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Recently, there has been much interest in the presence of "nanobubbles" on solid surfaces submerged in liquids. Nanobubbles have been imaged by scanning probe microscopy, and the effects of nanobubbles have also been seen in the surface forces apparatus, which measures small surface forces. The presence of nanobubbles can also be detected by Quartz Crystal Microbalances (QCM) submerged in liquids. QCM’s are high quality factor oscillators, and the QCM's used in this study oscillate transversely at frequencies of 5 MHz. When submerged in a liquid, a QCM will set up a transverse shear wave in the liquid. This shear wave is heavily damped, with decay lengths for water of approximately 250 nm. The damping of the oscillation by the fluid causes the resonance frequency of the QCM to shift, and a viscosity can be calculated from the frequency shift. As the shear wave generated is so short, this is a near-surface viscosity as opposed to a bulk viscosity. When bubbles cling to the QCM face, the viscosity of the liquid as measured by the QCM will be reduced from the true bulk viscosity of the liquid. We show that the size of the reduction is dictated by the total coverage of the QCM face by the bubbles, the location of the bubbles on the QCM face, and size of the bubbles as compared to the decay length of the shear wave. In this study, we have submerged QCM's with both hydrophilic (gold or silicon dioxide) and hydrophobic (octadecanethiol) surfaces in water distilled to varying degrees, including seltzer water, sonicated DI water, and vacuum distilled DI water. The seltzer water has visible, macroscale bubbles, and so we were able to correlate video acquired of the QCM submerged in the seltzer water with the viscosity data to directly link the presence or absence of a bubble to a change in the measured viscosity. For the sonicated DI water and the vacuum distilled DI water, we made simultaneous comparative measurements of the viscosity using two QCM's, one with a hydrophilic and one with a hydrophobic surface. For the vacuum distilled DI water, we acquired measurements over a period of two weeks post-distillation, as surrounding air diffused back into the distilled water, to observe any changes in the measured viscosity.
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