Adhesion Forces between Glass and Silicon Surfaces in Air Studied by AFM:  Effects of Relative Humidity, Particle Size, Roughness, and Surface Treatment

Abstract

Using the atomic force microscope (AFM), the pull-off forces between flat glass or silicon surfaces and silicon AFM tips or glass microspheres of different sizes have been extensively studied as a function of relative humidity (RH) in the range 5-90%, as model systems for the behavior of cohesive powders. The glass and silicon substrates were treated to render them either hydrophobic or hydrophilic. All the hydrophilic surfaces gave simple force curves and pull-off forces increasing uniformly with RH. Small contacts (R ∼ 20 nm) gave pull-off forces close to values predicted by simple Laplace-Kelvin theory (∼20 nN), but the values with microspheres (R ∼ 20 µm) fell well below predictions for sphere-flat or sphere-sphere geometry, due to roughness and asperity contacts. The hydrophobic silicon surfaces also exhibited simple behavior, with no significant RH dependence. The pull-off force again fell well below predicted values (Johnson-Kendall-Roberts contact mechanics theory) for the larger contacts. Hydrophobic glass gave similar adhesion to silicon over most of the RH range, but against both silicon tips and glass microspheres, there was an anomalously large adhesion in the RH range 20-40%, accompanied by a long-range noncontact force. The adhesion on fully hydrophilic surfaces and its RH dependence can be mostly explained by current theories of capillary bridges, but the interpretation is complicated by the sensitivity of theoretical predictions to contact geometry (and hence to roughness effects) and by uncertainties in the thickness of adsorbed water layers. The anomalous behavior on hydrophobic glass surfaces at intermediate values of RH is not fully understood, but possible causes are (1) dipole layers in the partially formed water film, giving rise to patch charges and long-range forces, or (2) fixed charges at a reactive glass surface, involving specific bonding reactions. The results may be useful in explaining the behavior of cohesive powders with different coatings or those which show a large humidity dependence (e.g., zeolites) or show electrostatic charging effects (e.g., silica aerogels).

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

“…(7.8), meniscus forces are expected to be maximal for hydrophilic surfaces (small contact angle) and to vanish for very hydrophobic surfaces. The expected decrease of the adhesion force with increasing hydrophobicity has indeed been observed by several authors [682][683][684][685][686][687][688]. An obvious limitation of Eq.…”

“…Often quite different dependencies of adhesion force on humidity are reported for the same system, e.g. for mica [700,706,707], and the increase of adhesion with humidity is either reported to be step-like [694,700], linearly increasing [686,708] or exhibiting a maximum at intermediate values [707,684]. These discrepancies need clarification.…”

“…Jones et al [686] found that adhesion on hydrophobic substrates increased uniformly with humidity and values for small contacts were predicted by simple Laplace-Kelvin theory. For microspheres, values were too small compared to theory, due to surface roughness.…”

On the Adhesion between Fine Particles and Nanocontacts: An Atomic Force Microscope Study

“…The adhesion force between particles or particle surface by using AFM colloidal probe technique depending on relative humidity [190,202,203]. Jones et al study of adhesion between flat surfaces of glass or silicon and silicon AFM tip or glass microspheres [204].…”

Adhesion Control for Micro- and Nanomanipulation

“…Current methods to measure micro/nanoforces between surfaces are the Surface Force Apparatus (SFA), 7,8 the Atomic Force Microscope (AFM), [9][10][11] capacitive force sensors 12 or nanoindentation testers. 13,14 The modeling of pull-off force is mainly based on two different approaches based on the surface energies on the contact, [15][16][17][18] or on the integration of the van der Waals forces between objects [19][20][21][22] and on some hybrid approaches between both. 23,24 The adhesion force reduction was already obtained in liquid and dry medium by surface structuring [24][25][26] or chemical functionalisation.…”