Abstract: Fluid Behavior Evaluation for varying Hydrophobicity and RPM Level in Centrifugal Step Emulsification Chips

In the microfluidics field, the manipulation of small fluid volumes, typically water-in-oil, is used to perform precise operations for various biological and chemical applications. Microfluidic chips have been particularly appealing for use in diagnostic testing because the small volumes allow for cell analysis to occur at a fast rate while maintaining high precision. To utilize these advantages in a chip, the droplets formed would have to have a low variation in their diameter, thus aiming for monodispersity. Due to the robustness of centrifugal step emulsification, the sizes of droplets generated are primarily impacted by the hydrophobicity of the chip material and flow rate. While there are a variety of materials and treatments applied in microfluidic chips, the effect of differing levels of hydrophobicity of the chip material on controlling droplet size has not been greatly studied. Here we show that microfluidic chips with a higher level of hydrophobicity centrifuged at a low rpm are most likely to result in reproducible droplet formation. The chips used in this project were developed previously and originally deemed less effective in developing monodisperse droplets since a decrease in contact angle, an indicator for hydrophobicity, was noted between the blank material COP and COP after undergoing chip bonding. Thus, we determined the level of hydrophobicity of the chip’s channel was most likely to affect reproducible droplet generation, with the rpm rate further affecting where in the chip droplet generation begins. Our findings demonstrate how the COP centrifugal step emulsification chips can be manipulated to improve the quality of droplet generation that occurs, even after the chips are fully fabricated. We anticipate our findings can be utilized to support the commercial-scale production of microfluidic chips, as the ideal hydrophobicity level and rpm for controllable aqueous droplet generation would increase the robustness of the device, thus making it more suitable for use in research and patient applications of diagnostic testing. Because centrifugal step emulsification is already considered a robust microfluidic method, further adjustments in the material properties and procedure in running the chip can greatly affect the type of droplets formed. Therefore, determining the parameters that result in easily controllable generation can lead to the commercial use of droplet generation in diagnostic testing applications, thus granting patients greater access to fast and accurate results.