  In this study, we investigated the formation of air-rich microbubbles through
local photothermal heating of non-degassed water. When non-degassed water is
locally heated, vapor-rich bubbles are initially formed. These bubbles have a
maximum radius of approximately 9 {\mu}m and stabilize while oscillating and
exhaling air-rich bubbles. However, when the exhaled bubbles fuse and grow,
they revert to vapor-rich bubbles on the heat source. The vapor-rich bubbles
are then exposed to a large amount of air, causing them to transition to
air-rich bubbles. In this paper, the motion of the exhaled air-rich bubble is
explained by the drag force owing to the flow and the Marangoni force owing to
the temperature gradient acting on the bubble. Because the drag force is
proportional to the bubble radius, and the Marangoni force is proportional to
the square of the bubble radius, the larger the bubble, the stronger the effect
of the Marangoni force. Thus, as the bubbles grow, they are drawn toward the
heat source against the flow created by the vapor-rich bubbles. These results
are useful for a better understanding of bubble growth and determining the
conditions for the stable formation of vapor-rich bubbles in non-degassed
water.
