Passive mixing in a three-dimensional serpentine microchannel

TLDR

The paper presents a three‑dimensional serpentine microchannel with a “C‑shaped” repeating unit designed to induce chaotic advection for passive fluid mixing. The device is fabricated on a silicon wafer by double‑sided KOH wet‑etching to realize the three‑dimensional channel geometry. Experiments with phenolphthalein/NaOH show that the serpentine channel mixes faster and more uniformly than diffusion or a square‑wave channel across Reynolds numbers 6–70, achieving ≥98 % reacted intensity after five segments for Re > 25, 16× the reaction of a straight channel and 1.6× that of a square‑wave channel at Re = 70, confirming chaotic advection and combined diffusion–stirring mixing.

Abstract

A three-dimensional serpentine microchannel design with a "C shaped" repeating unit is presented in this paper as a means of implementing chaotic advection to passively enhance fluid mixing. The device is fabricated in a silicon wafer using a double-sided KOH wet-etching technique to realize a three-dimensional channel geometry. Experiments using phenolphthalein and sodium hydroxide solutions demonstrate the ability of flow in this channel to mix faster and more uniformly than either pure molecular diffusion or flow in a "square-wave" channel for Reynolds numbers from 6 to 70. The mixing capability of the channel increases with increasing Reynolds number. At least 98% of the maximum intensity of reacted phenolphthalein is observed in the channel after five mixing segments for Reynolds numbers greater than 25. At a Reynolds number of 70, the serpentine channel produces 16 times more reacted phenolphthalein than a straight channel and 1.6 times more than the square-wave channel. Mixing rates in the serpentine channel at the higher Reynolds numbers are consistent with the occurrence of chaotic advection. Visualization of the interface formed in the channel between streams of water and ethyl alcohol indicates that the mixing is due to both diffusion and fluid stirring.