Three-Dimensional Microfluidic Measurements Using Optical Sectioning by Confocal Microscopy: Flow Around a Moving Air Bubble in a Micro-Channel

Abstract

Microscopic visualization of the transport phenomena associated with an air bubble entrained in liquid flowing through a microchannel has revealed a transition in the bubble shape and proximity to the channel walls with increasing flow rates. Experiments have been conducted to investigate the effect of channel geometry (three different microchannels with square cross-sections, 1.0-mm, 500-μm and 100-μm), and flow conditions (Capillary numbers ranging from 0 to 0.23) on the transition in bubble shape. In addition, micro-Particle Image Velocimetry (μ-PIV) has been used to map the detailed flow field around the bubble and an innovative attempt has been made to provide optically-sectioned image recordings using a state-of-the-art high-speed Confocal Laser Scanning Microscope (CLSM) technique. The research presented herein is directed towards characterizing transport phenomena in two-phase (gas-liquid) flows in microchannels. Of particular interest to this research is the determination of entrance and exit length regions in two-phase microchannel flows. The experimental techniques being utilized towards the objective of characterizing two-phase flow transport phenomena in microchannels are the focus of the following discussion.

Experimental Techniques for Bubble Dynamics Analysis in Microchannels: A Review

“…However, spinning disk confocal microscopes have thousands of moving pinholes on a scanner, which provide simultaneous scanning and higher temporal resolutions. An image acquisition rate of up to 120 fps with a spinning disk confocal microscope is reported by Park et al [93]. Optical sectioning and high axial resolution allow for accurate signal discrimination and avoid superimposing the structures on top of each other.…”

“…For the first time, Park et al [93] proposed combining CSLM with micro-PIV to obtain optically sectioned flow field mapping in microchannels. Velocity data for CSLM micro-PIV and widefield fluorescence micro-PIV for Poiseuille flow in circular microchannels have been directly compared [94].…”

“…The same technique is applied to determine the liquid-phase velocity field around the gaseous thread of the bubbles formed in a flow focusing device [2] and to characterize the bubble formation at a T-junction [43,104]. Park et al [93] showed that bubble shape and proximity to the channel walls undergo a transition with increasing flow rate. Using micro-PIV, the effects of channel dimensions and capillary number on the transition of the bubble shape as well as flow field ahead of the bubble was investigated.…”