Forces Measured between Hydrophobic Surfaces due to a Submicroscopic Bridging Bubble

Abstract

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...

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…All these theories predict an exponentially decaying force with a decay length of half the Debye length. Experiments, however, show that the hydrophobic force is independent of the ionic strength [509,[523][524][525].…”

“…However, routine force measurements with differently prepared hydrophobic surfaces became possible only with the AFM [504]. These include: silica, oxidized silicon wafers, and glass surfaces treated with octadecyl-trichloro-silane (OTS, CH 3 (CH 2 ) 17 SiCl 3 ) [355,496,505,506], trimethyl-chlorosilane [505,507], dichloro-dimethyl-silane [508], in fluorinated dichlorosilane [509], and hexamethyl-disilazane [180]; hydrophobic polymer surfaces such as polystyrene [189,510], polypropylene [511], polyethylene [512]; gold-alkanethiol coated surfaces [513]; silica, oxidized silicon wafers, and glass surfaces with physisorbed CTAB [514,515].…”

Effect of impurities in description of surface nanobubbles

“…The presence of the hydrophobic surface leads to the formation of spherical cap-like bubbles at the solid-liquid interface, called "surface nanobubbles". Over the years AFM techniques have been the most popular method in studying these surface nanobubbles [1][2][3][4][5]. Depending on the conditions that lead to their formation, different behaviors of the nanobubbles have been found by these studies: e.g., their spherical cap-like shape and chances of deviation from that shape [6][7][8], merging of two adjacently located nanobubbles [6,9], disappearance of nanobubbles in case the water is degassed [10], possible reappearances by exchange of solvents [7,[11][12][13][14][15] or increase of temperature [11], or electrolysis [9,16] etc.…”

Large-scale molecular-dynamics simulation of nanoscale hydrophobic interaction and nanobubble formation

“…[8][9][10][11][12][13][14] On the other hand, among mesoscopic-or macroscopic-sized hydrophobes, e.g., between two planar hydrophobic surfaces, hydrophobic interaction can be induced by dewetting transition or capillary cavitation, [15][16][17][18][19] or by the formation of vicinal microbubbles [20][21][22] and inverted clathratelike structures. 23,24 The crossover from molecular to macroscopic like hydrophobic interaction was predicted to occur at about 1 nm length scale of the size of hydrophobes.…”