Force modulation

The well defined contact geometry and the ionic structure of the mica surface favours observation of structural and solvation forces. Besides a monotonic entropic repulsion one may observe superimposed periodic force modulations. It is commonly believed that these modulations are due to a metastable layering at surface separations below some 3-10 molecular diameters. These diflftise layers are very difficult to observe with other teclmiques [92]. The periodicity of these oscillatory forces is regularly found to correspond to the characteristic molecular diameter. Figure Bl.20.7 shows a typical measurement of solvation forces in the case of ethanol between mica. 

Maivald, P, Butt, H.J., Gould, S.A., Prater, C.B., Drake, B., Gurley, J.A., Elings, VB. and Hansma, P.K.. Using force modulation to image surface elasticities with the atomic force microscope. Nanotechnology, 2, 103-106 (1991). 

Asif, S.A.S., Colton, R.J. and Wahl, K.J., Nanoscale surface mechanical property measurements Force modulation techniques applied to nanoindentation. In Ovemey, R.M. and Frommer, J.E. (Eds.), Interfacial Properties on the Submicron Scale. ACS/Oxford Press, Oxford, 2001. 

Asif, S.A.S., Wahl, K.J. and Colton, R.J., Nanoindentation and contact stiffness measurement using force modulation with a capacitive load-displacement transducer. Rev. Sci. Instrum., 70, 2408-2413 (1999). 

Davis JJ, Peters B, Xi W (2008) Force modulation and electrochemical gating of conductance in a cytochrome. J Phys Condens Matter 20 374123... 

Resch, R. Friedbacher, G Grasserbauer, M. Kanniainen, T. Lindroos, S. Leskela, M. Niinisto, L. 1997. Lateral force microscopy and force modulation microscopy on SILAR-grown lead sulfide samples. Appl. Surf. Sci. 120 51-57. 

Forced-vibration instruments, 21 745 Force field calculations, 16 742 Force field energy, 16 742 Force field performance, 16 745 Force fields, 16 743-745 Force field simulations, 16 746-747 programs for, 16 746 Force modulation microscopy, 3 332 Forces, exponents of dimensions in absolute, gravitational, and engineering systems, 8 584t Forchlorfenuron, 13 43t, 53 Ford nuclear reactor, 17 594... 

There are several other operation modes, such as the intermittent contact mode, (Fig. 6c) where the cantilever oscillates and the tip records the repulsive contact with the surface at the lowest point of the oscillation, and the force modulation mode (Fig. 6d) where the tip does not leave the surface at all during the oscillation cycle. 

Fig. 25. ATR spectra recorded during epoxidation of cyclohexene catalyzed by a Ti-Si aerogel with TBHP as the oxidant under the influence of forced modulation of the cyclohexene concentration (a) time-resolved spectra (reference recorded before modulation) (b) difference spectra obtained by subtracting one (arbitrarily chosen) spectrum (c) phase-resolved (demodulated) spectra. The data set for the spectra in (a)-(c) is the same (SO).

The so-called Force Modulation Mode (FMM) was introduced in 1991 as a modification of contact-mode SFM [123] and has since found a wide range of applications in heterogeneous polymer systems [124-129]. The FMM imaging is performed by z-modulating the probe (or sample) at a low frequency of about 5 to 20 kHz and an amplitude greater than 10 nm. The frequency is sig-... 

Fig. 14. Variation of the cantilever amplitude (A) recorded by force-modulation SFM where the z-position of the sample was modulated. The sample displays a heterogeneous surface consisting of soft and hard areas...

The linear dependence on ks is a strong advantage compared to the force-modulation SFM. Due to the inertia of the cantilever mass at high frequencies, the tip cannot follow completely the displacement of the sample. This results in smaller elastic deformations and low forces in the pN range, which are measured by accelerated mass md2zldt2. Recently, the SLAM technique has been advanced toward variable temperature experiments [138]. Before this development, temperature ramps have been used to perform local calorimetry [139]. 

Fig. 44. The SFM amplitude (a) and force modulation (b) maps of a cryogenic faced impact copolymer (ICP) composed of a polypropylene (PP) matrix with high ethylene (60 wt. %) ethylene-propylene copolymer (EP). Crystalline polyethylene (PE) phases are seen in the EP domains, which are surrounded by the PP matrix. Modulus contrast in the force modulation (drive amplitude 100 mV) image associated with the three polymers the stiff PP matrix is dark, the soft EP domains are light. The crystalline PE regions have modulus between the PP and the EP,thus an intermediate shade of grey is observed for the PE domains [128]...

Recently, dynamic mode SFM has been used for genltle and more controlled surface modification [464,465]. Depending on the substrate type, the loading force was varied with ca. 500 nN amplitude at a frequency of 0.1-10 kHz. Force modulation SFM was employed to write on polycarbonate surfaces with letters ca. 50 nm high (Fig. 50b) as the tip was in light contact with the surface and a modulation frequency was relatively low to ensure plastic deformation of the polymer [464]. 

General Experimental Protocols. As noted above, thermal mechanical analysis may be conducted in three separate modes standard, temperature-modulated, and force-modulated. Sample preparation requires dimensional stability, typically including either placement of the sample into a receptacle (useful for powders) or pressing into pellets or tablets. 

Low-amplitude displacement modulation can be superimposed to either the sample position or the position of the cantilever base see Fig. 2. Via the stiffness of the cantilever for bending and torsion, the displacement modulation translates to force modulation. More precisely, out-of-plane (i.e. in the z-direction) displacement modulation results in modulated compressive loading of the tip-sample contact [36], whereas in-plane (i.e., in the x- or y-directions) displacement modulation results in modulated shear loading... 

Well-established cleaning approaches such as that based on the effect of high-speed water jets can be Improved by subjecting the jet to ultrasonic forced modulation [79]. 

Pouderoux, P. and Kahrilas, P. J. 1995. Deglutitive tongue force modulation by volition, volume, and viscosity in humans. Gastroenterol, 108 1418-1426. 

Characterization of the copolymers containing 10 - 30 wt% of silicone macromonomers was done by GPC, DSC, SEM, TEM, surface analysis (SIMS, XPS, contact angle), and especially atomic force microscopy (both Tapping Mode and force modulation mode). 

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