B.M. Borkent

On the Shape of Surface Nanobubbles

Previous AFM experiments on surface nanobubbles have suggested an anomalously large contact angle theta of the bubbles (typically approximately 160 degrees measured through the water) and a possible size dependence theta(R). Here we determine theta(R) for nanobubbles on smooth, highly oriented pyrolytic graphite (HOPG) with a variety of different cantilevers. It is found that theta(R) is constant within experimental error, down to bubbles as small as R = 20 nm, and is equal to 119 +/- 4 degrees . This result, which is the lowest contact angle for surface nanobubbles found so far, is very reproducible and independent of the cantilever type used, provided that the cantilever is clean and the HOPG surface is smooth. In contrast, we find that, for a particular set of cantilevers, the surface can become relatively rough because of precipitated matter from the cantilever onto the substrate, in which case larger nanoscopic contact angles ( approximately 150 degrees ) show up. In addition, we address the issue of the set-point dependence. Once the set-point ratio is below roughly 95%, the obtained nanobubble shape changes and depends on both nanobubble size and cantilever properties (spring constant, material, and shape).

Nucleation threshold and deactivation mechanisms of nanoscopic cavitation nuclei

The acoustic nucleation threshold for bubbles trapped in cavities has theoretically been predicted within the crevice theory by Atchley and Prosperetti [“The crevice model of bubble nucleation,” J. Acoust. Soc. Am. 86, 1065 (1989)]. Here, we determine this threshold experimentally, by applying a single pressure pulse to bubbles trapped in cylindrical nanoscopic pits (“artificial crevices”) with radii down to 50 nm. By decreasing the minimum pressure stepwise, we observe the threshold for which the bubbles start to nucleate. The experimental results are quantitatively in good agreement with the theoretical predictions of Atchley and Prosperetti. In addition, we provide the mechanism which explains the deactivation of cavitation nuclei: gas diffusion together with an aspherical bubble collapse. Finally, we present superhydrophobic nuclei which cannot be deactivated, unless with a high-speed liquid jet directed into the pit.

Cassie-Baxter to Wenzel state wetting transition: scaling of the front velocity.

We experimentally study the dynamics of water in the Cassie-Baxter state to Wenzel state transition on surfaces decorated with assemblies of micrometer-size square pillars arranged on a square lattice. The transition on the micro-patterned superhydrophobic polymer surfaces is followed with a high-speed camera. Detailed analysis of the movement of the liquid during this transition reveals the wetting front velocity dependence on the geometry and material properties. We show that a decrease in gap size as well as an increase in pillar height and intrinsic material hydrophobicity result in a lower front velocity. Scaling arguments based on balancing surface forces and viscous dissipation allow us to derive a relation with which we can rescale all experimentally measured front velocities, obtained for various pattern geometries and materials, on one single curve.

Multiple time scale dynamics in the breakdown of superhydrophobicity

Drops deposited on rough and hydrophobic surfaces can stay suspended with gas pockets underneath the liquid, then showing very low hydrodynamic resistance. When this superhydrophobic state breaks down, the subsequent wetting process can show different dynamical properties. A suitable choice of the geometry can make the wetting front propagate in a stepwise manner leading to square-shaped wetted area: the front propagation is slow and the patterned surface fills by rows through a zipping mechanism. The multiple time scale scenario of this wetting process is experimentally characterized and compared to numerical simulations.Drops deposited on rough and hydrophobic surfaces can stay suspended with gas pockets underneath the liquid, then showing very low hydrodynamic resistance. When this superhydrophobic state breaks down, the subsequent wetting process can show different dynamical properties. A suitable choice of the geometry can make the wetting front propagate in a stepwise manner leading to {\it square-shaped} wetted area: the front propagation is slow and the patterned surface fills by rows through a {\it zipping} mechanism. The multiple time scale scenario of this wetting process is experimentally characterized and compared to numerical simulations.

Reproducible cavitation activity in water-particle suspensions

The study of cavitation inception in liquids rarely yields reproducible data, unless special control is taken on the cleanliness of the experimental environment. In this paper, an experimental technique is demonstrated which allows repeatable measurements of cavitation activity in liquid-particle suspensions. In addition, the method is noninvasive: cavitation bubbles are generated using a shock-wave generator, and they are photographed using a digital camera. The cavitation activity is obtained after suitable image processing steps. From these measurements, the importance of the particles surface structure and its chemical composition is revealed, with polystyrene and polyamide particles generating the highest yields. Further findings are that cavitation nuclei become depleted with an increasing number of experiments, and the existence of nuclei with varying negative pressure thresholds. Finally, a decrease of the cavitation yield is achieved by prepressurization of the suspension-indicating that the cavitation nuclei are gaseous.

The acceleration of solid particles subjected to cavitation nucleation

The cavity–particle dynamics at cavitation inception on the surface of spherical particles suspended in water and exposed to a strong tensile stress wave is experimentally studied with high-speed photography. Particles, which serve as nucleation sites for cavitation bubbles, are set into a fast translatory motion during the explosive growth of the cavity. They reach velocities of ~40 ms−1 and even higher. When the volume growth of the cavity slows down, the particle detaches from the cavity through a process of neck-breaking, and the particle is shot away. The experimental observations are simulated with (i) a spherical cavity model and (ii) with an axisymmetric boundary element method (BEM). The input for both models is a pressure pulse, which is obtained from the observed radial cavity dynamics during an individual experiment. The model then allows us to calculate the resulting particle trajectory. The cavity shapes obtained from the BEM calculations compare well with the photographs until neck formation occurs. In several cases we observed inception at two or more locations on a single particle. Moreover, after collapse of the primary cavity, a second inception was often observed. Finally, an example is presented to demonstrate the potential application of the cavity–particle system as a particle cannon, e.g. in the context of drug delivery into tissue.

Preferred sizes and ordering in surface nanobubble populations

Two types of homogeneous surface nanobubble populations, created by different means, are analyzed statistically on both their sizes and spatial positions. In the first type (created by droplet deposition, case A) the bubble size R is found to be distributed according to a generalized gamma law with a preferred radius R*=20 nm. The radial distribution function shows a preferred spacing at approximately 5.5R*. These characteristics do not show up in comparable Monte Carlo simulated configurations of random packings of hard disks with the same size distribution and the same density, suggesting a structuring effect in the nanobubble formation process. The nanobubble size distribution of the second population type (created by ethanol-water exchange, case B) is a mixture of two clearly separated distributions, hence, with two preferred radii. The local ordering is less significant, due to the looser packing of the nanobubbles.

Is there gas entrapped on submerged silicon wafers? Visualizing nano-scale bubbles with cavitation

Introduction With the ongoing downscaling of fluidic devices to the microand nanoscale in the semiconductor industry, cleaning on these scales becomes increasingly difficult. Shear flow along such microand nanostructures is a natural option for cleaning. However, this necessitates a thorough understanding of the flow boundary conditions. These may be affected by submicron bubbles on the solid/liquid interfaces. The existence of nanometer sized bubbles (“nanobubbles”) present at submerged solid surfaces has been revealed by a number of Atomic Force Microscopy (AFM) experiments [1-7]. Although the exact nature of these bubbles remains still unclear, they have been found on various flat hydrophobic surfaces (contact angle > 90) submerged in water, like silanized silicon-oxide [1,2], polystyrene [3] and gold [4]. Contra-intuitively also smooth and hydrophilic surfaces (contact angle < 90) like siliconoxide [5], mica [6] and HOPG [7] were found to be covered with nanobubbles, however only after in-situ replacement of ethanol by water. The effect of surface attached nanometer sized bubbles has not been studied in greater detail in the framework of ultraor megasonic cleaning yields.

Surface-Bubbles in Micro- and Nanofluidics

In this Proceeding we review and summarize recent findings of the Twente Physics of Fluids group on controlled cavitation of surface microbubbles [1-4] and surface nanobubbles [5] and in addition give some overview on the Twente findings on surface nanobubbles [6-9]. - Gas accumulation at liquid-solid interfaces can occur in the form of bubbles, for instance at a surface defect. The resulting micro- and nanoscale air pockets can act as nucleation sites in shockwave-induced cavitation experiments, leading to the random nature of cavitation, both in space and time. However, by controlling the size and the position of the defects, Bremond et al. [2; 3] succeeded to make cavitation perfectly reproducible both in time and space. This technique made it possible to study the dynamics of individual bubbles, bubble pairs, and bubble clusters emerging from the defects in an extremely reproducible fashion. In addition, by further minaturizing the pits down to diameters of 100nm, Borkent et al. [4] could measure the pressure at which these bubbles start to nucleate, giving results in perfect agreement with the theory by Atchley and Prosperetti [10]. In contrast, so-called surface nanobubbles, though comparible in size and volume to the bubbles in the nanopits, do not act as nucleation sites in cavitation experiments [5]. Finally, we report on some typical properties of surface nanobubbles [6] and report that they can be created in a reproducible and controllable way by electrolysis [8]. That suggests that these surface nanobubbles are in a dynamic equilibrium, i.e., gas in- and outflux are balanced. Such a dynamic stabilization has also been suggested for standard surface nanobubbles [11].

Surface cavitation on micro‐ and nanometer scales

Heterogeneous bubble nucleation at surfaces has been notorious because of its irreproducibility. Here controlled multibubble surface cavitation is achieved by heterogenous nucleation of bubbles on a hydrophobic surface patterned with microcavities. The expansion of the nuclei in the microcavities is triggered by an impulsive lowering of the liquid pressure. The procedure allows to control and fix the bubble‐distance within the bubble cluster. We observe a perfect quantitative reproducibility of the cavitation events where the inner bubbles in the two‐dimensional cluster are shielded by the outer ones, reflected by their later expansion and their delayed collapse. Apart from the final bubble collapse phase (when jetting flows directed towards the clusters center develop), the bubble dynamics can be quantitatively described by an extended Rayleigh‐Plesset equation, taking pressure modification through the surrounding bubbles into account. When repeating the same experiments with flat polyamide and hydropho...