Surface force device

The technological importance of thin films in snch areas as semicondnctor devices and sensors has led to a demand for mechanical property infonnation for these systems. Measuring the elastic modnlns for thin films is mnch harder than the corresponding measurement for bnlk samples, since the results obtained by traditional indentation methods are strongly perturbed by the properties of the substrate material. Additionally, the behaviour of the film under conditions of low load, which is necessary for the measnrement of thin-film properties, is strongly inflnenced by surface forces [75]. Since the force microscope is both sensitive to surface forces and has extremely high depth resolntion, it shows considerable promise as a teclnhqne for the mechanical characterization of thin films. 

The development of a host of scanning probe devices such as the atomic force microscope (AFM) [13-17] and the surface forces apparatus (SFA) [18-22], on the other hand, enables experimentalists to study almost routinely the behavior of soft condensed matter confined by such substrates to spaces of molecular dimensions. However, under conditions of severe confinement a direct study of the relation between material properties and the microscopic structure of confined phases still remains an experimental challenge. 

The surface force apparatus (SFA) is a device that detects the variations of normal and tangential forces resulting from the molecule interactions, as a function of normal distance between two curved surfaces in relative motion. SFA has been successfully used over the past years for investigating various surface phenomena, such as adhesion, rheology of confined liquid and polymers, colloid stability, and boundary friction. The first SFA was invented in 1969 by Tabor and Winterton [23] and was further developed in 1972 by Israela-chivili and Tabor [24]. The device was employed for direct measurement of the van der Waals forces in the air or vacuum between molecularly smooth mica surfaces in the distance range of 1.5-130 nm. The results confirmed the prediction of the Lifshitz theory on van der Waals interactions down to the separations as small as 1.5 nm. 

The various surface forces are seen to be responsible for capillary rise. The lower the surface tension, the lower the height of the column in the capillary. The magnitude of y is determined from the measured value h for a fluid with known pL. The magnitude of h can be measured directly by using a suitable device (e.g., a photograph image). 

The second device with which surface forces can be measured directly and relatively universally is the atomic force microscope (AFM) sometimes also called the scanning force microscope (Fig. 6.8) [143,144], In the atomic force microscope we measure the force between a sample surface and a microfabricated tip, placed at the end of an about 100 //,m long and 0.4-10 //,m thick cantilever. Alternatively, colloidal particles are fixed on the cantilever. This technique is called the colloidal probe technique . With the atomic force microscope the forces between surfaces and colloidal particles can be directly measured in a liquid [145,146], The practical advantage is that measurements are quick and simple. Even better, the interacting surfaces are substantially smaller than in the surface forces apparatus. Thus the problem of surface roughness, deformation, and contamination, is reduced. This again allows us to examine surfaces of different materials. 

A complete model for particle-surface interaction would include both (a) Huid mechanical effects as Lhe particle approaches the surface and surface forces. The Hu id mechanical calculations would take into account free molecule effects as the particle comes to within one mean free path of the surface. The presence of thin films of liquids and surface irregularities further complicate the situation. In practice, the design of cascade impactors (Chapter 6) and other devices in which rebound may be important is carried out empirically, by experimenting with various particle.s, coatings, and collecting surfaces. 

Multiphase/multifluid flows are ubiquitous in microsystems since, as sizes continue to reduce in such devices, surface to volume ratio increases and thus surface forces become dominant over the volume forces. It is therefore of great importance to understand and manipulate multiphase/multifluid flows in micro channels [1, 2]. 

In phase separation two immiscible fluids are physically separated. Microchannels offer the ability to separate phases in an orientation-independent manner, since capillary and surface tension forces are more dominant in these high-surface-area devices. Various microchannel phase separators have been developed to separate organic and aqueous phases for use in unit processes such as solvent extraction or reactions conducted at an aqueous organic interface [185-188]. The approach is to hydrophobize half of the channel with a non-polar agent so that the organic phase is constrained to the hydrophobic half and the aqueous phase to the hydrophilic half Phase separation is simply then a matter of splitting the flow at the hydrophobic-hydrophilic junction of the flow. 

Figure 6.30 AFM laser beam detection device Schematic illustration to show how force evolution between sample and tip may be detected by displacements in the position of a laser beam reflected from a mirror fixed to the rear of the cantilever. Movements in laser position are a very sensitive measure of tip to surface distance changes, Az, as a result of changes in tip to surface forces, ST-...

An interesting device for application in distillation was fabricated by milling on a silicon substrate [34]. The chamber was closed by a glass plate using anodic bonding. Methanol/water mixtures were used as a model system. In this device wall effects achieve separation of gas and liquid. The liquid is collected near the wall whereas the gas is withdrawn at a central cavity. Liquid moves to the wall by surface forces and gravity. The rectangular separation chamber was equipped with a liquid inlet, and a liquid outlet was located in the lower part of the device whereas the gas phase leaves at the top. 

Fig. 2 Apparatus to measure surface forces between two mica surfaces (SFA). (The figure is reproduced with kind permission from ]. Klein [33].) This particular device was built to study the behavior of liquid crystals confined in the narrow gap between the mica surfaces.

With any surface force-measuring device, the total force between two surfaces is determined. The origin of all surface forces is the interaction between electric... 

Several devices for measuring surface forces have been developed, including the surface force apparatus [13,14], the force balance [15], the osmotic stress method, and the total internal reflectance microscope [16]. But in all these methods there are limitations. 

The development of microfluidics-based Lab-on-Chip devices involves the incorporation of many of the necessary components and functionality of a typical laboratory onto a small chip-sized substrate. When experiments are carried out in a Lab-on-Qiip, forces are needed to drive liquids to flow through microchannels. Microfluidic flows are readily manipulated using many kinds of external field (pressure, electric, magnetic, capillary, and so on). As the dimensions shrink, the importance of surface forces relative to volume forces increases. Such... 

Surface forces tend to be inherently important for capillary-driven microfluidic devices as surface area-to-volume ratios are rather high and surface tension-induced interfacial curvature is significant enough to promote capillary wicking. For chemically patterned devices depending on hydrophobic/hydrophilic confinement in particular, the capillary number must be sufficiently low (i.e., Ca 1) in order to maintain fluid confinement within intricate geometry implying AF < ylw [1],... 

Shen, Y., Xi, N., Wejinya, U. and Li, W. (2004). High sensitivity 2-D force sensor for assembly of surface MEMS devices, in Proceedings of lEEE/RSJ International Conference on Intelligent Robots and Systems (Sendai, Japan), pp. 3363-3368. 

The study of fundamental adhesion has been hampered because standard Tests of adhesion provide a result that is a complicated combination of fundamental adhesion, the physical properties of the adherend and the viscoelastic/plastic character of the adhesive (see Adhesion - fundamental and practical, Peel tests). Our understanding of adhesion has been significantly improved with the advent of mechanical devices that are able to probe the forces of adhesion under conditions that minimize all of the confounding effects of adherend, viscoelasticity, and so on. The Surface Forces Apparatus (SFA) as developed by Israelachvili and Tabor is a mechanical device that has allowed adhesion scientists to directly measure the forces of adhesion under very low rate, light loading, almost equilibrium conditions. Attention is also drawn to Atomic force microscopy. 

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