Interaction capillary

The LOFO approach, based on capillary interactions induced by liquid-solid interfaces, is used for transferring prefabricated thin solid metal films onto molecu-larly modified solid substrates. In spite of the fact that the glass/metal pad during the lift-off process leaves a relatively rough (1 nm) surface, several types of device have been fabricated by LOFO [154-156]. 

P. A. Kralchevsky and K. Nagayama Capillary Interactions Between Particles Bound to Interfaces, Liquid Films and Biomembranes. Adv. Colloid Interface Sci. 85, 145 (2000). 

If one accepts the premise that self-assembly will be an important component of the formation of nanomaterials, it is clearly important to understand it as a process (or, better, class of processes). The fundamental thermodynamics, kinetics, and mechanisms of self-assembly are surprisingly poorly understood. The basic thermodynamic principles derived for molecules may be significantly different for those that apply (or do not apply) to nanostructures the numbers of particles involved may be small the relative influence of thermal motion, gravity, and capillary interactions may be different the time required to reach equilibrium may be sufficiently long that equilibrium is not easily achieved (or never reached) the processes that determine the rates of processes influencing many nanosystems are not defined. 

There are several ways to reduce or suppress the electroosmotic flow in capillaries. These methods involve either eliminating the zeta potential across the solution-solid interface or increasing the viscosity at this interface. One approach is to coat the capillary wall, physically, with a polymer such as methylcellulose or linear polyacrylamide. Because of the difficulty in deactivating the capillary surface reproducibly, however, alternative methods employing dynamic reduction of solute-capillary interactions have been developed. Dynamic reduction of these interactions include the addition of chemical reagents such as methylhydroxyethylcellulose, S-benzylthiouro-nium chloride, and Triton X-100. 

This chapter describes experimental and conceptual issues in mesoscale self-assembly (MESA), using examples from our work in the assembly of millimeter- and micron(micrometer)-sized polyhedral objects using capillary forces. In MESA, objects (from nm to mm in size) self-assemble into ordered arrays through noncovalent forces. Three systems that use capillary forces in MESA are described these involve the assembly of objects into two-dimensional arrays at the perfluorodecalin/H20 interface, into three-dimensional arrays at curved liquid/liquid interfaces, and into three-dimensional arrays from a suspension in water. The capillary interactions between objects can be viewed as a type of bond that is analogous to chemical bonds that act between atoms and molecules. 

By definition, any interaction between objects that tends to hold them together is a bond. In our work, capillary interactions provide the bond between objects. In general, a bond between two molecules or objects has two components one attractive, and one repulsive (Fig. 4.3). At some distance, these forces are equal... 

Transport in unsaturated porous media is sufficiently complex when only two fluid phases, air and water, are present flow becomes even more complicated when a third fluid phase, such as an immiscible organic fluid, is involved. This third fluid phase (NAPL) arises when liquid hydrocarbon fuels or solvents are spilled accidentally on the ground surface or when they leak from underground storage tanks. The resulting subsurface flow problem then involves three fluids, air, water, and NAPL, each having different interfacial tensions with each other, different viscosities, and different capillary interactions with the soil. The adequate description of three-phase flow is still a topic of active research, but a few qualitative generalizations can be drawn. 

As the sample proceeds through the capillary, interacting molecules are retained for longer periods of time than those that do not interact, resulting in a larger elution volume required for the interacting SMs. This allows for... 

FIGURE 5.18 Plot of the capillary interaction energy in kT units, AW/kT, vs. the radius, R, of two similar particles separated at a center-to-center distance L = 2R. 

In the case of interactions between inclusions in lipid bilayers (Figure 5.19) the elasticity of the bilayer interior must also be taken into account. The calculated energy of capillary interaction between integral membrane proteins turns out to be of the order of several Hence, this interaction can be a possible explanation of the observed aggregation of membrane proteins. The lateral capillary forces have been calculated also for the case of particles captured in a spherical (rather than planar) thin liquid film or vesicle. ... 

FIGURE 5.20 Special types of immersion capillary forces (a) The contact line attachment to an irregular edge on the particle surface produces undulations in the surrounding fluid interface, which give rise to lateral capillary force between the particles, (b) When the size of particles entrapped in a hquid film is much greater than the nonperturbed fihn thickness, the meniscus surfaces meet at a finite distance, r in this case, the capillary interaction begins at L < 2rp. 

At last, let us consider another type of capillary interactions — between particles snrronnded hy finite menisci. Such interactions appear when micrometer-sized or snb-micrometer particles are captured in a liquid film of much smaller thickness (Fignre 5.20b). If such particles are approaching each other, the interaction begins when the meiusci aronnd the two particles overlap, L < 2rp in Figure 5.20b. The capillary force in this case is nonmonotonic initially the attractive force increases with the increase of interparticle distance then it reaches a maximum and further decays. In addition, there are hysteresis effects the force is different on approach and separation at distances aronnd L = 2rp. ... 

FIGURE 5.21 Experimental setup for studying the capillary interaction between a floating particle (1) and a vertical hydrophobic plate (2) separated at a distance, x. The edge of the plate is at a distance, H, lower than the level of the horizontal liquid surface far from the plate (3) and (4) are micrometric table and screw (see References 249 and 250 for details). 

The sources of discrepancy are sHding friction, capillary interactions, and the fact that we have a multiasperity contact. Rolling (simultaneous release of contacts on one side and formation of new ones at the other) may also be part of the picture. 

Moreover, Shi and his group reported electrochemical deposition of PPy microcontainers onto soap bubbles associated with O2 gas released from the electrolysis of H2O in an aqueous solution of /3-naphthalenesulfonic acid (/3-NSA), camphorsulfonic acid (CSA), or poly(styrene sulfonic acid) (PSSA), which act both the surfactant and dopant [79-81]. Morphologies such as bowls, cups, and bottles could be controlled by electrochemical conditions (Figure 11.6). However, the microcontainers were randomly located on the electrode surface, which limited further applications, Shi and coworkers reported a linear arrangement of PPy microcontainers by self-assembly with gas bubbles acting as templates on a silicon electrode surface patterned by photolithography [82]. They found that capillary interactions between the gas bubbles and the polymer photoresist walls led the microcontainers to be arranged linearly. 

Yunker, P.J., Still, T., Lohr, M.A., Yodh, A.G. Suppression of the coffee-ring effect by shape-dependent capillary interactions. Nature 476, 308-311 (2011)... 

As already established in previous sections, the surface-directed approach to microfluidic system fabrication produces autonomously functioning devices with little rigor associated regarding implementation. This is a direct implication of the fact that the driving force for flow exploits capillary interactions producing spontaneous... 

Figure 6.2 Examples of static self-assembly, (a) Crystal structure of a ribosome, (b) Self-assembled peptide-amphiphile nanofibers, (c) An array of millimeter-sized polymeric plates assembled at a water/perfluorodecalin interface by capillary interactions, (d) Thin film of a nematic liquid crystal on an isotropic substrate, (e) Micrometer-sized metallic polyhedral folded from planar substrates, (f) A 3-D aggregate of micrometer plates assembled by capillary forces.

We see from these data that the values of surface tension calculated from known forces of adhesion (which are equal to the capillary forces) are low in comparison with the actual values. The point is that these investigators failed to account for the disjoining pressure of the thin layer of liquid (see Fig. IV.6.c), which weakens the capillary interaction. This is why their method for determining surface tension by measurement of adhesive force did not give accurate results. 

Micellar electrokinetic chromatography (MEKC). The electrolyte contains a surfactant [e.g., sodium dodecyl sulfate (SDS)] above its critical micelle concentration. Micelles present in the capillary interact with the sample components. Differences in the interactions between micelles and various analytes give rise to separation. Some non-ionogenic species can also interact with the surfactant micelles, thus promoting separation. Due to its resemblance to chromatography, the micellar solution in MEKC is sometimes referred to as pseudo stationary phase. 

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