Exploring the phase explosion of water using SOM-mediated micro-bubbles

Abstract

The phase explosion of water at a solid–water interface has been typically observed experimentally with the use of pulsed lasers inducing cavitation bubbles near an absorbing surface. Here we show that a tightly focused CW laser beam can be used to achieve phase explosion in a microscopic domain inside a thin film of water in contact with a specially created soft-oxometalate (SOM) coated glass surface. The laser beam induces a homogeneously nucleated micro-bubble at the water–glass (SOM-coated) interface due to high absorptivity of the SOMs at the laser wavelength, and the very high light intensity at the laser spot (∼10s' of MW cm⁻²). Increasing the laser power creates an interesting variation in the size of the bubble formed due to convective effects, until a certain power level is reached at which the bubble size increases very drastically. We demonstrate using a simulation based on a solution of the heat equation at the glass substrate–water interface, and by experimental consistency checks, that the size of the bubble essentially traces phase explosion in superheated water. The sudden increase in the bubble size occurs when we approach the critical point of water beyond which it cannot exist as a liquid. The size variation of the bubble at lower laser powers also serve as a probe to the microscopic flows of water around the bubble, and could help modulate the size of SOM microbubbles in the context of controlled lithography.