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...

The forces between spherical particles of cellulose (20−30 μm) have been measured in different solutions using an atomic force microscope, with a view to understanding the interactions in a model papermaking system. At low ionic strength (0.1 mM KBr), the interaction profile is dominated by a long range double layer force and shorter ranged electrosteric force. A qualititatively similar profile is observed at high pH, but in this case both the double layer force and electrosteric force increase as a consequence of cellulose charging. Conversely, the two force contributions both decrease in the presence of calcium ions. At high ionic strength (10 mM KBr) the electrosteric force is absent and the forces appear to be due solely to double layer forces. Overall, the results show that the surface is composed of looser chains that extend out into the solution, the conformation of which is highly sensitive to the solution conditions.

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The mechanism and geometry of force measurement with the atomic force microscope are analyzed in detail. The effective spring constant to be used in force measurement is given in terms of the cantilever spring constant. Particular attention is paid to possible dynamic effects. Theoretical calculations show that inertial effects may be neglected in most regimes, the exception being when relatively large colloidal probes are used. Model calculations of the effects of friction show that it can cause hysteresis in the constant compliance region and a shift in the zero of separation. Most surprising, friction can cause a significant diminution of the measured precontact force, and, if it actually pins the surfaces, it can change the sign of the calibration factor for the cantilever deflection, which would cause a precontact attraction to appear as a repulsion. Measurements are made of the van der Waals force between a silicon tip and a glass substrate in air. The evidence for friction and other dynamic effects is discussed. Interferometry is used to characterize the performance of the piezoelectric drive motor and position detector used in the atomic force microscope. It is shown that hysteresis in the former, and backlash in the latter, preclude a quantitative measurement of friction effects. The experimental data appear to underestimate the van der Waals attraction at high driving velocities, in qualitative agreement with the model friction calculations.

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