Large bubbles emerge from porous membranes with small pores because the emerging bubbles spread on the membrane surface before they detache from the surface. If the sessile bubble contact angle (theta) is less than 45degrees, no spreading occurs, and the bubble size is determined by the size of hole. Even in this case, the equivalent diameter (of spherical bubble) is significantly larger than the pore diameter due to the bubble formation mechanism. If theta > 45degrees, the spreading of the emerging bubble on the surface occurs, and the bubble size is determined by the size of base of a sessile bubble, which depends on the value of theta, and is independent of the pore size. The equivalent diameter of a bubble could be greater in order of magnitude than the pore diameter. The same principle applies to the bubble formation from an inclined surface; however, the bubble formation is more complicated because of drifting of a developing bubble out of the orifice and the merging of sliding bubbles. In order to create small bubbles, it is necessary to create small orifices on a hydrophilic surface, of which theta < 45&DEG;, and two orifices should be separated beyond the maximum diameter of emerging bubble. A horizontal flat surface is the best for the creation of small-sized bubbles. (C) 2003 Wiley Periodicals, Inc.