Implications of hydrophobic slippage for the dynamic measurements of hydrophobic forces

It has previously been shown that capillary measurements may be interpreted in favor of hydrophobic slippage. Here, the possible implications of this phenomenon for using the drainage technique for hydrophobic attractive force measurements are studied numerically. Attention is focused on investigating the role of various experimental conditions in the surface force apparatus. We demonstrate that under experimental conditions discernible deviations from the Reynolds theory may occur due to slippage. The misuse of Reynolds theory may lead to overestimation of the hydrophobic attractive force. We conclude that the apparent extra attraction has to depend on both the parameters of the surface force apparatus (configuration geometry, stiffness of the spring, and driving speed) and the types of surface interactions in the system. Apparently the only possibility of distinguishing between deviations from DLVO theory and deviations from Reynolds theory is to perform the drainage rate measurements at various driving speeds. However, sometimes the effect of slippage will be undistinguishable from that of the hydrophobic force, because the apparent hydrophobic force will be nearly independent of the driving speed.