Creating Isolated Liquid Compartments Using Photopatterned Obstacles in Microfluidics

Abstract

We propose a method to trap liquid (both oil and water) in a microchannel by sequentially injecting oil and water (or vice versa) over photopatterned obstacles with controlled wetting properties. We present a simple geometrical model to understand the liquid-entrapment process and predict the evolution of the water/oil interface over an obstacle. Our analysis provides an analytic solution that can successfully predict useful properties such as angular position of pinch-off and the amount of captured liquid. We show that we are able to obtain a liquid bridge over two circular obstacles, and we are also able to predict the condition for the formation of a bridge. We further demonstrate the effect of the obstacle shape and how a sudden change in gradient results in larger liquid entrapment. We also demonstrate the ability to entrap isolated liquid chambers for parallel experimentation.

Controlled liquid entrapment over patterned sidewalls in confined geometries

“…We do not account for any preferred values of γ 2 based on surface energies in our model. Last, a thin layer of oil that may be deposited in the photopatterned structure can alter the contact angle and surface properties of the structure [20]. Figure 5(b) summarizes results ofã for the sinusoidal patterns.…”

“…We observe that θ flat is relatively constant across different patterns. In contrast, the contact angle within the triangular pattern walls θ left and θ right is larger than θ flat due to the effect of third dimension [20,31,32]. Moreover, θ left and θ right are observed to be dependent on the pattern geometry, and thus we chose θ = θ flat .…”

“…We also define angle α such that tan(α) = 4mn. As we demonstrated in our previous work, significant simplifications are possible under certain physical conditions for an immiscible liquid displacement process [20]. We can neglect effects due to viscosity contrast since we use decane and water in our system for which N ≈ 1.…”

“…with 5% (volume/volume) photoinitiator 2-hydroxy-2-methylpropiophenone (SigmaAldrich). Using microscope-projection lithography, we photopattern a repetitive structure at one of the sidewalls [11,20,[26][27][28][29][30]. In this study, we pattern triangular and sinusoidal structures with 30 different combinations of wavelength (λ) and amplitude (A 0 ).…”

“…Though these studies are very useful for fundamental understanding of the oil drainage process, the investigators did not consider the effect of groove shape. Recently, we studied oil entrapment over isolated obstacles and demonstrated the importance of obstacle shape in controlling the entrapment process [20].…”