Boiling water one bubble at a time

The first step in boiling water is the formation of bubbles at the bottom of the pan. Those bubbles grow and leave the heated surface within a few milliseconds, which makes it difficult to study their formation in real time. Now Shalabh Maroo of Syracuse University and his colleagues have found a way to make the bubbles stick around longer. They took a container filled with room-temperature water and used a focused laser beam to locally heat a spot at the bottom of the container. A vapor bubble that forms on the spot can be held in place for hours by setting the laser power so that evaporation at the bubble’s base is balanced by condensation at its cooler parts. Thanks to that stability, the researchers could study at leisure how bubbles on a heated surface behave in different situations, including presence or absence of dissolved air, the use of hydrophilic or hydrophobic surface material, and increases in laser power. The figure shows a bubble, viewed from below, on a silicon dioxide surface in degassed water; the set-up is illuminated with light from a halogen lamp (upper) and a helium–neon laser (lower). Interference fringes revealed a thin liquid microlayer at the bubble’s base. In water with dissolved air, the bubbles steadily grew larger even at constant laser power due to the continuous release of air into the bubbles. Maroo and his colleagues measured the air release rate and thus the liquid evaporation rate in the microlayer. The results should help researchers better understand the dynamics of bubble growth during boiling. (A. Zou et al., Sci. Rep. 6, 20240, 2016, doi:10.1038/srep20240 .)