Formation of double emulsion micro-droplets in a microfluidic device using a partially hydrophilic-hydrophobic surface

Abstract

The objective of this paper is to propose a surface modification method for preparing PDMS microfluidic devices with partially hydrophilic-hydrophobic surfaces for generating double emulsion droplets. The device is designed to be easy to use without any complicated preparation process and also to achieve high droplet encapsulation efficiency compared to conventional devices. The key component of this preparation process is the permanent chemical coating for which the Pluronic surfactant is added into the bulk PDMS. The addition of Pluronic surfactant can modify the surface property of PDMS from a fully hydrophobic surface to a partially hydrophilic-hydrophobic surface whose property can be either hydrophilic or hydrophobic depending on the air- or water-treatment condition. In order to control the surface wettability, this microfluidic device with the partially hydrophilic-hydrophobic surface undergoes water treatment by injecting deionized water into the specific microchannels where their surface property changes to hydrophilic. This microfluidic device is tested by generating monodisperse water-in-oil-in-water (w/o/w) double emulsion micro-droplets for which the maximum droplet encapsulation efficiency of 92.4% is achieved with the average outer and inner diameters of 75.0 and 57.7 μm, respectively.

Fig. 1. Water contact angles of samples (a) type A: exposed to air and (b) type W: immersed in water. For WCA, the error bar is the corresponding standard deviation (n = 3) with the minimum and maximum values of 0.8% and 5.8%, respectively.

Fig. 2. Percent light transmission (%T) of PDMS samples (a) without surface modification and with surface modification using Pluronic F-127 surfactant solution of (b) 4 μL g⁻¹, (c) 8 μL g⁻¹ and (d) 12 μL g⁻¹.

Fig. 3. Hydrophobic surface recovery of modified PDMS with Pluronic solution of 4.0 μL g⁻¹: without water treatment (WT0) and with water treatment for 1, 2, 3, 7 and 14 days (WT1, WT2, WT3, WT7 and WT14).

Fig. 4. Water contact angles of the WT1 sample after the hydrophilic surface treatment within 24 hours.

Fig. 5. Process for surface preparation of a microfluidic device to generate water-in-oil-in-water double emulsion droplets. (a) Micro-channels with Pluronic surfactant solution with a concentration of 4.0 μL g⁻¹ after water treatment for 24 hours. (b) Micro-channels exposed to air longer than 9 days. (c) Micro-channels without hydrophilic surface treatment at the left junction and with hydrophilic surface treatment for more than 30 min at the right junction.

Fig. 6. Droplet generation in the microfluidic channel (a) junction 1 for water-in-oil droplets, (b) junction 2 for water-in-oil-in-water droplets and (c) locations where the images are captured.

Fig. 7. (a) Droplet encapsulation efficiency (primary vertical axis) and droplet diameter (secondary vertical axis) and (b) images of water-in-oil-in-water droplets at f_r = 0.73–1.30.

Fig. 8. Schematic diagram of the microfluidics device for double emulsion droplet generation with the flow-focusing micro-channel.