion-selective microgranule is placed. This device is built into a circular channel through which the electrolyte flows due to the electroosmotic flow caused by the difference in electrical potentials. Depending on the orientation of the inlet channel relative to gravity, there is an additional pressure of the water column in the inlet or outlet channel. As a result of selection of a suitable external electric field, it is possible to achieve a significant concentration of ions near the ion-selective microgranule. The enriched region can be carried away by the convective flow far into the outlet channel and then this device can be used to separate the flow into enriched and depleted, as happens with electrodialysis. The efficiency of the device increases with an increase in the intensity of the external field, however, starting from a certain critical value of the electric field strength, the steady state flow loses stability and an electroconvection is formed, which interferes with the concentration process. Due to the additional pressure, both the stability of the flow and the degree of concentration can be adjusted. The paper shows that by varying the values of the field strength and pressure gradient, it is possible to achieve maximum efficiency of the device.