A study of bubble wetting on surfaces

Abstract

In microfluidics, the formation of bubbles within devices obstructs flow and can damage the microfluidic chip or the samples contained therein. This thesis works toward a better understand of bubble wetting on surfaces, so that future microfluidics devices can be designed to be more robust and free of bubbles. Current wetting theory as applied to bubbles is examined, and two key areas for improvement are identified: disjoining pressure effects and gravitationaleffects. Wetting of textured surfaces is also analyzed for bubble application, leading to a prediction that a model based on a Cassie-Baxter analysis with knowledge of bubble wetting on a flat surface would be most accurate compared to other models. Dynamic and sessile bubble contact angles and droplet contact angles were measured on smooth acrylic, fluorosilanized silicon, glass, nylon, and silicon. These results were compared to the existing model, and the resulting error showed a strong correlation with a Pearson's correlation coefficient of 0.863 to the magnitude of the bubble contact angle hysteresis. Because contact angle hysteresis can be related to the disjoining pressure, these results were a good indicator that disjoining pressure should be considered in developing improved bubble wetting models. Dynamic and sessile bubble contact angles and droplet contact angles were also measured on four silicon samples with different surface textures. These results were compared to three existing wetting models as applied to bubble wetting, and it was found that the Cassie-Baxter model based on the bubble contact angle on a smooth silicon surface was most accurate, with an average percentage error of 0.8%. Finally, recommendations for further research to support developing models of bubble wetting are made.