Effects on wettability by surfactant accumulation/depletion in bulk polydimethylsiloxane (PDMS)

Polydimethylsiloxane (PDMS) is an appealing silicone elastomer as a base material for microfluidic biomedical applications. Its ability to easily cross-link as well as its favorable physical and chemical properties has enabled a large number of micro- and nano-cast applications. However, PDMS' hydrophobic surface can be problematic. Two methods presented here address the wettability of PDMS by accumulating or depleting surfactant in PDMS. The surfactant in the cross-linked PDMS matrix is released and activated upon contacting with an aqueous solution and assists the solution wetting of PDMS. Wettability was enhanced as a function of surfactant concentration. With the addition of 3% Triton TX-100 (a nonionic surfactant), the contact angle of deionized water on PDMS decreased up to 40° in 90 s, compared with a decrease of only 3° on unmodified PDMS. Dynamic measurements of contact angle and contact line radii showed wettability change due to the transfer of surfactant to the interface of the aqueous solution/PDMS. The stability of the modified PDMS was quantified by contact angle measurements over a 30-day duration. Immersing the modified PDMS in water for long time periods before the measurement depletes the surfactant; the resulting wettability becomes a function of immersion duration. The surface wettability can also be controlled with a function of immersion duration even after the PDMS is cured.