Passive valves based on hydrophobic microfluidics

Fluid-surface interactions can become dominant in microfluidics, which is a central technology in a number of miniaturized systems for chemical, biological and medical applications. In this paper, two kinds of hydrophobic valves in microfluidic applications were presented. One is based on special geometrical designs and chemical modification for silicon dioxide and glass microchannels. Silicon dioxide and Pyrex glass surfaces, which are hydrophilic originally, are modified with octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) to be hydrophobic, with the contact angles up to similar to 102 and 103degrees, respectively, for water. The formation of OTS SAMs takes <5 min. The OTS SAMs based stop valve can function well to enable stopping the flow of a liquid inside a microchannel in both directions. Tested for deionized water, the hydrophobic valve blocked successfully the liquid for many consecutive times and yielded pressure barrier up to similar to490 Pa, which is near to the theoretical prediction. The OTS SAMs, therefore the hydrophobic valve function, can be retained in a suitable environment for a long time and can rebirth conveniently when destroyed. The other kind of hydrophobic valve is based on hydrophobic pattern, which is formed by plasma depositing CHF3 patterns on the surfaces of silicon dioxide. The hydrophobic CHF3 patterns (measured for contact angle for water to be 102degrees) can block the liquid to flow forward. The theoretical analysis and the process design were presented. (C) 2003 Elsevier B.V. All rights reserved.