Investigations on Joule Heating and Associated Effects during Liquid Dielectrophoresis of Aqueous Droplets
Nampoothiri, Krishnadas Narayanan
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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 σ = 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. Heating and associated liquid flow were studied using simulations. Heating of droplets having a wide range of conductivity (0.3 mS/m to 20 mS/m) were simulated for 200 V within a frequency range of 1-1000 kHz. For calculating the body force and effectively simulate the flows, the change in the liquids electrical properties were characterized as a function of temperature. Using the temperature dependent permittivity and conductivity values obtained from the experiments, coupled flow simulations were performed. The simulation results were in good agreement with the experimental observations. The next part of the thesis demonstrates the applications of Joule heating effect. Since the simplified coplanar electrode structure can be used both for actuation and heating, the use of microheaters can be avoided. This significantly simplifies the design of digital microfluidic devices as the actuation electrodes themselves can be used for the thermal cycling. Using this heating technique biochemical reactions which require temperature cycling were demonstrated. The high frequency heating technique was also found to be effective for de-icing applications. Ice generated on interdigitated coplanar electrodes, was removed using 200 V at 50 kHz. The same electrodes were also used to characterize the extent of ice formation by measuring and comparing the impedance values for dry and iced substrate. Thus, the same electrode configuration can be used simultaneously for both ice detection and heating for de-icing. This significantly simplifies system design as compared to a conventional microheater based approach where sensing ice is not possible. The final part of the thesis investigates an interesting phenomenon where streams of microscale droplets were observed to be generated near the primary droplet that was actuated using L-DEP. Additionally, droplets flying outside the streams were also observed. These flying droplets, which were less in number and fly in an irregular pattern, were formed due to localized condensation on aerosol particles. For understanding the droplet streams, a series of experiments were performed to study the role of electric field and primary droplet temperature. Due to soft dielectric breakdown at higher electric fields, charged species were generated. These charged species act as additional nucleation sites for enhanced vapor condensation. Subsequently these condensed droplets were dragged along with the air convection currents, leading to the observation of streams. Air convection velocities was obtained from simulations which captured heat transfer, fluid flow and evaporation. Relationship between the stream velocity and the applied electric field was due to the reduction in microdroplet sizes with the increase in applied electric field. Since generation and transfer of droplets is of vital interest for many applications, monodispersed droplets in sub-picolitre volumes were transferred to oil-coated ITO electrodes using negative DC bias. The stream velocity was observed to increase with the bias voltage thus providing a method to control the impact dynamics on the top plate. In summary, a detailed study regarding the Joule heating during L-DEP of aqueous droplets have been done in this thesis. This heating technique has been applied for lab-on-chip and de-icing applications. The thesis has further examined the phenomena of generation of flying microscale droplet streams during L-DEP.