The filtration performance of filter media fabricated by foam coating technology is highly dependent upon their pore sizes. Therefore, we investigated the effect of the mixing ratio of combined coating components (PTFE emulsion, a foam stabilizer, a foaming agent, and a thickener) and operating parameters (feed rate of coating solution, air flow rate, mixing speed, and cooling temperature) on the bubble size distribution of generated foams in a foam generator. The mean bubble size and bubble size distributions were observed through microscopic observations with an optical microscope. We found that viscosity was the most critical parameter in preparing a coating solution, and the entire series of operating parameters as mentioned above influenced the bubble size in a foamed solution. The bubble size decreased with an increasing ratio of thickening agent up to 1.8%, mixing speed, and with decreasing solution feed rate, and cooling temperature. Furthermore, we had a minimum bubble size for the condition of air flow rate at 200 mL/min. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2062-2067, 2013