Flat nanobubbles elicit even more questions
Few secrets remain in the physics of air bubbles at the macroscale. Smaller bubbles at the nanometre scale, however, behave in a completely different manner because they are flat, for example. FOM PhD researcher Joost Weijs investigated the physical processes that underlie this at the University of Twente. Although the research has answered the question why nanobubbles are flat it has also elicited new questions. The results were published at the end of last week in the prestigious journal Physical Review Letters.
Surface nanobubbles are one of the major unsolved problems in fluid physics. These air bubbles are several tens of nanometres high and have been found with the aid of atomic force microscopy (AFM) on various materials in contact with water. Initially, other than being far smaller, these nanobubbles look just like 'normal' (macroscopic) bubbles on a surface in contact with liquids. Yet the nanobubbles behave in a fundamentally different manner from their macroscopic counterparts. For example, the pressure of the gas inside the bubbles is extremely high due to the strong curvature of the gas-liquid surface (several factors of ten higher than atmospheric pressure) and so the gas should dissolve in the liquid within a few microseconds. However, this does not happen and, stronger still, experimental observations have shown that nanobubbles can survive for several days.
The material properties of the gas, liquid and the substrate determine the angle of contact in normal bubbles and so the angle of contact is independent of the bubble size. Nevertheless nanobubbles are always far flatter than macroscopic bubbles.
The research
With the aid of molecular dynamics (MD) simulations, the researchers simulated the movements of independent atoms. This enabled them to make a detailed examination of what happens if these form nanobubbles on a surface. One finding is that nanobubbles form spontaneously when the gas concentration in the liquid is far higher than the saturation concentration. As a result of this oversaturation a bubble forms, which sometimes moves towards the substrate where it attaches itself.
Further the researchers saw that a gas adsorbent arises between the substrate and the liquid. As a result of this the bubbles became visibly flatter. This can also be explained quantitatively by the changing interaction between the bubble and the wall, which answers the question why nanobubbles are always flatter than macroscopic bubbles.
The simulated nanobubbles are not as stable as the real nanobubbles: they dissolve within the expected short period of time (microseconds). Although the physical mechanism underlying the nanobubbles could not be found in these simulations it was nevertheless clear that particles move through the bubble surface in a non-trivial manner. In some places the gas flows out of the bubble (as expected) but in the vicinity of the substrate a clear influx of gas can be measured although this is too small to stabilise the bubble. Further research is needed to demonstrate whether this influx can stabilise the bubble under specific conditions.