Surface forces balance

The texture and structure of foods is very delicate, therefore experimental methods which cause no or vety little structural damage has to be applied in their investigations. Such techniques, the surface force balance and back-light scattering methods and dielectrometry will be also discussed in the chapter. 

The structure and stability of foamed emulsions, such as whipped cream, ice cream or whipped toppings, strongly depend on the interparticle interactions and on the orientation of drops/particles at the foam films. Further development of the surface force balance and... 

Complementary to the SFA experiments, SFM techniques enabled direct, non-destructive and non-contact measurement of forces which can be as small as 1 pN. Compared to other probes such as optical tweezers, surface force balance and osmotic stress [378-380], the scanning force microscope has an advantage due to its ability in local force measurements on heterogeneous and rough surfaces with excellent spatial resolution [381]. Thus, a force-distance dependence measured from a small surface area provides a microscopic basis for understanding the macroscopic interfacial properties. Furthermore, lateral mapping... 

Figure 2. Surface force balance for measurements offerees between the quartz plates A, and A2. (From Peschel and Adifinger, 1970a).

DI) which is used to measure the film curvature (2). A sketch of the surface-force balance experimental set-up is shown in Fig. 6. For measuring oil-in-water emulsions the inner capillary of the cell is filled with oil phase, the bottom part of the outer capillary is filled with water phase, and the top part of the outer capillary if filled with oil phase. A curved film is formed by drawing the oil phase from the... 

The equation was originally derived for the homogeneous solid-liquid interface (no air pockets) (Fig. 6a) using the surface force balance and empirical considerations however, it was later put in a proper thermodynamic framework. It is important that according to Wenzel model the inherently hydrophilic flat surface will be more hydrophilic when rough, and inherently hydrophobic surface will become more hydrophobic. 

Intermolecular and surface forces have been studied for a long time using different methods, including osmotic stress," -" surface force balance (SFB)," " scanning probe techniques, "- optical tweezers, " "- magnetic traps, " and scattering methods. Each method has capabilities and linritarions, and the information obtained with each is often complementary to the others. 

The flow can be radial, that is, in or out through a hole in the center of one of the plates [75] the relationship between E and f (Eq. V-46) is independent of geometry. As an example, a streaming potential of 8 mV was measured for 2-cm-radius mica disks (one with a 3-mm exit hole) under an applied pressure of 20 cm H2 on QT M KCl at 21°C [75]. The i potentials of mica measured from the streaming potential correspond well to those obtained from force balance measurements (see Section V-6 and Chapter VI) for some univalent electrolytes however, important discrepancies arise for some monovalent and all multivalent ions. The streaming potential results generally support a single-site dissociation model for mica with Oo, Uff, and at defined by the surface site equilibrium [76]. 

The wettabiHty theory of adhesion is iaextricably related to the study of contact angles of Hquids on soHd surfaces. A force balance at the poiat of contact between the Hquid and the soHd can be written (3)... 

The most successful models are based on the finite element method. The flow is discretized into small subregions (elements) and mass and force balances are appHed in each. The result is a large system of equations, the solution of which usually gives the speed of the coating Hquid in each element, pressure, and the location of the unknown free surfaces. The smaller the elements, the more the equations which are often in the range of 10,000 to upward of 100,000. 

Forced-Recirculation Reboilers In forced-recirculation reboilers, a pump is used to ensure circiilation of the liquid past the heattransfer surface. Force-recirculation reboilers may be designed so that boiling occurs inside vertical tubes, inside horizontal tubes, or on the shell side. For forced boihng inside vertical tubes. Fair s method (loc. cit.) may be employed, making only the minor modification that the recirculation rate is fixed and does not need to be balanced against the pressure available in the downcomer. Excess pressure required to circiilate the two-phase fluid through the tubes and back into the column is supphed by the pump, which must develop a positive pressure increase in the hquid. 

Equilibrium is established when the attractive surface forces are balanced by elastic repulsion forces between the materials. The DMT model states that the elastic repulsion force is related to the attractive force within the contact region Fs by... 

As shown in the free-body diagram of Figure 2-15b, all the motion of the block is parallel to the surface of the ramp thus there is a static force balance in the y direction. 

For the flow of liquid of depth s down a plane surface of width w inclined at an angle 0 to the horizontal, as shown in Figure 3.19, a force balance in the X-direction (parallel to the surface) may be written. In an element of length dr the gravitational force acting on that part of the liquid which is at a distance greater than y from the surface is given by ... 

For a fluid film of thickness s flowing down a vertical surface of length l and width w, a force balance on the fluid at a distance greater than y from the surface (fluid depth a - v) gives ... 

Surface forces measurement directly determines interaction forces between two surfaces as a function of the surface separation (D) using a simple spring balance. Instruments employed are a surface forces apparatus (SFA), developed by Israelachivili and Tabor [17], and a colloidal probe atomic force microscope introduced by Ducker et al. [18] (Fig. 1). The former utilizes crossed cylinder geometry, and the latter uses the sphere-plate geometry. For both geometries, the measured force (F) normalized by the mean radius (R) of cylinders or a sphere, F/R, is known to be proportional to the interaction energy, Gf, between flat plates (Derjaguin approximation). 

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