Bulk nanobubbles are a novel type of nanoscale bubble system. Because of their extraordinary behavior, however, their existence is not widely accepted. In this paper, we shed light on the hypothesis that bulk nanobubbles do exist, they are filled with gas, and they survive for long periods of time, challenging present theories. An acoustic cavitation technique has been used to produce bulk nanobubbles in pure water in relatively large numbers approaching 10 bubble·mL with a typical diameter of 100-120 nm. We provide multiple evidence that the nanoentities observed in suspension are nanobubbles given that they disappear after freezing and thawing of the suspensions, their nucleation rate depends strongly on the amount of air dissolved in water, and they gradually disappear over time. The bulk nanobubble suspensions were stable over periods of many months during which time the mean diameter remained unchanged, suggesting the absence of significant bubble coalescence, bubble breakage, or Ostwald ripening effects. Measurements suggest that these nanobubbles are negatively charged and their zeta potential does not vary over time. The presence of such a constant charge on the nanobubble surfaces is probably responsible for their stability. The effects of pH, salt, and surfactant addition on their colloidal stability are similar to those reported in the literature for solid nanoparticle suspensions, that is, nanobubbles are more stable in an alkaline medium than in an acidic one; the addition of salt to a nanobubble suspension drives the negative zeta potential toward zero, thus reducing the repulsive electrostatic forces between nanobubbles; and the addition of an anionic surfactant increases the magnitude of the negative zeta potential, thus improving nanobubble electrostatic stabilization.

This paper elucidates parts of the mystery behind the interfacial and colloidal stability of the novel bubble system of bulk nanobubbles.

We investigate the existence and stability of bulk nanobubbles in various aqueous organic solvent mixtures. Bulk nanobubble suspensions generated via acoustic cavitation are characterised in terms of their bubble size distribution, bubble number density and their zeta potential. We show that bulk nanobubbles exist in pure water, but do not exist in pure organic solvents and they disappear at some organic solvent-water ratio. We monitor the nanobubble suspensions over a period of a few months and propose interpretations for the differences behind their long-term stability in pure water versus their long-term stability in aqueous organic solvent solutions. Bulk nanobubbles in pure water are stabilised by their substantial surface charge arising from the adsorption of hydroxyl ions produced by self-ionisation of water. Pure organic solvents do not auto-ionise and, therefore, nanobubbles cannot exist in concentrated aqueous organic solvent solutions. Due to preferential adsorption of organic solvent molecules at the nanobubble interfaces, the surface charge of the nanobubbles decreases with solvent content, but the strong hydrogen bonding near their interfaces ensures their stability. The mean bubble size increases monotonically with solvent content whilst the surface tension of the mixture is sharply reduced. This is in agreement with literature results on macro and microbubbles in aqueous organic solutions, but it stands in stark contrast to the behaviour of macro and microbubbles in aqueous surfactant solutions.

In this work, momentum and heat transfer characteristics of a heated sphere immersed in visco-plastic media have been studied over a wide range of conditions: plastic Reynolds number, 1 ≤ Re ≤ 100, Prandtl number, 1 ≤ Pr ≤ 100, and Bingham number, 0 ≤ Bn ≤ 104. The equations of motion and energy have been solved numerically to elucidate the influence of each of these dimensionless parameters on the local characteristics like streamlines, isotherms, kinematics of flow, local Nusselt number, etc. as well as on the gross engineering parameters such as drag coefficient and surface average Nusselt number. Furthermore, a detailed examination of the so-called yielded (fluid-like) and unyielded (solid-like) regions in the flow domain is carried out as a function of both the Reynolds and Bingham numbers. The present results have been compared with the scant results available in the literature and these are found to be in good agreement. Finally, the present numerical results for drag and Nusselt number (in the form of the so-called j-factor) have been correlated with the modified Bingham and Reynolds number via simple expressions thereby enabling their interpolation for the intermediate values of these dimensionless parameters.

The present study deals with the prediction of drag and forced convection heat transfer behavior of a heated sphere in shear-thinning yield-stress fluids over wide ranges of conditions: plastic Reynolds number, 1 ≤ Re ≤ 100; Prandtl number, 1 ≤ Pr ≤ 100; Bingham number, 10–3 ≤ Bn ≤ 10; and shear-thinning index, 0.2 ≤ n ≤ 1. The momentum and energy equations have been solved numerically together with the Papanastasiou regularization method for viscosity to circumvent the discontinuity inherent in the Herschel–Bulkley constitutive equation. Extensive results on the flow and heat transfer characteristics are presented in order to delineate the influence of the aforementioned dimensionless parameters. Thus, for instance, the flow characteristics are presented in terms of the streamlines, morphology of the yielded/unyielded regions, recirculation length, shear rate magnitude over the surface of the sphere, and drag coefficient. Similarly, heat transfer characteristics are examined in terms of isotherm contours in the close proximity of the sphere and the average Nusselt number as a function of the relevant dimensionless groups. Furthermore, the present results are compared with the available experimental and numerical results in order to establish the reliability and precision of the numerical solution methodology employed in this work. Finally, the average Nusselt number and drag values are correlated in terms of the shear-thinning index (n) and the modified Reynolds number (Re*) via simple expressions, thereby enabling their interpolation for intermediate values of the modified Reynolds number. All else being equal, in addition to Bingham number, shear-thinning behavior of yield stress fluids enhances the rate of heat transfer over and above that observed in Newtonian fluids.

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