Experimental Methods

2.1 Bubble column

The bubble column was made from cast acrylic to achieve strength and optical clarity; it was 1.2m in length with a 102mm internal diameter (D ). Figure 1 provides a schematic of the bubble column test facility. Tap water was passed through a cartridge filter (W10-BC, American Plumber, Pentair Residential Filtration, LCC) with 5μm nominal filtration. Surface tension of the filtered water and other tested liquids were measured with a force tesiometer (K6, Krüss GmbH) and platinum ring (RI0111-282438, Krüss GmbH). Over several days the surface tension of the filtered water was measured to be 72.6 ±0.4 mN/m, which is comparable to the nominal surface tension of the pure water (~72.8 mN/m). Liquid phase temperature was measured using a thermocouple (HSTC-TT-K-20S-120-SMPW-CC, Omega Engineering). Figure 1 also depicts the compressed airflow control panel. Airflow passes through a cartridge filter (SGY-AIR9JH, Kobalt, Lowe’s Companies, Inc.) with 5μm nominal filtration. The mass flow of air was controlled and monitored with a combination of a pressure regulator, rotameter (EW-32461-50, Cole-Palmer), and a thermocouple (5SC-TT-K-40-39, Omega Engineering). The rotameter measured the volumetric flow of air with an accuracy of 2% of the full scale (FS). The thermocouple measured the air temperature immediately upstream of the rotameter with accuracy of ±0.1°C. All tests were conducted with the air temperature between 20 °C and 22 °C, and temperature difference between the air and liquid phase was within ±2 °C. It is also worth mentioning that the height of liquid in the bubble column was kept constant at 9D following recommendations from Besagni et al.41 for studying the void fraction and bubble size independent of column aspect ratio.