NANOBUBBLES · Investigating surface nanobubbles

Investigating surface nanobubbles

A conceptually simple question to ask is when has a substrate been wetted? Pouring water into a glass beaker leads to much of the glass surface becoming wet, with the occasional micro or macroscopic air bubble trapped on the wall. Zooming into the'wetted'part of the surface with an optical microscope confirms the macroscopic observation: the surface is indeed wet. Zoom in once more, now with techniques that can access the nanoscale, and you may be surprised to find nanoscopic gaseous domains trapped at the interface. The surface has not been wetted after all and the previous coarse-grained observations could not give the full picture. These new nanoscopic gaseous domains come in two basic forms: surface nanobubbles and micropancakes. Surface nanobubbles are spherical cap-shaped, with typical heights and widths of 10–20 nm and 50–100 nm, respectively, whereas micropancakes are several microns in diameter but restricted to only 1–2 nm in height. Furthermore, the menagerie of new phenomena does not end here: nanobubble–micropancake composites have also been discovered, as have multi-layer micropancakes. The objectives of the present project were to address the following questions: 5. Is there an'ideal recipe'for nanobubble creation? 6. Can the size difference between micro-and nano-bubbles be bridged? 7. What is the mechanism that sustains nanobubbles? 8. How does the presence of nanobubbles affect slip? The'nanobubble'field is embedded within the broader topic of'nanoscopic gaseous domains', which, to date, is comprised of surface nanobubbles, micropancakes, and the coexistence of both nanobubbles and micropancakes. Surface nanobubbles are nanoscopic gas bubbles, with pressures that obey Laplace, whilst micropancakes are most probably dense adsorbates of gas molecules that have condensed out of solution. It turns out that to understand surface nanobubbles requires a thorough understanding of micropancakes, i. e., micropancakes and surface nanobubbles are inherently related. In this respect, the first major result of the project was the investigation of micropancake dynamics. Micropancakes are found to grow and rearrange in shape over time scales of several hours. Bearing in mind that micropancakes are adsorbed gas molecules, growth requires more gas to leave solution and adsorb on the wall, leading to an overall dynamic dewetting of the substrate from the liquid. We measured this dynamic dewetting on hydrophobised silicon, demonstrating that the growth was mediated by pinning on the substrate (also explaining why micropancakes are round on atomically flat graphite). Micropancakes are approximately one nanometre high, but extend laterally for microns. The surface coverage is thus perhaps three molecules thick (assuming molecular nitrogen as the gas). We noted several times in our measurements that micropancakes can be purposefully deformed and we then see nanobubbles in their place. Thus, the second major result of the project was to propose a new mechanism for heterogeneous bubble nucleation, i. e., surface nanobubbles form due to the decay (bulk desorption) of micropancakes. This new model for heterogeneous nucleation needed experimental validation, which provided the next major result. We showed that nanobubbles exist in a distinct region of liquid temperature/gas concentration phase space, in between the extensive regions of zero nucleation and micropancake nucleation. Technologically this is very important since it makes it now possible to purposefully generate nanobubbles (for the academic researcher) or, more importantly, guarantee that zero nanobubbles exist (for the industrialist worried about surface imperfections in their process). As well as the dependence of nucleation on temperature, we also investigated the dynamics of nanobubbles due to an in situ temperature change, showing that nanobubble size can be tuned through careful control and manipulation of the temperature. The fact that others had seen the coexistence of both micropancakes and nanobubbles is perhaps surprising, but we currently believe that micropancakes exist as an almost full monolayer on most wetted surfaces, and nanobubbles are just transients on the (long) route to equilibrium as the micropancakes break down. In fact, the current belief is that nanobubbles nucleate on top of the first layer of the micropancake, which explains nicely why nanobubble contact angle is very strongly gas type dependent but not substrate dependent. In this respect, a major (time wise) effort was undertaken to show that nanobubble sizes, population densities, and contact angles, are very dependent on gas type. The current stage of this branch of work is the investigation of micropancake growth and stability, using the experimental technique of ellipsometry. Once the nucleation mechanism of nanobubbles became understood, we moved on to investigate nanobubble stability. There are two bodies of work in the literature which provide possible answers and we were able to provide evidence for one of these. Principally, the gas within the nanobubble is able to leave due to dissolution, but remains local to the nanobubble and is captured by the near-wall potential and driven back into the bubble for replenishment. The experimental validation was to show that a non-zero force-field existed in the immediate vicinity of a nanobubble, which could be interpreted as being due to the streaming flow of liquid from the bubble apex to the three phase line. The next step was to try to utilise nanobubbles in a device. The system we chose was to investigate slip in a microfluidic device due to the presence of nanobubbles on the walls. The point here was to change the no-slip boundary condition to one of slip, thus making it simpler (cheaper) to drive the microfluidic flow. It turned out that this would be an entire project in itself, so, rather than finalising this body of work we provided much needed experimental tests as precursors to the measurement of slip. In particular, we showed that nanobubbles are stable in the presence of near-wall flow, plus their size was flow-speed dependent. The final two contributions were, although not providing results themselves, to summarise the field of nanoscopic gas domains in two invited review articles. Along with these, two other pieces of work were carried out in the general research area but not directly related to nanobubbles. The first of these was to understand whether cavitation could occur between an atomic force microscope probe and a substrate (the geometry and technique used in the principle research), albeit on a macroscopic test system. The second of these was to understand the roles of the various forces which exist between an atomic force microscope probe and a substrate.

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