Microwave Dielectric Heating of Drops in Microfluidic Devices

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Original entry by Sagar Bhandari, APPHY 225 Fall 2010

Reference

Microwave Dielectric Heating of Drops in Microfluidic Devices, David Issadore, Katherine J. Humphry, Keith A. Brown, Lori Sandberg, David Weitz, Robert M. Westervelt, Lab Chip. 2009 June 21; 9(12): 1701–1706.

Keywords

dielectric, microfluidics, heating, drops

Summary

In this paper, the authors present a technique to locally control the temperature of water droplets in a microfluidic device. Since water absorbs microwave power more efficiently than polymers, glass, and oils due to its permanent molecular dipole moment, microwave heating was employed for this purpose. The devices are fabricated using poly(dimethylsiloxane) (PDMS)-on-glass drop-based microfluidics. A schematic cross-section of the device is shown in Fig. 1 a.

Figure 1:

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Microwave power is locally delivered via metal electrodes that are directly integrated into the microfluidic device and that run parallel to the fluid channel.The microwaves are generated with a voltage controlled oscillator which is amplified to a maximum of 11.7 V peak to-peak with a maximum power of 26dBm. Using Finite Element simulations, the profile of electric field in the microwave heater was computed which is plotted in Fig. 1 b. Using this profile, the microwave power entering the drops was calculated. While, the temperature of the drops is obtained by observing the temperature-dependent fluorescence of CdSe nanocrystals embedded in the drops using a ccd camera. The profile of CdSe temperature dependence is show in Fig. 1 c. Using a flow focused geometry , drops of diameter 50 um were created and to ensure thermal isolation of the drops, they were floated on a low thermal conductivity hydrocarbon oil. Fluid flow is controlled via syringe pumps. To ensure appropriate fraction of water droplets in the device, the flow rate of oil was set to approximately ten times that of water.

Figure 2:

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Using this microfuidic device, the drops were heated upto 30 degrees above the base temperature in just 15 ms. The average temperature change of the drops as a function of time is plotted in Fig. 2b. As seen in Fig. 2 a, the temperature of the water drops so measured is linearly related to the microwave power applied.