2025 AIChE Annual Meeting

(648e) Intra-Slug Mixing Enhancement in Milli-Fluidic Liquid-Liquid Flow Systems Via Contact Angle Modification

Authors

Robert Tilton, Carnegie Mellon University
Enrique Lopez-Guajardo, Tecnologico de Monterrey/Carnegie Mellon University
Liquid-liquid slug flow has been widely used for carrying out different continuous chemical and biological processes under a laminar flow regime. Specifically, this type of flow pattern is used to improve mass transport to a reacting interface due to the formation of a pair of concentric (axial) vortices, also known as Taylor flow. The formation of the recirculating zones dampens the limiting effect of axial dispersion and decreases the radial diffusion of the components. However, the intra-vortex mixing within the slug is limited by the formation of stagnant zones that are governed by radial diffusion. Therefore, this work explores a passive technique to further improve the intra-slug mixing in milli-fluidic systems through a modification of the contact angle without a geometrical modification.

A series of computational fluid dynamic (CFD) simulations were performed to assess the effect of contact angle modification (CAM) in intra-slug mixing. The simulations consisted of a Lagrangian specification of field of a slug moving through a 2 mm channel that experiences a stepwise modification of the contact angle at a frequency of 1.3 s-1. The shape of the interface will adjust accordingly to the surface wettability (hydrophobic to hydrophilic and vice versa). A tracer with a concentration of 1 mol/m3 was introduced to the front half of the slug relative to the direction of motion, whereas the other half contained the same liquid phase without the tracer. The performance of the system was estimated by defining the intra slug-mixing index as Eq. 1.

Imix = 1 - (αt / α0), (1)

where αt and α0 are the standard deviations of the concentration profiles at a time t and time 0 s, respectively. A Imix = 0 indicates complete segregation (no mixing), while a Imix = 1 indicates a fully mixed system. The Imix of the CAM simulations were compared to a system without a modification in the contact angle (WCAM). Both simulations consider a slug with a fixed length and a constant flow rate (1 mL/min).

Results suggest that the strategy of CAM could improve the Imix due to the disruption of the constants stagnant zones (circulation patterns) inherent to Taylor flow within the slug (as seen in the figure attached) which would otherwise remain undisturbed. This disruption was observed as a result of a change in the slug interface shape that locally accelerates (transition from convex to concave interface) and decelerates (transition from concave to convex interface) the solute within the slug. As a result, by implementing CAM a 54% increase in Imix is obtained at 1.3 s, while a 46% improvement takes place at 3.7 s.

This passive technique effectively reduces concentration gradients (attached figure, t = 3.7 s) while comparing CAM and WCAM systems. Similar results could be achieved by the geometrical modification of the channels that alter the circulation patterns within the slug. However, this strategy could lead to a better distribution of the solute within the slug without requiring any geometrical changes to the system and without advanced manufacturing techniques for lower volumes and flow rates. CAM offers a simple yet effective way to enhance continuous chemical and biological processes.