James Richard Thorley Seddon

Knudsen Gas Provides Nanobubble Stability

We provide a model for the remarkable stability of surface nanobubbles to bulk dissolution. The key to the solution is that the gas in a nanobubble is of Knudsen type. This leads to the generation of a bulk liquid flow which effectively forces the diffusive gas to remain local. Our model predicts the presence of a vertical water jet immediately above a nanobubble, with an estimated speed of ∼3.3  m/s, in good agreement with our experimental atomic force microscopy measurement of ∼2.7  m/s. In addition, our model also predicts an upper bound for the size of nanobubbles, which is consistent with the available experimental data.

Surface Bubble Nucleation Stability

Recent research has revealed several different techniques for nanoscopic gas nucleation on submerged surfaces, with findings seemingly in contradiction with each other. In response to this, we have systematically investigated the occurrence of surface nanobubbles on a hydrophobized silicon substrate for various different liquid temperatures and gas concentrations, which we controlled independently. We found that nanobubbles occupy a distinct region of this parameter space, occurring for gas concentrations of approximately 100%-110%. Below the nanobubble region we did not detect any gaseous formations on the substrate, whereas micropancakes (micron wide, nanometer high gaseous domains) were found at higher temperatures and gas concentrations. We moreover find that supersaturation of dissolved gases is not a requirement for nucleation of bubbles.

A deliberation on nanobubbles at surfaces and in bulk

Surface and bulk nanobubbles are two types of nanoscopic gaseous domain that have recently been discovered in interfacial physics. Both are expected to be unstable to dissolution because of the high internal pressure driving diffusion and the surface tension which squeezes the gas out, but there is a rapidly growing body of experimental evidence that demonstrates both bubble types to be stable. However, the two types of bubbles also differ in many respects: surface nanobubble stability is most probably assisted by the nearby wall, which can repel the water (in the case of hydrophobicity), accept physisorbed gas molecules, and reduce the surface area through which outfluxing can occur; bulk nanobubbles, on the other hand, must stabilise themselves. This is perhaps through ionic shielding, perhaps through diffusive shielding, or perhaps through both. Herein, the features of both bubble types are described individually, their common and disparate features are discussed, and emerging applications are examined.

Diffusive Shielding Stabilizes Bulk Nanobubble Clusters

Using molecular dynamics, we study the nucleation and stability of bulk nanobubble clusters. We study the formation, growth, and final size of bulk nanobubbles. We find that, as long as the bubble-bubble interspacing is small enough, bulk nanobubbles are stable against dissolution. Simple diffusion calculations provide an excellent match with the simulation results, giving insight into the reason for the stability: nanobubbles in a cluster of bulk nanobubbles protect each other from diffusion by a shielding effect.

Surface nanobubbles as a function of gas type.

We experimentally investigate the nucleation of surface nanobubbles on PFDTS-coated silicon as a function of the specific gas dissolved in water. In each case, we restrict ourselves to equilibrium conditions (c = 100%, T(liquid) = T(substrate)). Not only is nanobubble nucleation a strong function of gas type, but there also exists an optimal system temperature of ∼35 -40 °C where nucleation is maximized, which is weakly dependent on gas type. We also find that the contact angle is a function of the nanobubble radius of curvature for all gas types investigated. Fitting this data allows us to describe a line tension that is dependent on the type of gas, indicating that the nanobubbles sit on top of adsorbed gas molecules. The average line tension was τ ≈ -0.8 nN.

Localized removal of layers of metal, polymer, or biomaterial by ultrasound cavitation bubbles

We present an ultrasonic device with the ability to locally remove deposited layers from a glass slide in a controlled and rapid manner. The cleaning takes place as the result of cavitating bubbles near the deposited layers and not due to acoustic streaming. The bubbles are ejected from air-filled cavities micromachined in a silicon surface, which, when vibrated ultrasonically at a frequency of 200 kHz, generate a stream of bubbles that travel to the layer deposited on an opposing glass slide. Depending on the pressure amplitude, the bubble clouds ejected from the micropits attain different shapes as a result of complex bubble interaction forces, leading to distinct shapes of the cleaned areas. We have determined the removal rates for several inorganic and organic materials and obtained an improved efficiency in cleaning when compared to conventional cleaning equipment. We also provide values of the force the bubbles are able to exert on an atomic force microscope tip.

Dynamic Dewetting through Micropancake Growth

We experimentally investigate the dynamics of nanometer-high, micrometer-wide gassy layers at the interface between a hydrophobic solid and bulk water. These micropancakes grow laterally in time, on the timescale of an hour, leading to partial dewetting of the solid. The growth is directional, mediated by chemical roughness on the substrate, and transient, occurring within the first hour after liquid deposition. We use circularity to measure the roundness of a micropancake (circularity C = 2(piA)(1/2)/L, where A is the surface area and L is the perimeter). The growth is anisotropic, as demonstrated by a decrease in circularity with time. However, once a micropancake reaches size saturation, its bulk rearranges its shape in order to minimize the length of its three-phase line. We interpret this combination of growth followed by bulk rearrangement as dynamic dewetting.

Temperature Dependence of Surface Nanobubbles

The temperature dependence of nanobubbles was investigated experimentally using atomic force microscopy. By scanning the same area of the surface at temperatures from 51 °C to 25 °C it was possible to track geometrical changes of individual nanobubbles as the temperature was decreased. Interestingly, nanobubbles of the same size react differently to this temperature change; some grow whilst others shrink. This effect cannot be attributed to Ostwald ripening, since the growth and shrinkage of nanobubbles appears to occur in distinct patches on the substrate. The total nanobubble volume per unit area shows a maximum around 33 °C, which is comparable with literature where experiments were carried out with increasing temperature. This underlines the stability of surface nanobubbles.

Directional Liquid Spreading over Chemically Defined Radial Wettability Gradients

We investigate the motion of liquid droplets on chemically defined radial wettability gradients. The patterns consist of hydrophobic fluorinated self-assembled monolayers (SAMs) on oxidized silicon substrates. The design comprises a central hydrophobic circle of unpatterned SAMs surrounded by annular regions of radially oriented stripes of alternating wettability, i.e., hydrophilic and hydrophobic. Variation in the relative width of the stripes allows control over the macroscopic wettability. When a droplet is deposited in the middle, it will start to move over to the radially defined wettability gradient, away from the center because of the increasing relative surface area of hydrophilic matter for larger radii in the pattern. The focus of this article is on a qualitative description of the characteristic motion on such types of anisotropic patterns. The influence of design parameters such as pattern dimensions, steepness of the gradient, and connection between different areas on the behavior of the liquid are analyzed and discussed in terms of advancing and receding contact lines, contact angles, spatial extent, and overall velocity of the motion.

Particle tracking around surface nanobubbles

The exceptionally long lifetime of surface nanobubbles remains one of the biggest questions in the field. One of the proposed mechanisms for producing the stability is the dynamic equilibrium model, which describes a constant flux of gas in and out of the bubble. Here, we describe results from particle tracking experiments carried out to measure this flow. The results are analysed by measuring the Voronoï cell size distribution, the diffusion, and the speed of the tracer particles. We show that there is no detectable difference in the movement of particles above nanobubble-laden surfaces as compared to ones above nanobubble-free surfaces.