Nonintrusive Optical Visualization of Surface Nanobubbles

Individual surface nanobubbles are visualized with nonintrusive optical interference-enhanced reflection microscopy, demonstrating that their formation is not a consequence of the hitherto used intrusive atomic force microscopy technique. We then use this new and fast technique to demonstrate that surface nanobubbles form in less than a few seconds after ethanol-water exchange, which is the standard procedure for their preparation, and examine how they react to temperature variations.

Figure 1

Schematic diagram of the optical interference-enhanced reflection microscopy technique. The locally reflected light intensity is the sum of the reflected beams. It is different for surfaces with and without adsorbed transparent objects (e.g., nanobubbles). An appropriate thickness of the ${\mathrm{SiO}}_2$ layer (300 nm thick) and a suitable wavelength significantly enhance the contrast. Because the footprint diameter of the nanobubbles is usually smaller than the wavelength, they also act as scattering objects (corresponding light paths not shown in the figure). Thus, the imaging does not necessarily show the geometry of the nanobubbles, and the nanobubbles typically appear as dark spots, as the scattered light is lost for the imaging.

Figure 2

Comparison between the imaging of surface nanobubbles by (a) interference-enhanced reflection microscopy and (b) AFM. Identically prepared samples were imaged under the same conditions (same surface preparation and same liquid exchange protocol, but the images do not show identical surface areas). Inserts are $10\mu \mathrm{m}\times 10\mu \mathrm{m}$ zooms of the images. The AFM measurements were performed with an Agilent 5100 [tapping mode with $4.5\mathrm{N}/\mathrm{m}$, 10 nm noncontact silicon conical probes (MikroMasch), and dry resonance 150 kHz]. The height range in the AFM images is 40 nm. (c) Distribution of nanobubble footprint diameters measured with both techniques. Optically, only diameters above 300 nm [$\lambda /(2\times \mathrm{numerical}\mathrm{aperture})$] can safely be quantified. Within these limitations, the size distributions are in good agreement.

Figure 3

Interference-enhanced optical microscopy images of a hydrophobic interface wetted by water (super)saturated with ${\mathrm{N}}_2$ after ethanol-water exchange. The bright dots originate from nanobubbles. These act as scattering objects, and in the original imaging they appear as darker spots compared to the reflecting bare surface; for better visibility here, we inverted the contrast. The nanobubbles are only visible after the ethanol-water exchange. Background images without nanobubbles, which were used for background subtraction (for contrast enhancement), are featureless except for surface artifacts (scratches, dust particles). The three frames show the surface after various times or treatments as indicated (see the main text). The images of the top row show roughly the same surface region. The magnified sections show the exact same surface regions for all the three cases. This is possible thanks to the dark or bright “artifact pairs” that originate from persistent surface artifacts and serve as position markers. The different relative orientation or distance of the dark or bright artifact spots results from the different relative positions of the sample in each experiment as well as the background recording. For the description of the specifically numbered nanobubbles (1, 2, and 3), we refer to the main text.