Force rate measurements

Equivalent circuit of ferroelectret film sensor Force rate measurements Oscilloscope... 

Fig. 7 Force measurement with a ferroelectret sensor coupled to a charge amplifier top). A rectangular varying compressive force is applied to the sample. Force rate measurement, when the ferroelectret sensor is connected to a digital oscilloscope bottom). Measurement signals can be calibrated to determine the piezoelectric coefficient of the ferroelectret foam sensor...

G. H. W. Sanders, J. Booth, and R. G. Compton, Quantitative Rate Measurement of the Hydroxide Driven Dissolution of an Enteric Drug Coating Using Atomic Force Microscopy. Langmuir 13 (1997), 3080-3083. 

Compressive strength, or ability of a specimen to resist a crushing force, is measured by crushing a cylindrical specimen (ASTM-D-695) as shown in Figure 14.14. Here a sample of specified dimensions is placed between two heads, one movable and one set. Force is applied to the movable head moving it at a constant rate. The ultimate compression strength is equal to the load that causes failure divided by the minimal cross-sectional area. Since many materials do not fail in compression, strength reflective of specific deformation is often reported. 

Figure 2. (Left) Experimental setup in force flow measurements. Optical tweezers are used to trap beads but forces are applied on the RNApol-DNA molecular complex using the Stokes drag force acting on the left bead immersed in the flow. In this setup, force assists RNA transcription as the DNA tether between beads increases in length as a function of time, (a) The contour length of the DNA tether as a function of time and (b) the transcription rate as a function of the contour length. Pauses (temporary arrests of transcription) are shown as vertical arrows. (From Ref. 25.) (See color insert.)...

Modeling and measuring the generation of mechanical force (rate and magnitude) by gels in terms of accessible parameters like modulus and solubility parameters. 

In the second plot, the energy required by the primary compressor is shown to be sensitive to the pressure in the evaporator-freezer and condenser-melter. Because both the evaporation and condensation are by direct contact of the refrigerant with water or ice, the opportunity exists for realistic achievement of a low over-all driving force. Experimentally, the evaporation has been found to occur so rapidly that a rate measurement has not been practical. The melting of the ice is more of a problem because the condensate film is a heat transfer barrier. However, the barrier is a fluid and not a stationary wall as in a distillation unit, wherein steam is condensed against water or boiling water. 

Equations 3.12, 3.13, and the final Equation 3.14 are all forms of the classic engineering expression [Rate = (Driving force)/Resistance] where the driving force is expressed as concentration differences. The overall resistance (1 /K ) can be controlled by a low value of either individual coefficient. The mass transfer coefficient (kd) controls crystallization when the reaction is very rapid relative to diffusion, but the reaction coefficient ( r) controls crystallization when diffusion is much more rapid than reaction. In such cases the overall coefficient K may be approximated by the smaller k value. However, the concentrations in the driving force remain measurable (c) or calculable (ceq) rather than non-measureable (c ). 

Here, max and jrm n denote, respectively, the maximum and the minimum values of the muscular activation, a determines the slope of the feedback curve, S is the displacement of the curve along the flow axis, and Fneno is a normalization value for the Henle flow. The relation between the glomerular filtration and the flow into the loop of Henle can be obtained from open-loop experiments in which a paraffin block is inserted into the proximal tubule and the rate of glomerular filtration (or, alternatively, the so-called tubular stop pressure at which the filtration ceases) is measured as a function of an externally forced rate of flow of artificial tubular fluid into the loop of Henle. Translation of the experimental results into a relation between muscular activation and Henle flow is performed by means of the model, i.e., the relation is adjusted such that it can reproduce the experimentally observed steady state relation. We have previously discussed the significance of the feedback gain a in controlling the dynamics of the system, a is one of the parameters that differ between hypertensive and normotensive rats, and a will also be one of the control parameters in our analysis of the simulation results. 

Fig. 8. (A) Force normalised by radius as a function of surface separation for a streptavidin surface interacting with 5% (o) and 0.5% ( ) biotin surface in 0.3 mM salt at pH 7.2 and 33°C, at approach rates higher than 10 A/s. The equilibrium force profile measured at approach rates less than 10 A/s is shown in the inset. It demonstrates the absence of time-dependent steric barrier. (B) Illustration of the biotin and streptavidin molecular arrangement during approach and in strong adhesive contact. Redrawn with permission from Ref. [109]. 1994, American Chemical Society.

In these experiments, the tensile force is measured as a function of time, so that at a constant rate of deformation e it is possible to calculate the true tensile stress and the extensional viscosity r/c elastic properties of the deformation can be determined by measuring the elastic strain e. 

Rate measurements for these two compounds are very revealing. We can force them to react by SN.l by using methanol as the solvent (p. 414). If we set the rate of substitution of the benzyl compound with methanol at 25 °C at 1.0, then the 4-MeO benzyl compound reacts about 2500 times faster and the 4-N02 benzyl compound about 3000 times more slowly. 

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