Whole Chip Temperature Measurements Using Thin-Film PDMS/Rhodamine B for Microfluidic Chip Design

A novel method for in-situ temperature measurements of microfluidic devices using thin-film poly(dimethylsiloxane) (PDMS) saturated with Rhodamine B dye is reported. Rhodamine B, a dye with temperature dependent fluorescent intensity, is frequently injected into the working fluid for on-chip temperature field visualization of glass and silicon based microfluidic devices. However, such a visualization method results in unreliable temperature measurements due to high absorption and adsorption for polymeric devices such as PDMS. Thus, an inexpensive temperature measurement technique is developed in which a thin PDMS layer (∼30 μm) is fabricated and submersed for several days into a Rhodamine B solution. To prevent backward diffusion of the dye into the working fluid during operation, a glass barrier (∼150 μm) is bonded between the thin film and the PDMS mold containing the microchannel design of interest. Temperature measurements are made by utilizing standard method of measuring changes in the normalized fluorescent intensity. For verification purposes, a new calibration curve is developed and the thin film is tested with a microchannel subjected to joule heating. The resulting temperature field along the axial direction of the channel for different applied powers compares well with numerical simulations. Analysis of dye intensities before and after experiments provides temperature deviation estimates due to photobleaching. Errors in temperature measurement due to the film thickness are discussed.