Hydrophobic Attraction between Silanated Silica Surfaces in the Absence of Bridging Bubbles

Abstract

The interaction forces between silanated silica surfaces on which there were neither nanobubbles nor a gas phase were measured using colloidal probe atomic force microscopy (AFM). To obtain hydrophobic surfaces without attached nanobubbles, an aqueous solution was introduced between the surfaces after an exchange process involving several solvents. In the approaching force curves obtained, an attractive force was observed from a distance of 10-25 nm, indicating the existence of an additional attractive force stronger than the van der Waals attraction. In the retracting force curves, a strong adhesion force was observed, and the value of this force was comparable to that of the capillary bridging force. The data clearly showed that although the bridging of nanobubbles is responsible for long-range hydrophobic attraction, there also exists an additional attractive force larger than the van der Waals attraction between hydrophobic surfaces without nanobubbles. Both the ionic strength and the temperature of the solution had little influence on the force. The possible origin of the force is discussed on the basis of the obtained results.

Surface nanobubbles and nanodroplets

“…Lou et al (2000Lou et al ( , 2002, Zhang, Maeda, and Craig (2006) flow rate influences the concentration gradient of ethanol and hence the transit saturation level of the gas, as the solubility of atmospheric gases decreases (nonlinearly) with the decrease of ethanol concentration in water (Pollack, 1991). In case that the exchange was performed stepwise, that is, ethanol was replaced subsequently by an ethanol aqueous solution of 80%, 60%, 40% etc., no nanobubbles formed on the surface (Ishida, Kusaka, and Ushijima, 2012). The mixing pattern is also related to the shear on the surface.…”

“…The solvent exchange process has meanwhile successfully been applied in many research groups to produce surface nanobubbles; see, e.g., Martinez and Stroeve (2007), S. Yang et al ( , 2008, Hampton, Donose, and Nguyen (2008), Palmer, Cookson, and Lamb (2011), Chan and Ohl (2012), Ishida, Kusaka, and Ushijima (2012), Karpitschka et al (2012), and Belova et al (2013). We discuss many of these papers later in this section and in Sec.…”

Direct Measurement of Interaction Forces between Surfaces in Liquids Using Atomic Force Microscopy

“…Indeed, the hydrophobic attraction has been found between highly hydrophobic surfaces in the absence of nanobubbles or electrostatic attraction (Ishida et al, 2012), as shown in Fig. 9.…”

Hydrophobicity of Minerals Determined by Atomic Force Microscopy – A Tool for Flotation Research

“…CP-AFM is espe-cially useful in the investigation of the so-called hydrophobic effect or the occurrence of small gas layers on hydrophobic surfaces, the nanobubbles (more generally they are gas structures on partially wetted surfaces), responsible for the hydrophobic effect [15 -26]. There still is, however, some controversy in the science community about whether gas layers are only responsible for the long range hydrophobic effect [19,27]. It was emphasized by Heinrich Schubert that the hydrophobic effect caused by nanobubbles is, besides the hydrodynamics in a flotation cell, the most essential microprocess for separating minerals by flotation [28].…”