Effect of added salt on preformed surface nanobubbles: a scaling estimate.

In this paper we propose a scaling argument to quantify the role of added electrolyte salt in affecting the stability and the morphology of preformed surface nanobubbles on hydrophobic substrates like the water-OTS-silicon or the water-HOPG interfaces. The added salt controls the electric double layer formation as well as affects the zeta (ζ) potential at the air-water and solid-water interfaces. The resulting electrostatic wetting tension acts in conjunction with the air-water surface tension (analogous to electrowetting scenarios), thereby affecting the nanobubble morphologies. Weak ζ potential of the water-HOPG interface or the water-OTS-silicon interface at acidic pH ensures that the added salt will have imperceptible effect on the corresponding preformed surface nanobubbles, validating the experimental observations. However, at alkaline buffer pH for the OTS-silicon substrate, under certain system conditions, salt-induced ζ potential can be substantially high so that the properties of preformed surface nanobubbles will be affected. This paper will thus readdress the long-held universal notion that added salt, no matter in what concentration, will not influence the properties of preformed surface nanobubbles.