Atomic force microscope, measurement

There have now been attempts to determine rheological properties on the nanoscale. The nano-rheological properties (surface viscoelasticity) of emulsion droplets have been estimated through modelling based on data from atomic force microscope measurements [371]. 

Associative polymers, rheology effect, 9 Atomic force microscope, measurement of steric exclusion forces, 266-277... 

Despite the complexity of the characterisation, it is not contradictory to stress the necessity of making every effort to achieve the best possible knowledge of the system. The definition of the structure of the coating, in the widest sense, requires the use of a number of spectroscopic, microscopic, morphological, and compositional measurements to complement the electrochemical ones. However, it must be stressed that, in many cases, the deposits have been studied in the dry state, i.e., not really in the conditions under which they are actually used. As an example, only a few studies are found in which atomic force microscopic measurements are made in the solution phase, where the deposit is created and subsequently used. On the other hand, the Pt and Au substratelpolymeric deposit interface has been studied in situ by surface-enhanced Raman spectroscopy. 

What are STM and AFM The scanning timneling microscope measures the electrical current of electrons that tunnel across a small gap between sample and probe. The atomic force microscope measures the force arising from intermolecular interactions between the probe tip and sample surface. Both methods are used to map—or even to alter—the structure of a surface with single-atom precision. 

Carpick R W, Agrait N, Ogletree D F and Salmeron M 1996 Measurement of interfacial shear (friction) with an ultrahigh vacuum atomic force microscope J. Vac. Sc/. Technol. B 14 1289... 

Ducker W A, Senden T J and Pashley R M 1991 Direct measurement of colloidal forces using an atomic force microscope Nature 353 239... 

Thundat T, Zheng X-Y, Chen G Y, Sharp S L, Warmack R J and Schowalter L J 1993 Characterization of atomic force microscope tips by adhesion force measurements App/. Phys. Lett. 63 2150... 

Meyer G and Amer N M 1990 Simultaneous measurement of lateral and normal forces with an optical-beam-deflection atomic force microscope Appl. Phys. Lett. 57 2089... 

Overney R M, Meyer E, Frommer J, Brodbeck D, Luthi R, Flowald L, Guntherodt Fl-J, Fu]ihara M, Takano FI and Gotoh Y 1992 Friction measurements of phase separated thin films with a modified atomic force microscope Nature... 

Jarvis S P and Tokumoto FI 1997 Measurement and interpretation of forces in the atomic force microscope Probe Microscopy 1 65... 

The atomic force microscope (ATM) provides one approach to the measurement of friction in well defined systems. The ATM allows measurement of friction between a surface and a tip with a radius of the order of 5-10 nm figure C2.9.3 a)). It is the tme realization of a single asperity contact with a flat surface which, in its ultimate fonn, would measure friction between a single atom and a surface. The ATM allows friction measurements on surfaces that are well defined in tenns of both composition and stmcture. It is limited by the fact that the characteristics of the tip itself are often poorly understood. It is very difficult to detennine the radius, stmcture and composition of the tip however, these limitations are being resolved. The AFM has already allowed the spatial resolution of friction forces that exlribit atomic periodicity and chemical specificity [3, K), 13]. 

Figure C2.9.3 Schematic diagrams of the interfaces reaiized by (a) tire atomic force microscope, (b) tire surface forces apparatus and (c) tire quartz crystai microbaiance for achieving fundamentai measurements of friction in weii defined systems.

All this being said, perhaps the most definitive study of the relative roles of electrostatic and van der Waals forces was performed by Gady et al. [86,101,102]. In their studies, they attached a spherical polystyrene particle, having a radius between 3 and 6 p.m, to the cantilever of an atomic force microscope. They then conducted three distinct measurements that allowed them to distinguish between electrostatic and van der Waals forces that attracted the particle to various conducting, smooth substrates. 

Burnham, N.A. and Colton, R.J., Measuring the nanomechanical properties and surface forces of materials using an atomic force microscope. J. Vac. Sci. Technol. A Vac. Surf. Films, 7(4), 2906-2913 (1989). 

Weisenhom, A.L., Maivald, P, Butt, H.J. and Hansma, P.K., Measuring adhesion, allrac-lion, and repulsion between surfaces in liquids with an atomic-force microscope. P/ry.v. Rev. B Condens. Matter, 45(19), 11226-11232 (1992). 

Butt, H.J., Measuring electrostatic. Van der waals, and hydration forces in electrolyte-solutions with an atomic force microscope. Biophys. J., 60(6), 1438-1444 (1991). 

A very similar technique is atomic force microscope (AFM) [38] where the force between the tip and the surface is measured. The interaction is usually much less localized and the lateral resolution with polymers is mostly of the order of 0.5 nm or worse. In some cases of polymer crystals atomic resolution is reported [39], The big advantage for polymers is, however, that non-conducting surfaces can be investigated. Chemical recognition by the use of specific tips is possible and by dynamic techniques a distinction between forces of different types (van der Waals, electrostatic, magnetic etc.) can be made. The resolution of AFM does not, at this moment, reach the atomic resolution of STM and, in particular, defects and localized structures on the atomic scale are difficult to see by AFM. The technique, however, will be developed further and one can expect a large potential for polymer applications. 

A most recent commercial Nano Indenter (Nano Indenter XP (MTS, 2001)) consists of three major components [66] the indenter head, an optical/atomic force microscope, and x-y-z motorized precision table for positioning and transporting the sample between the optical microscopy and indenter (Fig. 28). The load on the indenter is generated using a voice coil in permanent magnet assembly, attached to the top of the indenter column. The displacement of the indenter is measured using a three plate capacitive displacement sensor. At the bottom of the indenter rod, a three-sided... 

Mayer, G. and Amer, N. M., Simultaneous Measurement of Lateral and Normal Forces with an Optical-Beam-Deflection Atomic Force Microscope, AppZ. Phys. Lett., Vol. 57, 1990, pp. 2089-2091. 

Sahin, O., Magonov, S., Su, C., Quate, C., and Solgard, O., An atomic force microscope tip designed to measure time-varying nanomechanical forces. Nature Nanotechnology, 336, 1037, 2007. 

Surface forces measurement directly determines interaction forces between two surfaces as a function of the surface separation (D) using a simple spring balance. Instruments employed are a surface forces apparatus (SFA), developed by Israelachivili and Tabor [17], and a colloidal probe atomic force microscope introduced by Ducker et al. [18] (Fig. 1). The former utilizes crossed cylinder geometry, and the latter uses the sphere-plate geometry. For both geometries, the measured force (F) normalized by the mean radius (R) of cylinders or a sphere, F/R, is known to be proportional to the interaction energy, Gf, between flat plates (Derjaguin approximation). 

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