Manufacturing and Test of Split and Recombine Micromixers with a Variable Cross-Section
Authors: Martínez-López J. Israel , Mendoza-Buenrostro Christian , Celis Retana Pavel , Betancourt HA , Vazquez Elisa , Siller Hector R. , Rodriguez Ciro A. 
Published: February 2020

The methodology for manufacturing array of microdevices that features a varying cross-section is introduced towards the development of complex devices that are more prone to be adapted in-field. In this paper stereolithography or digital light, processing is assessed for developing asymmetric split-and-recombine micromixers that can be tested under a machine vision system. To test the proposed manufacturing methodology an array of 6 microdevices with a varying depth from 100 to 1000 micrometers were characterized using an Infinite Focus Measurement to verify the feasibility of the proposed methodology for a large range of length-to-depth ratios. Surface metrology showed that molds were produced within 4 to 10 mm of absolute deviational error.
Split and Recombine (SAR) micromixers devices were manufactured employing microfabrication methods derived from the semiconductor industry to engrave polydimethylsiloxane (PDMS) such as photolithography, or to a lesser extent micromachining or laser ablation of other materials. However, these methodologies have several limitations; they are often expensive, require special infrastructure such as clean rooms and may have long production cycles for new designs. Moreover, devices are manufactured using a soft lithography technology that is limited to a singular cross-section depth. An Envisiontec Perfactory P3 Mini Multilens additive manufacturing equipment that uses a 60 mm lens system and with a work tray of 84 x 63 x 230 mm and with a pixel resolution of up to 50 μm for the used material. The equipment was loaded with a white Flex Series ABS resin characterized by a yield stress of 65MPa and a Young's modulus of 1772 MPa. An Otoflash pulse curing chamber from the same supplier with a wavelength between 300 and 700 nanometers, with a lamp of 11 watts of power and 10 pulses per second.
An array of six assymetric asymmetric SAR micromixers (ASAR) were developed with a width of 1000 µm and a varying depth of 100, 250, 500, 750, 900 and 1000 µm for depth-to-width ratio of 1:10, 1:4, 1:2, 3:4, 9:10 and 1:1 respectively.
For characterization of the molds, an Alicona Infinite Focus Measurement Machine has been employed for surface characterization. This device allows the acquisition of datasets at a high depth of focus similar to a Scanning Electronic Microscope. Infinite Focus Microscopy (IFM) has shown to be capable of capturing images with a lateral resolution down to 400 nm providing 3D datasets with very accurate results. For this work, each mold depth array element was measured twice for a total number of 36 samples.
The molds were used to pour polydimethylsiloxane (PDMS) to produce the micromixing structures.
A syringe pump and two plastic syringes were used to individually supply fluorescent particles and a solution of blue dye and distilled water through the micromixer inlets at same flow rate. A flow rate of 1 ml/min was applied to the microdevice to test the device sealing. Two machine vision setups were employed: an inverted microscope with a ultra high-speed camera and an inverted microfluidic microscope were implemented to evaluate used to assess the performance of a micromixer using the concentration field including parameters such as mean concentration (Cm), standard deviation (σ), coefficient of variation (CoV) or mixing index (M).
Methodology for the production of an array of micromixers with a variable cross-section in a single step was successfully implemented.

Figure 1