Transparency of PDMS based microfluidic devices under temperature gradients

Polydimethylsiloxane (PDMS) is one the most popular materials for rapid prototyping of microfluidic devices. It is highly regarded for its optically transparency and chemical inertness in aqueous environments. Recently, we have observed a unique phenomenon that a PDMS microfluidic channel filled with water loses transparency upon exposure to a temperature gradient. This paper describes a characterization of this phenomenon and proposes a mechanism to explain its cause. Specifically, we have characterized the transparency loss over a variety of gradient magnitudes, absolute temperatures and exposure durations. Observing the low-transparency device with an optical microscope reveals droplets within the channel region (CH) but not within bulk PDMS. We hypothesize that the droplets form as a result of low molecular weight (LMW) PDMS chains entering the aqueous channel, driven by a thermophoretic response, and separating into a different phase from the aqueous medium. This hypothesis is supported by a strong correlation between the magnitude of temperature gradients and degree of transparency loss experienced by the device, while heating or cooling the device uniformly without a temperature gradient does not induce the transparency change. Additionally, we report an ability to restore the optical transparency by heating the device to evaporate water in the channel. The loss and recovery of optical transparency is repeatable. Substance extracted from the low-transparency channel demonstrates fingerprint features of PDMS, supporting our reasoning of the mechanism. To our knowledge, this is the first time such phenomenon is reported and systematically studied in the literature. This work provides some insight about thermophoresis of PDMS and the phenomenon could find applications in micro-sensors and micro-actuators. (paper)