Normal force measurement

The viscoelastic samples to be tested by this method may be in different forms. The simplest to work with is a soft or liquid-like viscoelastic material such as mayonnaise or other food emulsions. These are easy samples to work with terms of sample loading. More solid-like samples such as cheese or food gels are more difficult to load onto the instrument in a consistent matter. The degree of compression of soft samples should ideally be controlled using a normal force measure or force rebalance system. Slippage is also a concern and roughened plates or even adhesives may be needed if slip is an issue. As this protocol is a general one, it is assumed that the sample is already loaded on the rheometer and has achieved equilibrium in terms of temperature and viscoelastic stmcture (time-dependent behavior). 

By stud)dng the structure of LB monolayers as a function of transfer pressure different phases have been visualized, mostly on a mesoscopic scale ). Domain changes and phase transitions, e.g., as a function of temperature, have also been studied ). If the roughness of the underlying substrate precludes domain imaging by normal force measurements, domains might be imaged through lateral force measurements. The inner structure of condensed-phase domains czm be visual-... 

However, the pressure and normal stress contributions in normal force measurements on surfaces cannot be separated. Hence, the only quantities of experimental interest are the shear stress and two normal stress differences. Assuming that the flow is in the x direction, the stresses usually used in conjunction with shear flow are as follows. 

Figure 14 Normalized force measured between hydropho-bized mica surfaces in crossed cylinder geometry coated with, P-casein in a solution containing 0.1 mgP-casein/ml (pH = 7 1 mM NaCl) ( , 0) and after dilution with 1 mMNaCl ( , o)- Filled and imfiUed symbols represent the force measured on compression and decompression, respectively represent the force measured between hy-drophobized mica surfaces across a 0.1 mM NaCl solution at pH 5.6 containing 0.2 mg proteoheparan sulfate/ml. The inset shows the measured forces between adsorbed layers of P-casein before and after dilution with 1 mM NaCl on an expanded scale. (From Ref. 26, with permission.)...

In the work reported by Jackson and Kaye [1966] the spacing, h, between a cone and a plate was varied and the normal force measured as a function of gap size. The same method was used by Marsh and Pearson [1968] who showed that. 

An interesting experimental observation was noted by Magda et al, [32] regarding the effect of smoothness of the rheometer plates. For a solution of flexible polymers, two sets of plates gave identical linear normal stress profiles, as required to extract from a combination of point normal force measurements and total thrust measurements. However for the lyotropic solutions, only the smoother plate yielded a linear profile. They concluded that although it is possible to impose... 

Pin-on-disk devices consist of a rotating disk onto which a pin or ball is pressed at a constant normal force. Measurement of the normal force and of the angular momentum permits the determination the friction coefficient. 

Because of this usefulness it is important to have relatively simple experiments available to obtain the VL function. Kearsley and Zapas (97) have shown that either simple extension combined with simple compression or torsion with normal force measurements can be used to obtain the VL function. Valanis and Landel (98) used pure shear measurements. The equations for the torsional measurements arise from the relationship between w X) and Wi and W2 given by... 

Fig. 32. The VL function (98) derivative vs stretch for natural rubber samples cured with 1, 5, and 15 parts per hundred dicumyl peroxide, as indicated. The VL function was obtained from torque and normal force measurements, o APHRl A APHR5 0 APHR15. After McKenna et al. (102).

Fig. 56. Values of the time and strain-dependent strain energy function derivatives dW/dli and 9W/9/2 for a glassy PMMA determined from torque and normal force measurements in single-step stress relaxation torsional experiments. After McKenna (114).

Fig. 57. Values of the time and strain-dependent strain energy function derivatives Wi = 9W/9/1 and W2 = 9W/9/2 for a glassy polycarbonate determined from torque and normal force measurements in single-step stress relaxation torsional experiments, (a) >/ 0.017 0.033 A 0.050 v 0.067 0.083 O 0.10. (b) y A 0.017 0.033 o 0.050 T 0.067 v 0.083 0.10. After Pesce and McKenna (146).

Quantitative measurements of nano-scale frictional properties of pure and mixed SAMs on Au (111) were achieved by in situ normal and lateral force calibration of AFM/FFM. For pure SAMs, the friction coefficients for the same alkanethiol system but with different tips, differ by less than 15%, indicating the reliability of nano-scale frictional and normal force measurements using a scanning force microscope. The friction coefficient increases as the chain length decreases as also found previously by other workers. Tip-based molecular dynamics simulations were carried out to interpret the chain length dependence on frictional properties of alkanethiols. Simulation results show that AFM/FFM tip penetrates deeper into films formed by shorter chain SAMs, causing higher friction. 

Harsch, M., Karger-Kocsis, J., Herzog, F., Fejos, M., 2011. Effect of cure regime on internal strain and stress development in a filled epoxy resin assessed by fiber Bragg-grating optical strain and normal force measurements. Journal of Reinforced Plastics and Composites 30, 1417-1427. 

Due to enormous measurement difficulties, the elasticity of melt polymeric systems has not been studied to the same extent as shear measurements (Dealy and Wissbum 1990 Shenoy 1999). Some of the common methods of evaluation of the elasticity include normal force measurements, capillary flow entrance pressure drop and extrudate swell. The approach by Rahman et al. was to measure Ni, as an indication of the elasticity of melt polymers in steady shear experiment. 

Niemiec, J. M., Pesce, J.-J, McKenna, G. B. Anomalies in the normal force measurement when using a force rebalance transducer. /. Rheol. (1996) 40, pp. 323-334... 

Figure 30 shows typical torque and normal force measurements for a natural rubber sample as functions of the deformation (as x/rR) in a double logarithmic plot (96). The data for T, N, and their derivatives can then be obtained and used in equations 47 and 48 to solve for Wi and W2. Figure 31 shows typical data for the values of Wi and W2 as functions of deformation for a cross-linked natural rubber obtained from such measurements (94,97). Importantly, these values are related to the damping function of the DE model discussed subsequently. They represent the deformation dependence of the shear modulus in this instance. 

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