Water behavior on different surfaces is a topic of special interest for Lab on a Chip applications. In general, Poly(dimethyl)siloxane (PDMS) is used to fabricate microchannels due to its good elastic, optical and chemical properties. PDMS is hydrophobic, with water contact angle between 100° and 110° (1). Nevertheless, in order to form water drops in an oil continuous phase, surfactants are added to the oil phase, which ensures the existence of an oil lubrication film that avoids drops sticking to the PDMS walls. The aim of this investigation is to bring an alternative tool that does no require the use of surfactants for the formation of water drops in microchannels.

There have been several investigations exploring treatments to PDMS with the aim of convert it from hydrophobic to superhydrophobic (1)(2)(3).

Using optical and soft lithography we fabricated modified PDMS surfaces, consisting on square arrays of micro-holes of diameter ranging between 5 µm and 60 µm, depth between 5 µm and 30 µm and periodicity between 10 µm and 120 µm, endowing the PDMS surface with superhydrophobicity. The contact angle of millimetric water drops deposited onto the surfaces exceeded 160° in some cases. Then, we used the same technique to fabricate microchannels with micro-holes on their top and bottom walls. The possibility of forming water-in-oil drops in the absence of surfactants was assessed depending on the dimensions and distances between the holes.

(1)Jin, Meihua, et al. "Super‐hydrophobic PDMS surface with ultra‐low adhesive force." Macromolecular rapid communications 26.22 (2005): 1805-1809.

(2)Sabbah, Abbas, Ayman Youssef, and Pascal Damman. "Superhydrophobic surfaces created by elastic instability of PDMS." Applied Sciences 6.5 (2016): 152.

(3)Stanton, Morgan M., et al. "Super-hydrophobic, highly adhesive, polydimethylsiloxane (PDMS) surfaces." Journal of colloid and interface science 367.1 (2012): 502-508.