Fluid interface

The first inhalation of a newborn baby requires an extraordinary effort the lungs become inflated and a large fluid/air interface is created. This is opposed by the interfacial tension of the alveoli. To overcome this difficulty, a special lung surfactant is released from alveolar cells and is spread to form a mono-layer at the surface of the alveoli. It reduces the interfacial tension and provides the alveoli with the proper interfacial rheological characteristics that allow easy breathing. 

The notion monolayer usually refers to a layer of amphiphilic molecules at a fluid interface, being either a liquid/liquid or a liquid/gas interface. Even when the layer is incomplete or it is more than one molecular layer thick, it is still called a monolayer. The term monolayer may as well be used in case of adsorption at solid surfaces to distinguish it from a bilayer or multilayer. 

This chapter deals primarily with monolayers of surfactants at fluid interfaces, but some attention is also given to (nano) coatings such as Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) films, self-assembled monolayers (SAMs), and layers obtained by alternating polyelectrolyte deposition. Such coatings may be applied for the functionalization of surfaces, for instance, to achieve biocompatibility of biomaterials, improve specificity and selectivity of biosensors and membranes, and control immobilization of enzymes or cells in bioreactors. 

Monolayers demonstrate well how interactions on a molecular level affect macroscopic phenomena. For instance, a monolayer of oil on the surface of an ocean damps the waves, and precious water supplies in ponds and lakes may be conserved by a monolayer that retards evaporation of the water underneath. 

Studying monolayers provides information on the orientation and association of the amphiphiles at interfaces. This information may also be useful for understanding self-assembled structures of such amphiphilic compounds (Chapter 11) as well as the role they play in the formation and stabilization of emulsions and foams (Chapter 18). 

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Surface tension arises at a fluid to fluid interface as a result of the unequal attraction between molecules of the same fluid and the adjacent fluid. For example, the molecules of water in a water droplet surrounded by air have a larger attraction to each other than to the adjacent air molecules. The imbalance of forces creates an inward pull which causes the droplet to become spherical, as the droplet minimises its surface area. A surface tension exists at the interface of the water and air, and a pressure differential exists between the water phase and the air. The pressure on the water side is greater due to the net inward forces... 

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

S. Toxvaerd. The structure and thermodynamics of a solid-fluid interface. J Chem Phys 74 1998-2005, 1981. 

P. Tarazona, U. Marini Bettolo Marconi, R. Evans. Phase equilibria of fluid interfaces and confined fluids. Non-local versus local density functionals. Mol. Phys (50 573-595, 1987. 

D. E. Sulhvan, M. M. Telo da Gama. Wetting transition and multilayer adsorption at fluid interfaces. In C. A. Croxton, ed. Fluid Interfaeial Phenomena. New York Wiley, 1986. 

R. Gatignol, P. Seppecher. Modelisation of fluid-fluid interfaces with material properties. Journal de Mecanique Theorique et Appliquee, Numero Special, 1986, pp. 225-247. 

T. L. Chester and J. D. Pinkston, Pressure-regulating fluid interface and phase behavior considerations in the coupling of packed-column supercritical fluid chromatography with low-pressure detectors , ]. Chromatogr. 807 265-273 (1998). 

In general, adsorption is a surface phenomenon, where gas or liquid is concentrated on the surface of solid particles or fluid interfaces. There are many adsorption systems. 

The area of colloids, surfactants, and fluid interfaces is large in scope. It encompasses all fluid-fluid and fluid-solid systems in which interfacial properties play a dominant role in determining the behavior of the overall system. Such systems are often characterized by large surface-to-volume ratios (e.g., thin films, sols, and foams) and by the formation of macroscopic assembhes of molecules (e.g., colloids, micelles, vesicles, and Langmuir-Blodgett films). The peculiar properties of the interfaces in such media give rise to these otherwise unlikely (and often inherently unstable) structures. 

The current status and the emerging opportunities in the science of colloids, surfactants, and fluid interfaces can be addressed conveniently by considering a threefold hierarchy of systems as follows ... 

This last category may also include fluid interface systems with unstructured bulk phases and/or moderate surface-to-volume ratios. 

Mathpati, C.S. and Joshi, J.B. (2007) Insight into theories of heat and mass transfer at the solid/fluid interface using direct numerical simulation and large eddy simulation. Joint 6th International Symposium on Catalysis in Multiphase Reactors/5th International Symposium on Multifunctional Reactors (CAMURE-6/ISMR-5-), 2007, Pune. 

We discuss the application of atomic scale computer models to bulk crystal growth and the formation of thin films. The structure of the crystal-fluid interface and the mobility of the material at this interface are discussed in some detail. The influence of strain on thin film perfection and stability is also examined. 

The mapping (7) introduces the unknown interface shape explicitly into the equation set and fixes the boundary shapes. The shape function h(x,t) is viewed as an auxiliary function determined by an added condition at the melt/crystal interface. The Gibbs-Thomson condition is distinguished as this condition. This approach is similar to methods used for liquid/fluid interface problems that include interfacial tension (30) and preserves the inherent accuracy of the finite element approximation to the field equation (27)... 

Detachment includes two processes erosion and sloughing. Sloughing is a process in which large pieces of biofilm are rapidly removed, frequently exposing the surface. The causes are not well understood. Biofilm erosion is defined as continuous removal of single cells or small groups of cells from the biofilm surface and is related to shear stress at the biofilm/fluid interface. An increase in shear stress increases the erosion rate and decreases the biofilm accumulation rate. Empirical observations indicate that the erosion rate is related to biofilm thickness and density. 

In order to describe correctly the dynamic evolution of a fluid/fluid interface, a number of boundary conditions have to be implemented into the computational models. 

Condition (2) is also quite common. For instance, in crystals it results in a reduced sound velocity, v q) when q approaches a boundary of the Brillouin zone [93,96], a direct result of the periodicity of a crystal lattice. In addition, interaction between modes can lead to creation of soft mode with qi O and corresponding structural transitions [97,98]. The importance of nonlocality at fluid interfaces and the corresponding softening of surface modes has been demonstrated recently, both theoretically [99] and experimentally [100]. 

Other possible direct probes are optical experiments similar to studies [113] of vesicles but expanded towards shorter A (20-30 A). Alternatively neutron spin-echo studies of stacked bilayer arrays, which can probe the 10-30 A range [114], might possibly be applicable here. Finally, the x-ray grazing-incidence technique has been shown to be a powerful tool for studying short wavelength fluctuations at fluid interfaces [100]. The application of this technique to the investigation of membrane surface fluctuations can reasonably be expected in the near future [115,116]. 

Biofilms adhere to surfaces, hence in nearly all systems of interest, whether a medical device or geological media, transport of mass from bulk fluid to the biofilm-fluid interface is impacted by the velocity field [24, 25]. Coupling of the velocity field to mass transport is a fundamental aspect of mass conservation [2]. The concentration of a species c(r,t) satisfies the advection diffusion equation... 

Fig. 5.1.5 Quantitative data on the correlation of biofilm and velocity for a slice perpendicular to the flow axis. The images on the left are from top to bottom T2 map, z velocity component, x velocity component and y velocity component. One dimensional profiles through lines A, in bulk fluid, and B, intersecting biofilm fluid interface, are shown on the right. The biofilm signal indicator, dotted grey line, has been normalized so that zero corresponds to no biomass and 1 corresponds to the highest...

The study of transport between immiscible phases requires a device which (1) yields contacting surfaces with known contact area, (2) has a stationary interface, and (3) has known hydrodynamics. Stowe and Shaewitz [6] developed and evaluated a cell which meets these criteria. Each liquid is stirred with a rotating disk. As noted by these investigators, if the fluids are rotated in opposite directions by the rotating disks, then the torque on the interface cancels. The criterion for operation of the stirred cell with a stationary fluid interface is... 

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