(418b) The Effect of Step-Down Pressure and Wettability on Pressure-Driven Bubble Nucleation for Dissolved Gas Separation

Dissolved gas separation from supersaturated liquid solutions takes place in a wide range of natural and industrial processes. The liberation of dissolved gas from a liquid occurs either by diffusion through the gas-liquid interface and/or by bubble nucleation at the solid-liquid interface. This research aims at studying the effect of step-down pressure (or supersaturation level) and solid-surface wetting on bubble nucleation of, sparingly soluble gases, methane and nitrogen in water. Surface chemical treatments on glass slides and vials were performed using chlorinated polydimethylsiloxane (CM), chlorinated fluoroalkylmethylsiloxane (CF), and (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethoxysilane (HT) to obtain surfaces of different degrees of wettabilities. Contact angle and atomic force microscopy (AFM) measurements were conducted for the treated and untreated glass substrates to study wettability, and surface roughness characteristics, respectively. The average air-water contact angle of untreated glass sample was 33.91° ± 0.4°, whereas the corresponding contact angles for CM, CF and HT samples were 90.8° ± 0.8°, 97.8° ± 0.4° and 114.6° ± 0.75°, respectively. The average roughness (R_a) values of the untreated and treated surfaces were in the range of 1.1 – 7.4 nm. Bubble nucleation experiments were carried out with the saturated liquids in the hydrophilic (untreated) and hydrophobic (treated) glass vials. 5 mm height of deionized water, in a 12 mm diameter and 25 mm height glass vial placed in a pressure cell, was saturated with methane or nitrogen gas at 6000 mbar for 24 hours. The saturation times of the gases in water were estimated using a semi-infinite diffusion model for planar geometry. To initiate the pressure-driven bubble nucleation process, 100 or 500 mbar step-down pressure was applied to the pressure cell until the bubble nucleation was initiated. A digital microscope was used to observe the bubble nucleation process. All the experiments were performed at room temperature (~22 °C). No gas bubble nucleation was observed in the hydrophilic vials even when the pressure was reduced to atmospheric pressure with either of the step-down pressures. Surprisingly, with 100 mbar step-down pressure neither methane nor nitrogen bubble nucleation occurred in the CM, CF and HT hydrophobic vials. However, both methane and nitrogen gas bubble nucleation occurred in the hydrophobic vials when the step-down pressure was increased to 500 mbar. For the 500 mbar step-down pressure, average onset pressures for methane bubble nucleation on CM, CF and HT surfaces were found to be 4167 ± 289, 4250 ± 500 and 4833 ± 289 mbar, respectively. Similarly, the average onset pressures for nitrogen bubble nucleation on CM, CF and HT surfaces were 2750 ± 866, 4167 ± 289 and 4833 ± 289 mbar, respectively. The above experimental findings revealed the importance of step-down pressure and surface wetting nature on sparingly soluble gas bubble nucleation.