Direct force measurement

Larson I, Drummond C J, Chan D Y C and Grieser F 1993 Direct force measurements between TI02 surfaces J. Am. Chem. Soc. 11511 885... 

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. 

Figure Bl.20.1. Direct force measurement via deflection of an elastic spring—essential design features of a direct force measurement apparatus.

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. 

Interactions between macromolecules (protems, lipids, DNA,.. . ) or biological structures (e.g. membranes) are considerably more complex than the interactions described m the two preceding paragraphs. The sum of all biological mteractions at the molecular level is the basis of the complex mechanisms of life. In addition to computer simulations, direct force measurements [98], especially the surface forces apparatus, represent an invaluable tool to help understand the molecular interactions in biological systems. 

Kelly T W ef a/1998 Direct force measurements at polymer brush surfaces by atomic force microscopy Macromoiecuies 31 4297-300... 

Leckband D ef a/1994 Direct force measurements of specific and nonspecific protein interactions S/oc/rem/sfry 33 4611-23... 

P. Kekicheff, P. Richetti, H. K. Christenson. Structure and elastic properties of lamellar mesophases from direct force measurements. Langmuir 7 1874-1879, 1991. 

Comprehension of the interactions among microstructures composed of tethered chains is central to the understanding of many of their important properties. Their ability to impart stability against flocculation to suspensions of colloidal particles [52, 124, 125] or to induce repulsions that lead to colloidal crystallization [126] are examples of practical properties arising from interactions among tethered chains many more are conceivable but not yet realized, such as effects on adhesion, entanglement or on the assembly of new block copolymer microstructures. We will be rather brief in our treatment of interactions between tethered chains since a comprehensive review has been published recently of direct force measurements on interacting layers of tethered chains [127]. 

The statics and dynamics of microstructures are governed by the forces that create or maintain them. Rarely can the forces be measured directly. But forces between special surfaces immersed in fluid can now be accurately gauged at separations down to 0.1 nm with the direct force measurement apparatus, an ingenious combination of a differential spring, a piezoelectric crystal, an interferometer, and crossed cyhndrical surfaces covered by atomically smooth layers of cleaved mica (Figure 9.4). This recent development is finding more and more applications in research on liquid and semiliquid microstructures, thin films, and adsorbed layers. 

FIGURE 9.4 The direct force measurement apparatus shown here ean measure the forees between two eurved molecularly smooth surfaces in liquids. Mica surfaces, either raw or eoated, are the primary surfaees used in this apparatus. The separation between the surfaces is measured by optieal teehniques to better than 10 nm. The distance between the two surfaces is controlled by a three-stage meehanism that ineludes a voltage-driven piezoelectric crystal tube supporting the upper mica surface this crystal tube can be displaced less than 10 nm in a controlled fashion. A force-measuring spring is attached to the lower mica surface and its stiffness can be varied by a factor of 1,000 by shifting the position of a movable clamp. Reprinted with permission from Proc. Natl. Acad Sci. USA, 84, July 1987, 4722. 

P. Omarjee Direct Force Measurements Between Polymer Stabilized Colloids. Ph.D thesis, Bordeaux I University (1999). 

As pointed out by Israelachvili (1991), the principle of direct force measurements is usually very straightforward, but the challenge is in measuring very weak forces at very small intermo-lecular or surface separations that must be controlled and measured to within 0.1 nm. Following Israelachvili (1991), we divide our description into two parts, namely, surface force measurements and interatomic force measurements. 

The JKR theory predicts correct contact radii for relative soft surfaces with effective radii larger than 100 /an. This was shown in direct force measurements by the surface forces apparatus [217, 218] or specifically designed systems. For smaller spheres it was verified using the colloidal probe technique [219],... 

Israelachvili, J. 1987. Solvation forces and liquid structure, as probed by direct force measurements. Acc. Chem. Res. 20 415-421. 

Wang, A.F., Jiang, L.P., Mao, G.Z. and Liu, Y.H. (2002) Direct force measurement of silicone-and hydrocarbon-based ABA triblock surfactants in alcoholic media by atomic force microscopy. /. Colloid Interface Sci., 256(2), 331 40. 

Direct Force Measurement at Liquid/Liquid Interfaces... 

Dispersion behaviour in systems with liquid/liquid or liquid/gas interfaces (i.e. droplet or bubbles) has traditionally been described in terms of rheological properties, wetting properties, including contact angle and interfacial tensions, or phase behaviour and stability measurements. Direct force measurements provide a means to fundamentally probe the interactions between deformable interfaces that significantly impact the dispersion (or emulsion) behaviour. 

DIRECT FORCE MEASUREMENT AT UQUID/LIQUID INTERFACES... 

A variety of interaction behaviours can be observed between liquid/liquid interfaces based on the types of colloidal forces present. In general, they can be separated into static and dynamic forces. Static forces include electrostatic, steric, van der Waals and hydrophobic forces, relevant to stable shelf life and coalescence of emulsions or dispersions. Dynamic forces arise ftom flow in the system, for instance during shear of an emulsion or dispersion. EHrect force measurements tend to center on static force measurements, and while there is a large body of work on the study of film drainage between both liquid or solid interfaces, there are very few direct force measurements in the dynamic range between liquid interfaces. Below are general descriptions of some of the types of force observed and brief discussions of their origins. 

FIGURE 4.2. A schematic of an atomic force microscope, comprising a piezo mbe, a cantilever, a sample or substrate to image and an optical lever using a laser and four-quadrant photodiode. A colloidal probe (radius 5 pm) is added to the end of the cantilever for direct force measurement. For imaging application, the probe is absent and the tip is rastered across the surface. 

---