Surface force-distance measurements

The measurement of surface forces calls for a rigid apparatus that exhibits a high force sensitivity as well as distance measurement and control on a subnanometre scale [38]. Most SFAs make use of an optical interference teclmique to measure distances and hence forces between surfaces. Alternative distance measurements have been developed in recent years—predominantly capacitive techniques, which allow for faster and simpler acquisition of an averaged distance [H, 39, 40] or even allow for simultaneous dielectric loss measurements at a confined interface. 

Frantz P, Agrait N and Salmeron M 1996 Use of capacitance to measure surface forces. 1. Measuring distance of separation with enhanced spacial and time resolution Langmuir 12 3289-94... 

Several experimental parameters have been used to describe the conformation of a polymer adsorbed at the solid-solution interface these include the thickness of the adsorbed layer (photon correlation spectroscopy(J ) (p.c.s.), small angle neutron scattering (2) (s.a.n.s.), ellipsometry (3) and force-distance measurements between adsorbed layers (A), and the surface bound fraction (e.s.r. (5), n.m.r. ( 6), calorimetry (7) and i.r. (8)). However, it is very difficult to describe the adsorbed layer with a single parameter and ideally the segment density profile of the adsorbed chain is required. Recently s.a.n.s. (9) has been used to obtain segment density profiles for polyethylene oxide (PEO) and partially hydrolysed polyvinyl alcohol adsorbed on polystyrene latex. For PEO, two types of system were examined one where the chains were terminally-anchored and the other where the polymer was physically adsorbed from solution. The profiles for these two... 

Subsequently, the scan size is increased as much as is required to image the entire crystal. Then a FV scan is started using a z ramp rate of > a few Hz. An array of 64 x 64 points2 (4,096 consecutive force-distance measurements) is acquired to provide an FV image of the hydrophobic PE crystal on the more hydrophilic mica. Because of the high surface energy of (clean) mica and the sample preparation procedure, which involves hydrophobic solvents, the mica surface will be covered with a thin contamination layer. Nevertheless, the adhesive properties differ markedly and the crystal can be visualized as shown in Fig. 4.6b. This FV image is scaled... 

Single Molecule Force Spectroscopy - Force-Distance Measurements Measuring the force as a function of tip-sample distance can yield information on the elasticity of individual supermolecules, on conformational transitions (e.g. of proteins), on the mechanical stability of chemical bonds and secondary structures, as well as of the desorption of the molecules from the solid substrate [82, 92-94]. Moreover, information on the chemical bond formation of the tip cluster with a particular bonding site on the sample surface can be obtained [95]. 

In our laboratory, two techniques have been extensively used for studying protein behavior at various interfaces. The first technique censists of in situ measurement of protein adsorption with labeled proteins the second technique based on multiple-beam interferometry measures surface forces between two mica sheets with adsorbed proteins (Tabor-Israelachvili technique). While the in situ measurements enable quantitation of protein adsorption, force-distance measurements provide direct experimental data on the extension of adsorbed protein layers towards the solution and on their conformation. 

Adhesive forces were measured to analyze the interaction between Si atomic force microscope (AFM) tips and the deposited films from the force-distance measurement with the AFM. The force-distance curve is expected to elucidate the adsorption behavior of PAM on the deposited films. In the force-distance measurement, there is no interaction until the tip is close enough to be attracted to the surface. As the tip approaches to the surface, it contacts with the surface. After the contact, the tip is retracted from the surface, the cantilever is bent, and a repulsive force (positive) is measured. When the tip is being retracted, an attractive force is measured (negative). When the critical force is reached, the tip is separate from the surface and this point is called the pull-off point. Therefore, the pull-off point, which corresponds to the point of the critical force, is determined by the degree of the adhesive force between the tip and the surface. The higher the adhesive force between the tips and the films, the lower the pull-off point. Figure 6.14a shows the force-distance curves of the tips and poly Si film at pH 10 as a function of the PAM concentration. In the absence of the absorbed PAM molecule, an adhesive force was observed at approximately 20 nm of separation distance. It is of interest that there is no significant difference between the surface forces of the tip and poly Si film even with the presence of PAM. This result is almost the same for all samples, irrespective of the concentration, which means that PAM is scarcely adsorbed on poly Si film. 

New techniques are being developed that are able to measure the force-distance relationships of single molecules as they are stretched. Such techniques include atomic force microscopy and optical tweezers. Atomic force microscopy utilizes a microscale cantilever that has a probe to scan the specimen surface. Force is measured in piconewtons and the distance in nanometers. An example of some results from this technique is shown in Figure 6.5, where the force required to stretch poly(ethyleneglycol) molecules of different lengths is shown (Oesterhelt, Rief, and Gaub 1999). 

Environmental scanning electron microscopy (ESEM, Philips ESEM-FEG XL30, FEI, Netherlands) was used to image the AFM tip used for force-distance measurements and the surface of the CoCrMo discs. 

An example of the effect of different media on force-distance measurements is shown in Figure 1 for the case of a PMMA surface scanned with a SiO,t probe in water, 2-propanol, and PFD. Since both the PMMA surface and the SiO probe are negatively charged at neutral pH, the force-distance measurements in water exhibit strong double-layer interaction forces Most polymer surfaces acquire a charge in water,and this is a common feature... 

To explore the use of PFD as a contrast-enhancing medium in force distance measurements on polymer surfaces, two series of polymers were chosen for the experiments a first series, consisting of apolar polymers with different refractive indexes, and a second series, in which polymers of different hydrophobicities/hydrophi-licities were selected. 

Figure 1 Schematic illustration of single-molecule force-distance measurement of a supramolecular host-guest pair exploiting a surface-attached single-polymer chain. The conversion of the recorded cantilever deflection-piezo displacement signal (a) into a force-extension curve of the single-polymer chain (b) is shown. (Redrawn from Ref. 21. Wiley-VCH, 2006.)...

The strongest attractive disj rsion interaction is anticijmted in vacuum or air. However, force-distance measurements in air are dominated by capillaiy forces (22) due to the presence of contaminant layers on the surfaces of probe and sample. For this reason, force-distance measurements with SPM techniques are often performed in vacuum or under liquids. 

RLA occurs when repulsive forces are stronger than attractive forces. In addition to the electrical double layer force, under certain conditions of pH and electrolyte concentration, a thin hydration layer develops around the particle surface and results in short-range hydration repulsion, which could cause a reduction in collision efficiency between particles and leads to slow aggregation kinetics [44, 45]. Based on empirical models derived from force-distance measurements using atomic force microscopy, Runkana et al. [24] derived the following expression for hydration... 

It should be emphasized that using the MBI method the absolute distance is measured between the surfaces. This distance measurement can be extremely precise (better than 1 A), is not affected by instrumental drift, and gives an absolute value of separation distance D. It includes the determination of absolute zero distance, which means it can be detected if the mica sheets get into molecular contact or are still separated by adsorbed layers. In contrast, the forces acting between the surfaces are deduced from the difference between measured absolute distance D and movement O of the actuator. Thus, the precision of the force measurement depends on precise knowledge of the spring constant and the true actuator movement. The calibration of the lever arm spring constant in the SFA is straightforward... 

Much of the fundaments we know about surface forces are based on experiments with the SFA. With the SFA, surface forces are measured between two atomically smooth mica surfaces. Distance is measured interferometrically, which allows absolute determination of separation distance with a resolution of typically 0.1 nm (with down to 25 pm achievable). Its absolute force sensitivity is not as high as in several other methods, but in terms of the usually more relevant force per unit area, its sensitivity is excellent. Lateral (friction) forces can be measured in addition to normal standard force versus distance measurements and have contributed much to our understanding of lubrication by thin films. Additional information such as refractive index and contact area can be obtained. The main reason for the limited number of groups using this instrument is the difficult operation of such a system that needs a very experienced and skillful expert. The large interaction areas demand a contamination-free surface preparation and can lead to significant hydrodynamic forces in highly viscous media, which could make equilibrium measurements hard to achieve. 

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. 

The accurate and absolute measurement of the distance, D, between the surfaces is central to the SFA teclmique. In a typical experiment, the SFA controls the base position, z, of the spring and simultaneously measures D, while the spring constant, k, is a known quantity. Ideally, the simple relationship A F(D) = IcA (D-z ) applies. Since surface forces are of limited range, one can set F(D = go) = 0 to obtain an absolute scale for the force. Furthennore, SF(D = cc)/8D 0 so that one can readily obtain a calibration of the distance control at large distances relying on an accurate measurement of D. Therefore, D and F are obtained at high accuracy to yield F(D), the so-called force versus distance cur >e. 

Fig. 8. (i) Surface force.s apparatus (SFA). The force between the two surfaces is measured by measuring the deflection of the leaf spring on which one of the surfaces is mounted. The distance between the surfaces is determined by measuring the wavelengths of interference fringes. 

The surface force apparatus (SFA) is a device that detects the variations of normal and tangential forces resulting from the molecule interactions, as a function of normal distance between two curved surfaces in relative motion. SFA has been successfully used over the past years for investigating various surface phenomena, such as adhesion, rheology of confined liquid and polymers, colloid stability, and boundary friction. The first SFA was invented in 1969 by Tabor and Winterton [23] and was further developed in 1972 by Israela-chivili and Tabor [24]. The device was employed for direct measurement of the van der Waals forces in the air or vacuum between molecularly smooth mica surfaces in the distance range of 1.5-130 nm. The results confirmed the prediction of the Lifshitz theory on van der Waals interactions down to the separations as small as 1.5 nm. 

The ratio (p/G) has the units of time and is known as the elastic time constant, te, of the material. Little information exists in the published literature on the rheomechanical parameters, p, and G for biomaterials. An exception is red blood cells for which the shear modulus of elasticity and viscosity have been measured by using micro-pipette techniques 166,68,70,72]. The shear modulus of elasticity data is usually given in units of N m and is sometimes compared with the interfacial tension of liquids. However, these properties are not the same. Interfacial tension originates from an imbalance of surface forces whereas the shear modulus of elasticity is an interaction force closely related to the slope of the force-distance plot (Fig. 3). Typical reported values of the shear modulus of elasticity and viscosity of red blood cells are 6 x 10 N m and 10 Pa s respectively 1701. Red blood cells typically have a mean length scale of the order of 7 pm, thus G is of the order of 10 N m and the elastic time constant (p/G) is of the order of 10 s. 

Surface forces measurement is a unique tool for surface characterization. It can directly monitor the distance (D) dependence of surface properties, which is difficult to obtain by other techniques. One of the simplest examples is the case of the electric double-layer force. The repulsion observed between charged surfaces describes the counterion distribution in the vicinity of surfaces and is known as the electric double-layer force (repulsion). In a similar manner, we should be able to study various, more complex surface phenomena and obtain new insight into them. Indeed, based on observation by surface forces measurement and Fourier transform infrared (FTIR) spectroscopy, we have found the formation of a novel molecular architecture, an alcohol macrocluster, at the solid-liquid interface. 

---