Long-term behavior of nonionic surfactant-added PDMS for self-driven microchips

The outstanding characteristics of polydimethylsiloxane (PDMS) owe its extensive use to the fact that it is a base material for the microfluidic devices manufacturers’. In spite of favorable physical and chemical properties, the hydrophobic surface of PDMS is a handicap when pumping aqueous solutions through microchannels using only capillary forces. There are several techniques to achieve a hydrophilic behavior of PDMS, but most of them face the problem of hydrophobic recovery after a short period of time while most commercial microdevices require long storage and distribution times. The use of surfactant-added PDMS provides a novel method to overcome hydrophobicity and to control the hydrophobic recovery over a long period of time. There are many different types of surfactants and not a deep methodology to choose one in terms of efficiency, clearance and duration of the hydrophilic behavior. This paper has compared three non-ionic surfactants with different critical micelle concentration and chemical composition: Triton X-100, Brij 35 and Tween 20. Short and long-term studies were done using 5-μL deionized water droplet on the surface of the prepared surfactant-added PDMS. The experiments demonstrated that Triton X-100 is more efficient than Brij 35 and Tween 20 since with less concentration it achieves a maximum contact angle of around 23.5°. In terms of hydrophobic recovery, the experiments showed that using surfactants and controlling humidity of samples, hydrophobic recovery of the surfactant-added PDMS was negligible after 2 months. According to these results, the use of PDMS with Triton X-100 and Brij 35 provides a good potential for building capillary driven devices without the need of tedious preprocessing techniques.