Force measurement indirect methods

Methods used to demonstrate the existence of membrane phospholipid asymmetry, such as chemical labelling and susceptibility to hydrolysis or modification by phospholipases and other enzymes, are rmsuitable for dynamic studies because the rates of chemical and biochemical reactions are of a different order compared to the transmembrane translocahon of the phospholipids. Indirect methods have therefore been developed to measure the translocation rate which are consequent on the loss of membrane phospholipid asymmetry. Thus time scales appropriate to rates of lipid scrambling under resting conditions or when the forces preserving the asymmetric phospholipid distribution are disturbed can be monitored. Generally the methods rely on detecting the appearance of phosphatidylserine on the surface of cells. Methods of demonstrating Upid translocation in mammalian cells has been the subject of a recent review (Bevers etal., 1999). 

Indirect methods are those in which we use some macroscopic property or phenomenon to deduce information about (colloidal or intermolecular) interaction forces. These methods are known as indirect since additional assumptions or approximations about the relation between the forces and the property or phenomenon measured are needed. Some examples of these methods are illustrated in Figure 1.26 and are described below (Israelachvili 1991). 

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. 

Determination of the activity coefficients of the non-volatile solute in a solution is difficult. If electrolytes (ions) are present, the activities can be obtained from experimental electromotive force (EMF) measurements. However, for non-electrolyte and non-volatile solutes an indirect method is applied to find initially the activity of the solvent over a range of solute concentrations, and then the Gibbs-Duhem equation is integrated to find the solute activity. If the solution is saturated, then it is easy to calculate the activity coefficient... 

The surface molecules of solids are practically fixed in position, and contrary to the behavior of liquid molecules, they cannot move to any other place. Individual atoms and molecules are only able to vibrate around their mean position. As a result, solid surfaces cannot spontaneously contract to minimize their surface area, and a non-equilibrium surface structure forms. This situation is quite distinct from that of a liquid surface, which attains equilibrium almost as soon as it is formed because of the mobility of the surface molecules. However, this does not mean that surface tension is absent in solids. In principle, surface tension also exists in all solids and the inward pull on the solid surface atoms is always present, owing to cohesion, exactly as in liquids. Nevertheless, the changes of surface shape due to surface tension are very much slower in solids than in liquids this is not because the cohesion forces are smaller but because the mobility of the surface molecules (or atoms) is very much less. For this reason, measurement of the solid surface tension is a difficult and ambiguous procedure, and indirect methods are mostly applied (see Section... 

Basically two different types of experimental approaches have been used to study the boundary shp local (direct) [45,60] and effective (indirect) methods [49-52,61]. The first group of methods is based on apphcation of optical techniques using tracer particles or molecules to determine the flow field. These techniques have a resolution of less than lOOnm, so they cannot distinguish small differences in slip lengths. The effective methods assume the boundary conditions (Eq. 18) or similar ones to hold at the substrate surface and infer the slip length by measuring macroscopic quantities. These methods have been the most popular so far and they include atomic force microscopy (AFM), surface force apparatus (SEA), capillary techniques, and QCM. 

Tangential force coefficients can be determined by direct measurement of the tangential tractive force and the normal loading force of the system, but the ratio A /A is usually obtained by indirect methods. McFarlane and Tabor [9] found that the adhesion of indium to steel under the influence of a normal preload was so strong that the tractive force to initiate sliding after removal of the preload was essentially a measure of the shear force in indium. From this measurement they calculated a value of a =3.3 for indium. Courtney-Pratt and Eisner [10] evaluated A /A by electrical resistance and obtained a value of a = 11.66 for platinum. 

In conclusion, it should be pointed out that none of the physicochemical techniques discussed above permits the direct measurement of the elements of the polymeric materials porous structure we measure the properties of the systems where the polymers interact with certain test substances (nitrogen, mercury, water, polystyrene standards, ions, etc.), and not the dimensions of the pores or other supramolecular elements of the material. Therefore, the evaluation of the surface area and diameters of pores available to the molecules of these substances must be considered as indirect methods of examining the porous structure. Because of this, all calculations are based on assuming certain models of the structure of the material and accepting certain assumptions as to the mechanism of interaction between the material and test molecules. Only transmittance, scanning, and, in particular, atomic force microscopy can be considered as direct methods of measuring dimensions and distances. However, up to now the last technique has not been appHed to microporous hypercrosslinked polymers. 

In this section we summarize experimental methods that enable measuring (depletion) interaction potentials between particles [64]. We distinguish pair interactions (Sects. 2.6.1-2.6.3) and many-body interactions (Sect. 2.6.4). The latter can be measured indirectly using scattering techniques or microscopy, whereas for pair interactions direct methods are available. Common instruments for investigating such pair interactions are the surface force apparams (SFA) [65], optical tweezers [66, 67], atomic force microscopy (AFM) [68], and total internal reflection microscopy (TIRM) [69, 70]. 

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.

---