Patterning microfluidic device wettability using flow confinement
Original entry: Darren Yang, AP225, Fall 2010
A.R. Abate, J. Thiele, M. Weinhart, D.A. Weitz, “Patterning microfluidic device wettability using flow confinement,” Lab on a Chip, 10, 14 (2010).
Monodisperse Double Emulsions, Multiple Emulsions, Poly(Dimethylsiloxane),
The authors present a versatile method to spatially pattern the surface properties of microfluidic devices using flow confinement. This technique allows surface patterning with micron-scale resolution, and to demonstrate its effectiveness, they use it to pattern wettability to form Water/Oil/Water and Oil/Water/Oil double emulsions.
The formation of multiple emulsions, which consist of large drops with smaller drops inside, requires microfludics channels with patterned surface properties. The current methods to spatially pattern the wettability of microfluidics devices are difficult to use and of limited versatility. For example, one of the current approaches uses a polymerization reaction that is initiated by exposure to ultraviolet (UV) light to specific part of the channels. However, since micron-scale resolution is required, sophisticated optics and a powerful UV-light source are needed.
In this paper, the authors present an alternative method for patterning surface wettability that is versatile and easy to use. First they use an inert fluid to physically confine a chemical treatment that alters wettability in selected regions of the microfluidic channel. Since spatial control is achieved by physical confinement of the reaction, this chemical treatment method allowing many different surface treatments to be used.
To illustrate novel technique, the authors use photo-initiated and thermal-initiated surface treatments. Specifically, they demonstrate the effectiveness of this chemical treatment method by patterning the wettability of microfluidics devices to form both Water/Oil/Water and Oil/Water/Oil double emulsions.
To control the wettability of the microfluidic devices, the authors use a sol-gel approach. First they design a sol-gel coating that is intrinsically hydrophobic, but can be converted to hydrophilic after a chemical treatment. The authors first perform contact angle measurements of sol-gel-coated glass sides with water drops. The surface is intrinsically hydrophobic as the sol-gel surface is incorporated with fluorosilanes (Figure 1A). The sol-gel is can be converted to hydrophilic by attaching poly(acrylic acid) (PAA) to the surface through a polymerization reaction (Figure 1B).
To form Water/Oil/Water (W/O/W) double emulsions, the upper portion needs to be hydrophobic and the lower portion hydrophilic (Figure 2A). To create this pattern a reactive surface treatment solution is injected into the device outlet and an inert blockers solution is injected in the inner and middle-phase inlets (Figure 2B). To form Oil/Water/Oil (O/W/O) double emulsions, the pattern is inverted (Figure 2C), and this is achieved by switching the inlets into the reactive and inert solution are injected (Figure 2 D).
Finally, to form W/O/W double emulsions, the author use flow confinement to pattern the wettability of a double emulsion device. The first drop maker is hydrophobic and the second hydrophilic (Figure 3A). To form O/W/O, the pattern is inverted. The image samples collected from both device confirm the formation of double emulsions. The scale bars denote 100 um.