Expansion of a paraffin oil droplet in a gelatin gel was explained by an equation given by Gent and Tompkins which takes into account the effects of internal pressure, elasticity, and surface tension. The equation suggests that large bubbles monotonously expand as the internal pressure increases but small bubbles have to surpass maxima in the internal pressure for expansion. It was shown that there was a characteristic value in the initial bubble radius, r'_c, above which the bubbles expand monotonously as the internal pressure increased, and it was given as r'_c = 3σ/E (σ: surface tension, E: elasticity). The calculated value of r'_c, agreed with the minimum of radii of bubbles reported in the preceding paper, which expanded even at a constant temperature, absorbing the air released from other shrinking small bubbles. However, for the critical radius of bubble expansion at an elevated temperature, further study is needed.

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