(267e) Rapid Mixing in Microfluidic Devices with Induced-Charge Electro-Osmosis for Improved Control over Precipitation Reactions

Precipitation involves the rapid formation of particles, which is frequently used in various chemical industries. Product quality attributes such as the particle size distribution often need to be controlled tightly to optimize downstream processes and to attain a desired final product functionality. However, control of product quality is often difficult to achieve in precipitation processes due to the entangled kinetics of mixing, nucleation, growth and aggregation at usually high supersaturation levels[1], [2]. The particles may nucleate and grow before complete mixing is achieved, which can lead to a non-uniform product quality that is difficult to predict. Thus, achieving rapid mixing in the order of the induction time for nucleation of the particles is needed to create uniform and predictable conditions for precipitation[3]. Micromixers are attractive for precipitation processes as they offer better reproducibility and controlled reaction environments compared to conventional stirred tank reactors[4]. Microfluidic devices usually need to be integrated with either passive or active mixing techniques to avoid relying on slow diffusive mixing only in the laminar flow regime. Passive mixers exploit favorable flow conditions created by specific structural designs, whereas active mixers employ external energy fields such as magnetic, electric, or pressure from, for example, ultrasound to manipulate mixing during operation[5].

Electrokinetically driven active micromixers utilizing induced-charge electro-osmosis (ICEO) have gained attention due to their ability to provide rapid and uniform mixing at relatively low AC voltages[6]. The working principle is based on the application of an electric field that drives double layer formation and a non-linear fluid flow near a polarizable conducting surface by creating microvortices[7]. Apart from mixing applications, ICEO has been utilized for fluid pumping, particle sorting and separation process in microfluidic devices[8]–[10]. Moreover, other electric-field-assisted mixing methods such as AC-electrothermal[11] and electrohydrodynamic flows[12] have been explored for mixing of multiple fluid streams in nanoparticle synthesis and liposome synthesis, respectively. Among these electric-field-assisted mixing techniques, ICEO is an attractive choice, as it can provide efficient mixing without requiring any electrical conductivity gradients or high voltage amplitudes[13], [14]. However, use of ICEO for improved control over precipitation processes has not been reported yet.

The objective of this work is to design and characterize a microfluidic device to obtain improved control over precipitation processes through rapid mixing based on ICEO-driven flows. A triple-electrode design was used to enhance mixing through the formation of asymmetric microvortices. The design was analogous to a field effect transistor with a floating gate electrode in the center[15]. CFD simulations were used to scale up the device so that clogging during the continuous precipitation processes was avoided while maintaining good mixing performance. The CFD simulations also revealed that the mixing length required to achieve complete mixing was drastically reduced in the presence of ICEO. Next, the mixing performance of the device was experimentally characterized for different fluid flow rates and electrical field parameters such as fluid conductivity, field frequency, voltage amplitude, and the phase difference between the driving electrodes for the precipitation of silver chloride that was adopted as a model system. The ICEO-based micromixer provided best mixing for fluids with low electrical conductivity when antiphase voltage was applied to the driving electrodes at an intermediate AC-frequency. Furthermore, the supersaturation profiles, mixing lengths and mixing patterns inside the microfluidic device were determined numerically at the optimal mixing conditions, which were compared with the mixing conditions without ICEO. The particle size distribution of the silver chloride nanoparticles was characterized at different mixing conditions, flow rates, and supersaturation ratios. The silver chloride nanoparticles produced with ICEO-driven mixing were smaller and exhibited a narrower size distribution compared to the corresponding case without ICEO under all the tested conditions. The ability to make small particles of uniform size demonstrates the good potential of the mixer for precipitation processes. Finally, the feasibility of the concept for application to a cocrystallization system was demonstrated for the carbamazepine-succinic acid cocrystal model system. Cocrystallization systems are an attractive class of applications due to their low conductivities. The yield of the cocrystals was higher when using ICEO, which was attributed to the improved mixing.

Acknowledgement: The work described in this abstract was supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, People’s Republic of China (Project No. 16214418).

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