Measurements of the forces in water between neutral hydrophobic surfaces prepared by covalent modification of glass are presented. The surfaces are stable under a variety of conditions including high temperature, high salt concentrations and with added ethanol. The forces between these surfaces have been studied under all of these different conditions. In water the force is attractive at very large surface separations, and discontinuities or steps are present in the force curves. It is suggested that the steps at the onset of the force are due to the bridging of submicroscopic bubbles or cavities between the surfaces and that it is their consequent growth with decreasing separation that causes the long-range attraction between hydrophobic surfaces. Electrolyte has a negligible effect on the range and strength of the measured forces, except at very high salt concentrations where the strength of the attractive forces and the adhesion between the surfaces increases slightly. The addition of ethanol reduces both the strength of the long range forces and the adhesion between the surfaces. On the basis of the comparison between these results and earlier measurements, it appears that the attraction does not obey the Derjaguin approximation. Forces were also measured in the presence of a microscopic vapor cavity created by first bringing the surfaces into contact. IntroductionDirect measurements of the interactions between macroscopic hydrophobic surfaces have revealed the presence of strong attractions of much longer range than the classical van der Waals force.'-14 The force is in some cases measurable at separations greater than 100 nm. The existence of an interaction at such distances challenges fundamental notions of liquid structure and surface forces, and despite considerable theoretical effort the molecular origin of this long-range attraction remains controversial.A number of explanations of the range and the strength of the interaction have been proposed. It has been shown that the force might originate from a perturbation in the ordering of water propagating through the liquid between two hydrophobic surfaces,lS a hypothesis that is difficult to test by experimental or

Imaging of hydrophobic surfaces in water with tapping mode atomic force microscopy reveals them to be covered with soft domains, apparently nanobubbles, that are close packed and irregular in cross section, have a radius of curvature of the order of 100 nm, and a height above the substrate of 20-30 nm. Complementary force measurements show features seen in previous measurements of the long-range hydrophobic attraction, including a jump into a soft contact and a prejump repulsion. The distance of the jump is correlated with the height of the images. The morphology of the nanobubbles and the time scale for their formation suggest the origin of their stability.

Atomic force microscopy on hydrophobic microspheres in water reveals a strong attraction with a range of 20 -200 nm, following an initial steep repulsion at long range. The data are consistent with a single submicroscopic bubble between the surfaces, with the attraction due to its attachment and lateral spread, and the repulsion dependent on film drainage and the electric double layer. The results provide direct experimental evidence of the existence of long-lived submicron bubbles, and of their bridging as the cause of the measured long-range attractions between macroscopic hydrophobic surfaces.[S0031-9007(98)06357-1] PACS numbers: 61.16.Ch, 68.10.Cr, 68.15. + e, 82.65.Dp In the early 1970s Blake and Kitchener [1] measured the rupture of the water film between a hydrophobic surface and an approaching bubble, and concluded that a long-ranged attraction existed. The force between two macroscopic hydrophobic surfaces has since been directly measured, and, although the quantitative details vary, the measurements confirm a strong attraction that is much larger than the van der Waals force (see Ref.[2]). The extreme range of the force (measurable at 300 nm [3]) challenges conventional theories of surfaces forces and the liquid state. Comparisons with polywater are not entirely uncalled for, following the early suggestion [4] that the force was due to extended, surface-induced, water structure.Most consensus for the underlying physical mechanism has focused on long-range electrostatic forces, following the proposal by Attard [5] that the two surfaces coupled via correlated fluctuations. This idea and its various modifications [6-9] all predict a strong dependence on the electrolyte concentration, which experiments variously confirm [10 -12] and refute [3,[13][14][15].Alternatively, it has been suggested [3,16] that the force is due to the presence of submicroscopic bubbles adhering to the surfaces (Harvey nuclei), with the attraction due to the attachment to the other surface and subsequent lateral spreading. The proposal was based on the observation of steps or discontinuities in the force data at large separations [3], which were taken to be due to the bridging of multiple bubbles. The idea is supported by the fact that the force tends to be more short ranged when measured in de-aerated water [15,17], and when measured between surfaces that had never been exposed to the atmosphere [17], presumably due to the attachment of bubbles to defects in the surfaces when they were taken through the air-water interface.What is attractive about bridging bubbles as a mechanism for these long-ranged forces is that the range of the force is set by the physical size of the bubble, and one avoids a putative surface-induced structure in the liquid that extends over thousands of molecular diameters. The main difficulty with the proposal is that, according to macroscopic thermodynamics, bubbles are metastable [16]; the Laplace equation predicts a high internal gas pressure for submicroscopic bubbles that should make them dissolve [18...

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