Force measurement direct methods

Film pressure is often measured directly by means of a film balance. The principle of the method involves the direct measurement of the horizontal force on a float separating the film from clean solvent surface. The film balance has been considerably refined since the crude model used by Langmuir and in many... 

There are some exceptions to this method. For example, it is possible to apply forces directly to the end of the cantilever rather than displacements to the sample in order to control the approach and separation of the two surfaces [29,30]. This more direct method reduces unwanted relative lateral motion between the tip and the surface. The application of direct forces in this way also has alternative uses, such as enabling sensitive dynamic measurements to be made [29],... 

We introduced the technique for measuring the weak interaction forces acting between two particles using the photon force measurement method. Compared with the previous typically used methods, such as cross-correlation analysis, this technique makes it possible to evaluate the interaction forces without a priori information, such as media viscosity, particle mass and size. In this chapter, we focused especially on the hydrodynamic force as the interaction between particles and measured the interaction force by the potential analysis method when changing the distance between particles. As a result, when the particles were dose to each other, the two-dimensional plots of the kinetic potentials for each particle were distorted in the diagonal direction due to the increase in the interaction force. From the results, we evaluated the interaction coeffidents and confirmed that the dependence of the... 

In 1916 Langmuir published his well-known paper on The fimdamental properties of liquids, in which he accepted the conclusion that films of oils on water were unimolecular and made further experiments on the subject by a direct method. Pockels and Rayleigh had measured the surface tension of their films by determining the force necessary to break them, a method open to some obvious objections. The French investigators did not as a rule measure the tensions of the films at all but determined their... 

Three types of measurements are usually used for the study of thermomechanical behaviour of polymers the temperature changes resulting from the instantaneous loading of the sample, the temperature dependence of the stress or force and direct calorimetric measurements of heat effects in various deformation modes. Detailed discussion of the first and second types of thermomechanical measurements may readily be found in the literature and, therefore, we confine ourselves to a brief description of typical procedures. As to the third method, we will describe it in... 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

Up to date, besides the SFA, several non-interferometric techniques have been developed for direct measurements of surface forces between solid surfaces. The most popular and widespread is atomic force microscopy, AFM [14]. This technique has been refined for surface forces measurements by introducing the colloidal probe technique [15,16], The AFM colloidal probe method is, compared to the SFA, rapid and allows for considerable flexibility with respect to the used substrates, taken into account that there is no requirement for the surfaces to be neither transparent, nor atomically smooth over macroscopic areas. However, it suffers an inherent drawback as compared to the SFA It is not possible to determine the absolute distance between the surfaces, which is a serious limitation, especially in studies of soft interfaces, such as, e.g., polymer adsorption layers. Another interesting surface forces technique that deserves attention is measurement and analysis of surface and interaction forces (MASIF), developed by Parker [17]. This technique allows measurement of interaction between two macroscopic surfaces and uses a bimorph as a force sensor. In analogy to the AFM, this technique allows for rapid measurements and expands flexibility with respect to substrate choice however, it fails if the absolute distance resolution is required. 

The interferometric SFA has served as an invaluable tool in studying the hydrophobic attraction among other things due to the fact that it is the only technique available today that enables direct observation of occurrence of cavitation. For instance, recently Lin et al. [89] employed a dynamic surface forces measurement method to study interactions between DODAB LB coated surfaces. High-speed camera images of FECO revealed that there are no bubbles on the surfaces prior to contact. However, short-lived cavities, typically lasting 3 ps before disappearing, have been observed to form upon separation (Fig. 6). 

The phenomenon of cavitation was also observed by Christenson and Claesson [82], around 20 years ago, who used DODAB LB films and the equilibrium method for forces measurements. The direct measurement of forces between hydrophobhic surfaces in water were rather recently thoroughly reviewed by the above authors [94] and the reader who is interested in this intriguing topic is referred to this review and many references therein. 

Until fairly recently, the theories described in Secs. II and III for particle-surface interactions could not be verified by direct measurement, although plate-plate interactions could be studied by using the surface forces apparatus (SFA) [61,62]. However, in the past decade two techniques have been developed that specifically allow one to examine particles near surfaces, those being total internal reflection microscopy (TIRM) and an adapted version of atomic force microscopy (AFM). These two methods are, in a sense, complementary. In TIRM, one measures the position of a force-and torque-free, colloidal particle approximately 7-15 fim in dimension as it interacts with a nearby surface. In the AFM method, a small (3.5-10 jam) sphere is attached to the cantilever tip of an atomic force microscope, and when the tip is placed near a surface, the force measured is exactly the particle-surface interaction force. Hence, in TIRM one measures the position of a force-free particle, while in AFM one measures the force on a particle held at a fixed position. 

The advent of the atomic force microscope has allowed surface properties at nearly molecular length scales to be measured directly for the first time. Recently, a method has been proposed whereby a small ( 3.5 /nn) particle is attached to the cantilever tip of the commercially available, Nanoscope II AFM [67,68]. The particles are attached with an epoxy resin. When the cantilever tip is placed close to a planar surface, the AFM measures directly the interaction force between the particle and the surface. A primary difference between this technique and the surface forces apparatus (SFA) is the size of the substrates, since the SFA generally requires smooth surfaces approximately 2 cm in diameter. Other differences are discussed by Ducker et al. [68]. For our purposes, it suffices to note that the AFM method explicitly incorporates the particle-wall geometry that is the focus of this chapter. 

FTMA is a forced vibration test method based on direct measurement of stress and strain spectra. As with all forced vibration methods, FTMA is subject to spurious wave effects at high frequencies. The lower frequency limit is determined by transducers, signal conditioners, etc. The lower limit in this research was 35 Hz as determined by the inherent properties of the piezoelectric transducers. With different transducers (for example load cell for the force and LVDT for displacement measurements) and signal conditioners, FTMA should measure material properties down to much lower frequencies. 

Unlike the surface force apparatus the AFM measurement does not have a direct method to detect the distance between the surfaces. Instead, one will take the linear signal of cantilever deflection against sample displacement as the origin of the surface distance The linear signal results because the two surfaces are in contact and move together. This manner of wall detection usually works well. However, what would result if the freezing point elevation is the case ... 

The heat evolved on combustion, i.e. on the complete oxidation of combustible substances, is of particular interest. From the heat of combustion of an organic compound we can calculate its heat of formation. This quantity is of considerable importance, but cannot be measured directly, as the preparation of organic compounds from the elements is either impossible, or only possible under conditions which cannot be reproduced inside a calorimeter. It is necessary to emphasise the fact that only very rapid reactions can be treated calorimetrically, for the interchange of heat with the surroundings during a slow reaction becomes so great that even approximate measurements are impossible. As most organic reactions are very slow, we are forced to determine the heat of reaction by an indirect method, viz. from the determination of the heat of combustion. 

The force at the barrier may be measured directly by a calibrated torsion wire that is mechanically attached to the barrier. However, nowadays the surface tensions at both sides of the barrier are measured Independently using one of the appropriate methods in sec. 1.8, mostly by the static Wilhelmy plate technique. The latter method has the advantage that einy leakage of monolayer material across the barrier can be easUy detected. 

Fig. 1.2. Determining the surface tension of a liquid by direct force measurements a extended wire of diameter 2R, b ring method according to Lecomte du NoOy (1919)...

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