Experimental Analysis of Multiple Emulsion Compound Droplet Dynamics in a Stenosed Microchannel

The dynamics of (O/W)/O multiple emulsion droplets navigating through stenosed microchannels are critical for understanding droplet behaviour in confined geometries, with implications for drug delivery, controlled emulsification, and biomedical modelling. This study explores how factors such as flow rate ratio (\(\:{Q}_{r}\)), droplet size (both inner and outer), channel wall compliance, and the inherent size variation of the inner droplets influence the transit phenomena. Experimental results reveal that the size and transit time of the compound droplets varies exponentially with an increase in flow rate ratio (\(\:3\:\le\:\:{Q}_{r}\le\:\:5\)). Material optimization showed that Xanthan gum (XG) concentration (\(\:0.16\--0.2\%\:\)w/v) and intermediate oil concentrations (\(\:\sim15\%\) v/v) were found to stabilize the inner emulsions while maintaining consistent droplet behaviour. For larger inner droplets (0.65 ≤ \(\:{D}_{i,\:max}^{*}\) ≤ 1.11), mean transit times (for droplet passage through stenosed microchannel) were \(\:52.27\:\pm\:\:1.07\) ms, approximately 8% higher than for smaller droplets (\(\:0.21\le\:{D}_{i,\:max}^{*}\) \(\:\le\:0.25\)) at 48.35 ± 0.96 ms. Crucially, wall compliance significantly altered droplet behaviour, leading to earlier droplet formation and consistently longer transit times due to curvature effects arising from wall deformation. The compliant channel exhibits a maximum \(\:{t}_{total}\) increase of 35.07% compared to the rigid channel at \(\:{Q}_{r}=\:3.333\). Additionally, this work practical preparation method for mineral oil emulsions using an XG+DI solution, which enables stable, foam-free visualization and controlled encapsulation. This novel preparation technique, combined with experimental insights, highlights the importance of both internal droplet structure and wall compliance in microfluidic droplet dynamics.