Stability of Nanobubbles Formed at the Interface between Cold Water and Hot Highly Oriented Pyrolytic Graphite

Abstract

For the wider application of nanobubbles, a simple and reproducible nucleation process is not readily available. Here we describe a method for nucleating nanobubbles using only the most basic of conditions: depositing cold water at 4 °C on heated highly oriented pyrolytic graphite substrates. This method thus avoids the need, as in previous studies, to use secondary liquids, salts, or electrolysis to nucleate the nanobubbles and provides a pure system in which the properties of nanobubbles can be studied. The nanobubbles generated with this method are observed to survive for at least 5 days, barely changing their contact angles or heights after the first few hours. The stability of the nanobubbles in our system is discussed within the framework of some recently published theories.

The interplay among gas, liquid and solid interactions determines the stability of surface nanobubbles

“…The sample we considered in our simulation consists of a 5-layer thick slab of graphite with an ABAB pattern 34 in contact with TIP4P/2005 water 35 containing a single N 2 or O 2 molecule. The molecule interacts with the surface and the liquid through a Lennard-Jones (LJ) potential v(r ij ) = 4ε αβ ((σ αβ /r ij ) 12 − (σ αβ /r ij ) 6 ), where r ij is the distance between an atom of the molecule and one of the surface or water. ε αβ sets the strength of the interaction between atoms of type α and β and σ αβ the corresponding range.…”

“…Consistently with the TIP4P/2005 model, 35 interaction between graphite and water is modeled by a modified LJ potential, v(r ij ) = 4ε CO ((σ CO /r ij ) 12 − c(σ CO /r ij ) 6 ), in which the attractive term is scaled by a factor c. 36 c can be tuned to control the hydrophobicity of the surface. In practice, we compute the Young contact angle θ Y formed by a cylindrical droplet deposited on the graphite slab ( Fig.…”

“…The model requires that ζ > 0, i.e. that gas concentration in the bulk liquid is higher than the saturation value, which is contradicted by the well-established experimental observation of surface nanobubbles in equilibrated open systems (ζ = 0), 6 and even under undersaturated conditions (ζ < 0). [7][8][9] Indeed, results reported in ref.…”

“…Although these studies were successful at upholding the importance of contact line pinning to stability, they gave a limited contribution in identifying other key elements to achieve nanobubble stability. Indeed, perhaps influenced by the theoretical hypothesis that nanobubbles are stable only under oversaturated conditions, 4,5 but in contrast with experimental evidence, [6][7][8][9] these works focused only on ζ ≥ 0 conditions. Moreover, standard molecular dynamics techniques adopted in these pioneering studies allow to span a limited timescale, typically few tens of nanoseconds, 11-12 orders of magnitude shorter than the experimentally reported lifetime of nanobubbles, hours to days.…”

Interfacial nanobubbles produced by long-time preserved cold water

“…This phenomenon may be attributed to the observation that the growth and stabilization of nanobubbles in our methods are much slower and moderate (about 30 min) while in other methods the environment near the HOPG surface is severely disturbed by the exchange process [69] or high substrate temperature. [60] 1.0 µm 1.0 µm 1.0 µm 1.0 µm…”