Design and numerical analysis of serpentine microchannel integrated with inner-wall ridges for enhanced droplet mixing

A numerical model is developed for three-dimensional droplet mixing in serpentine microchannels with inner-wall ridges(referred to as SMR microchannels).The underlying mechanisms of mixing in SMR microchannels are revealed,and the effects of the ridge width and ridge number on mixing inside droplets of different sizes are clarified.The results indicate that SMR microchannels are capable of enhancing the mixing efficiency by 3.6%–12.5%compared with that in smooth serpentine microchannels,particularly when the droplet length is larger than the microchannel width and the width ratio of the ridge to the microchannel is 0.25–1.The above mixing enhancement is determined by two main factors:the combined effects of the vortex pair and reverse flow,and non-horizontal vortices.Moreover,when the width ratio is 1,there is a maximum 16.0%increase in mixing efficiency for small droplets with a length equal to the microchannel width.However,for small droplets,mixing in an SMR microchannel is worse than in a smooth serpentine microchannel when the width ratio is less than 0.75.With increasing the ridge number,when the width ratio is 0.25,the mixing efficiency is reduced for small droplets but increased for larger droplets.