Microfluidics-Based Fabrication of a Hele-Shaw Cell Device for Drop Coalescence Imaging

The Hele-Shaw cell has been well used for various flow studies because it can simplify analyzing the flow of interest to two-dimensional creeping one. The Hele-Shaw cell consists of two parallel flat plates with a very small gap between them. When a working fluid is injected through this narrow gap, two-dimensional Stokes flow is generated with an extremely low Reynolds number and negligible flow velocity normal to the plates. Applications of Hele-Shaw cell devices include experimental studies of drop coalescence, Stokes flow, and confining cells to a 2D plane, and they usually depend on cameras to capture phenomena of interest from the experiment. The quality of the collected images largely depends on the optical quality of the Hele-Shaw cell. Therefore, it is favorable to reduce the degree of light attenuation through Hele-Shaw cells. Previously we fabricated a Hele-Shaw cell using polydimethylsiloxane (PDMS) and soft lithography, which are common in microfluidic fabrication, to study drop coalescence with high-speed imaging. It was found that the frame rate of high-speed imaging was limited by light intensity, which was affected by light attenuation through PDMS, and thus the very initial stage of the neck growth during coalescence was not captured. To overcome this limitation, an advanced fabrication method of the Hele-Shaw cell has been developed to incorporate a glass observation window in the PDMS part of the cell. The previous and newly developed Hele-Shaw cell devices were compared in terms of local intensity and contrast, and improvement in image quality was confirmed. Our method has the following advantages. First, the thickness of the PDMS layers is controllable and thus the attenuation of light intensity can be reduced. Second, the glass-based observation window can enable clearer images with reduced image defects caused by PDMS. Therefore, the suggested microfluidics-based fabrication method of the Hele-Shaw cell has the potential to increase the resolution of high-speed imaging and to reduce the difficulty of post image processing.