Surface force contact forces

In the last 40 years, techniques to directly measure surface forces and force laws (force vs. separation distance between surfaces) have been developed such as the surface forces apparatus (SFA) [6] and AFM. Surface forces are responsible for the work required when two contacting bodies (such as an AFM tip in contact with a solid surface) are separated from contact to infinite distance. Although the physical origin of all relevant surface forces can be derived from fundamental electromagnetic interactions, it is customary to group these in categories based on characteristic features that dominate the relevant physical behavior. Thus, one speaks of ionic (monopole), dipole—dipole, ion—dipole interactions, electrostatic multipole forces (e.g., quadrupole), induced dipolar forces, van der Waals (London dispersive) interactions, hydrophobic and hydrophilic solvation, structural and hydration forces,... 

In contact AFM mode (also known as repulsive mode), the AFM tip makes soft physical contact with the sample. As the scanner traces the tip across the surface, the contact force causes the cantilever to bend to accommodate changes in topography. From Fig. 2 it can be seen that the slope on the left side of the graph is relatively steep suggesting the force increases significantly for a very small movement towards the surface. In other words, when the cantilever pushes the tip against the surface, the cantilever will bend rather than force the tip against the sample. 

The force Fna is not operative in those cases in which all projections (asperities) of the surface other than those projections making contact are located (in relation to the points of the opposing surface) at a distance exceeding the range of action of surface forces. The force Fna is eliminated in the case in which the actual contact area is equal to the nominal contact area, this case occuring in full-film lubrication. In all other cases, the force Fna does have an effect on adhesion. It may exceed the force Fq since the sections of the surface having direct contact may be considerably smaller in area than the sections where the force Fna is acting. 

The molecular forces, which are present between the surface molecules, are found to be different from the forces acting on molecules in the bulk phase or the gas phase. Accordingly, these forces are called surface forces. Surface forces make the liquid surface behave like a stretched elastic membrane in that it tends to contract. One cannot see this phenomenon directly, but it is observed through indirect experimental observations (both qualitatively and quantitatively ). The latter arises from the observation that when one empties a beaker with a liquid, the liquid breaks up into spherical drops. This indicates that drops are being created under some forces, which must be present at the surface of the newly formed interface. These surface forces become even more important when a liquid is in contact with a solid (such as ground water, oil reservoir). The flow of liquid (e.g., water or oil) through small pores in the underground is... 

Force can be exerted externally on a body in two particular ways. Gravity and inertia can be thought of as body forces since they act directly on all the individual particles in the body. The other type are surface or contact forces... 

Compared witii other direct force measurement teclmiques, a unique aspect of the surface forces apparatus (SFA) is to allow quantitative measurement of surface forces and intermolecular potentials. This is made possible by essentially tliree measures (i) well defined contact geometry, (ii) high-resolution interferometric distance measurement and (iii) precise mechanics to control the separation between the surfaces. 

In accordance with equation (Bl.20.1). one can plot the so-called surface force parameter, P = F(D) / 2 i R, versus D. This allows comparison of different direct force measurements in temis of intemiolecular potentials fV(D), i.e. independent of a particular contact geometry. Figure B 1.20.2 shows an example of the attractive van der Waals force measured between two curved mica surfaces of radius i 10 nun. 

Frantz P ef al 1997 Use of capacitance to measure surface forces. 2. Application to the study of contact mechanics Langmuir 2 5957-61... 

The separation of two surfaces in contact is resisted by adhesive forces. As the nonnal force is decreased, the contact regions pass from conditions of compressive to tensile stress. As revealed by JKR theory, surface tension alone is sufficient to ensure that there is a finite contact area between the two at zero nonnal force. One contribution to adhesion is the work that must be done to increase surface area during separation. If the surfaces have undergone plastic defonnation, the contact area will be even greater at zero nonnal force than predicted by JKR theory. In reality, continued plastic defonnation can occur during separation and also contributes to adhesive work. 

The pores in question can represent only a small fraction of the pore system since the amount of enhanced adsorption is invariably small. Plausible models are solids composed of packed spheres, or of plate-like particles. In the former model, pendulate rings of liquid remain around points of contact of the spheres after evaporation of the majority of the condensate if the spheres are small enough this liquid will lie wholly within the range of the surface forces of the solid. In wedge-shaped pores, which are associated with plate-like particles, the residual liquid held in the apex of the wedge will also be under the influence of surface forces. 

Particle-Bubble Attachment. In the above, principles leading to creation of desired hydrophobicity/hydrophihcity of the particles has been discussed. The next step is to create conditions for particle-bubble contact, attachment, and their removal, which is simply described as a combination of three stochastic events with which are associated the probability of particle-bubble colhsion, probabihty of attachment, and probability of retention of attachment. The first term is controlled by the hydrodynamic conditions prevaihng in the flotation unit. The second is determined by the surface forces. The third is dependent on the s irvival of the laden bubble by liq ud t irbulence and impacts by the other suspended particles. A detailed description of the hydrodynamic and other physical aspects of flotation is found in the monograph by Schulze (19 ). 

Coagulation, i.e., the process by which discrete particles come in contact with each other in the air and remain joined together by surface forces, represents another way in which aerosol diameter will increase. However, it does not alter the mass of material in the coagulated particle. 

The van der Waals and other non-covalent interactions are universally present in any adhesive bond, and the contribution of these forces is quantified in terms of two material properties, namely, the surface and interfacial energies. The surface and interfacial energies are macroscopic intrinsic material properties. The surface energy of a material, y, is the energy required to create a unit area of the surface of a material in a thermodynamically reversible manner. As per the definition of Dupre [14], the surface and interfacial properties determine the intrinsic or thermodynamic work of adhesion, W, of an interface. For two identical surfaces in contact ... 

Section 4.1 briefly describes some of the commonly employed experimental tools and procedures. Chaudhury et al., Israelachvili et al. and Tirrell et al. employed contact mechanics based approach to estimate surface energies of different self-assembled monolayers and polymers. In these studies, the results of these measurements were compared to the results of contact angle measurements. These measurements are reviewed in Section 4.2. The JKR type measurements are discussed in Section 4.2.1, and the measurements done using the surface forces apparatus (SFA) are reviewed in Section 4.2.2. 

It has been also shown that when a thin polymer film is directly coated onto a substrate with a low modulus ( < 10 MPa), if the contact radius to layer thickness ratio is large (afh> 20), the surface layer will make a negligible contribution to the stiffness of the system and the layered solid system acts as a homogeneous half-space of substrate material while the surface and interfacial properties are governed by those of the layer [32,33]. The extension of the JKR theory to such layered bodies has two important implications. Firstly, hard and opaque materials can be coated on soft and clear substrates which deform more readily by small surface forces. Secondly, viscoelastic materials can be coated on soft elastic substrates, thereby reducing their time-dependent effects. 

There are two different scales of deformation in any adhesive contact (1) the bulk scale of deformation which is characterized by the radius a of contact area over which the compressive forces are significant and (2) the zone of action of surface forces or the cohesive zone at the edge of the contact, characterized by the length d over which the tensile forces are dominant. When the contact boundary is moving with a speed u, the two scales of deformation translate into two time scales, one on the order of a/ v) and the other of the order of (d/v). 

Equilibrium is established when the attractive surface forces are balanced by elastic repulsion forces between the materials. The DMT model states that the elastic repulsion force is related to the attractive force within the contact region Fs by... 

---