Investigations on Joule Heating and Associated Effects during Liquid Dielectrophoresis of Aqueous Droplets

Abstract

Liquid dielectrophoresis (L-DEP) is an electrokinetic phenomenon in which a dielectric liquid with higher permittivity than the surrounding medium, when placed in a non-uniform electric field, moves towards the regions of higher electric fields. L-DEP is considered as a promising digital microfluidic technique as it offers a method to manipulate both low-conductivity and non-conducting liquid droplets. In these devices droplets are manipulated on the dielectric-coated coplanar electrodes. Different droplet functionalities such as droplet generation, mixing and sensing are achieved by application of spatially modulated electric fields. Due to the high frequency ohmic currents, Joule heating occurs inside the drop. Non-uniform temperature distribution leads to a spatial variation of electrical and mechanical properties of the liquid within the droplet. This in addition to the non-uniform electric field sets up an electrothermal flow inside the droplet. Joule heating and electrothermal flows leads to observation of several other interesting phenomena. These phenomena have not been significantly studied for practical applications. The aim of the thesis is to experimentally study such phenomena and demonstrate their applications. The initial part of the thesis deals with the investigation of Joule heating effect inside aqueous drop during L-DEP. A maximum temperature rise (difference between observed maximum surface temperature of the drop and the room temperature) of 38 °C was observed when 10 µl water droplet of conductivity and#963; = 0.2 mS/m was actuated using V = 460 V at f = 50kHz. For a fluid of any given conductivity, Joule heating was observed to be effective only within a frequency range. Same temperature rise was achieved at lower voltages for higher conductivity NaCl solutions. However, as the droplet conductivity increases, the frequency range for Joule heating shifted to higher values. The performance of this heating technique was experimentally compared to that of a conventional microheater...