36 results
Age-related decline in 5-HT2A and 5-HTU sites in the rhesus monkey hypothalamus
- E.K. Hamlyn, D.E. Roberts, J.A. Pugh, D.L. Rosene
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- European Psychiatry / Volume 22 / Issue S1 / March 2007
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- 16 April 2020, p. S314
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Serotonin 2A receptors (5-HT2A), and serotonin reuptake transporters (5-HTU) are involved in regulating some autonomic and cognitive processes. While the pre-synaptic and post-synaptic distribution of 5-HT2A receptors is unknown in the primate hypothalamus, in cortex, the majority of 5-HT2A receptors are located post-synaptically on pyramidal and glial cells. The density of 5-HT2A and 5-HTU sites declines with age in the primate and rodent hippocampus and frontal lobe but such changes have not been documented in the hypothalamus. To assess age-related changes in the density of 5-HT2A and 5-HTU binding sites in the rhesus monkey (Macaca mulatta) hypothalamus, autoradiographic ligand binding was utilized within the anterior, tuberal, and posterior hypothalamus, and the mammillary body (MMB) of 11-17 monkeys (4.4-31.8 yo). 5-HTU binding was assayed with tritiated citalopram and 5-HT2A with iodinated dimethoxyaminopropane (DOI). The density of 5-HTU binding was significantly reduced with age in the anterior (R= -0.57, N= 16, P=0.021), tuberal (R= -0.627, N= 17, P= 0.007), and posterior (R= -0.053, N= 15, P= 0.042) hypothalamus. Conversely, only the MMB displayed a significantly lower 5-HT2A density in aged animals (R=- 0.631, N= 11, P= 0.037). These results show a significant age-related decline in CIT binding throughout the hypothalamus, suggesting an age-related reduction in its serotonergic innervation. While we were unable to evaluate 5-HT U binding in the MMB, our results show a significant decline in DOI binding in this nucleus. Future studies are needed to determine the 5-HT2A receptor distribution in the monkey hypothalamus. (Supported by NIH Grant-P01-AG00001-29 and RR-00165).
Chlamydia gallinacea: a widespread emerging Chlamydia agent with zoonotic potential in backyard poultry
- L. LI, M. LUTHER, K. MACKLIN, D. PUGH, J. LI, J. ZHANG, J. ROBERTS, B. KALTENBOECK, C. WANG
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- Epidemiology & Infection / Volume 145 / Issue 13 / October 2017
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- 03 August 2017, pp. 2701-2703
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Chlamydia gallinacea, a new chlamydial agent, has been reported in four European countries as well as Argentina and China. Experimentally infected chickens with C. gallinacea in previous study showed no clinical signs but had significantly reduced gains in body weight (6·5–11·4%). Slaughterhouse workers exposed to infected chickens have developed atypical pneumonia, indicating C. gallinacea is likely a zoonotic agent. In this study, FRET-PCR confirmed that C. gallinacea was present in 12·4% (66/531) of oral–pharyngeal samples from Alabama backyard poultry. Phylogenetic comparisons based on ompA variable domain showed that 16 sequenced samples represented 14 biotypes. We report for the first time the presence of C. gallinacea in North America, and this warrants further research on the organism's pathogenicity, hosts, transmission, and zoonotic potential.
List of symbols
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10 - Antifoaming and defoaming
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Summary
Fed officials used to think there was little they could do to prevent bubbles from inflating. Their strategy was to mop up after a bubble burst with lower interest rates.
Jon Hilsenrath, Wall Street Journal, 2 Dec, 2009.Background and types of antifoamers and defoamers
Although foams are thermodynamically unstable, under practical conditions they can remain fairly stable for a considerable period of time, and it is often necessary to add chemicals to prevent foaming or to destroy the foam. Early definitions of antifoamers referred to the chemicals or materials pre-dispersed in the liquid phase prior to processing to prevent foam formation (produce low foamability) while defoamers were added to eliminate existing stable foams (produce low foam stability) by a shock effect. However, today this distinction is confusing since most chemical additives cover several roles and the nomenclature varies according to the industry where they are used. In fact, they are often referred to as foam control agents, foam inhibitors, foam suppressants and air release agents.
Foaming causes problems throughout a range of industrial processes, for example, in the production and processing of paper, pharmaceuticals, materials, textiles, coatings, crude oil, washing, leather, paints, adhesives, lubrication, fuels, heat transfer fluids and so on. In the processing of food and beverages such as sugar beet, orange and tomato juice, beer, wine and mashed potatoes, foaming problems caused by soluble proteins and starch are commonly encountered. Food containers are washed and recycled and again foaming must be prevented during these processes. It is also frequently necessary to break foam in storage vessels to increase the capacity (such as beer), and foam breaking is necessary to increase the efficiency of distillation or evaporation processes. There are numerous reviews of the antifoaming/defoaming area and a comprehensive book by Garrett (1) in 1993 covers the basic physical chemistry and most of the industrial uses of antifoamers. A more recent publication by Garrett (2) in 2015 summarizes further developments associated with the mode of action and also the mechanical aspects of defoaming are reviewed. Early publications by Owen (3) classify different products, and Kerner (4) lists the antifoaming products supplied by major companies. There are over 100 suppliers, if smaller companies are included, and many international suppliers have manufacturing capacity whereas smaller companies specialize in formulations for particular industries or processes.
6 - Coalescence of bubbles in surfactant solutions
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Preface
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Summary
The aim of this book is to provide a comprehensive, well-structured insight into the physical chemistry of liquid foams which can be used by both academics and industrialists. Liquid foams may occur naturally or by design and may be desirable or undesirable. Generally, there is a multitude of complex causes of foaming and antifoaming and the text is structured to give clarity to the field by providing an up-to-date, state-of-the-art guide explaining the chemistry of real foam systems. It is hoped that the reader will achieve a reasonably clear understanding of why foaming occurs, how it can be measured and how it can be prevented. As the use of foams spans different disciplines, some introductory aspects of physics, chemical engineering and material science of foams are included but this is relatively easy to follow. This book is orientated toward the descriptive rather than the theoretical and contains many diagrams. It is also a rich source of information and references, arranged in a way which the reader should find useful and also provides an historical prospect to the area of foams and foaming.
The most popular academic books dealing solely with foams include the classics Foams by J. J. Bikerman (1973), published by Springer-Verlag, Berlin and The Physics of Foams by D. Weaire and S. Hutzler (1999), published by Clarendon Press, Oxford. Both of these books ran into several updated editions but considerable advancements in the field have been made since their publication. Other early texts are Foams and Biliquid Foams-Aphrons by F. Sebba (1987), published by Wiley and the two books – Antifoaming (edited by P. Garrett, 1993) and Foams (edited by R. K. Prud'homme and S. A. Kahn, 1996) – published in the Surfactant Science Series (Marcel Dekker). These are fairly well-read books but are essentially a collection of viewpoints which describe many varied aspects of foaming and antifoaming science. Foam and Foam Films by D. Exerowa and P. M. Kriglyako (1997), published by Elsevier in the Studies in Interfacial Science Series, has been well received but presents a strongly fundamental text with the main emphasis on thin films. More recently is the book Foam Engineering, edited by P. Stevenson (2012) and published by Wiley, covers rheology, flow and foam processing and is aimed toward the chemical engineering community.
5 - Generation of bubbles and foams
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Summary
You don't start to play your guitar thinking you're going to be running an organization that will maybe generate millions.
Keith Richards, www.quoteauthors.com/keith-richards-quotes/Introduction
There are a number of methods for generating foams, and these have been reasonably well documented throughout the literature. They may be classified into two groups; the first involves the entrapment of air bubbles from the atmosphere and this can be achieved by relatively simple techniques such as shaking, pouring, circulation, sparging (introduction of gas using frits), etc. The second method involves artificially producing gas bubbles by physical methods (e.g. by nucleation or electrolysis) or chemical methods, which are commonly exploited in the production of polymer foams and involve the use of so-called blowing agents. These are chemical compounds that decompose or react to produce gas bubbles. While it is difficult to control the bubble size using physical methods of bubble formation, with chemical methods it is much easier to achieve a narrow-size distribution along with a high generation rate. Many different types of gases are used for foam generation, but it is important to recognize that foams generated with less soluble gases, such as N2 or air, will coarsen more slowly than foams produced with more soluble gases such as CO2 since gas diffusion through the soap films is largely determined by the gas solubility and the diffusion coefficient. In traditional foaming processes such as froth flotation, mechanical air entrapment methods are frequently used since they are relatively inexpensive, whereas in the production of material foams more sophisticated chemical processes have been developed.
The adsorption of surfactant on the freshly generated bubbles
The initial step in the generation of bubbles and foams involves the formation of a gas/liquid interface. This process involves work which can be quantified as the product of the interfacial tension and the increase in area of the interface; it be expressed by the equation
where ΔA is the created interfacial area and γ is the surface tension of the freshly produced bubbles. In water, bubbles have a high interfacial energy and become instantly unstable. Therefore, it is essential that surfactant adsorbs at the interface and reduces the surface tension and stabilizes the bubble. The adsorption kinetics plays an important role in the stabilization of the bubble, and the surfactant molecules need to rapidly diffuse from the bulk solution to the bubble interface.
Index
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9 - Foaming in non-aqueous liquids
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Summary
Nernst was a great admirer of Shakespeare, and it is said that in a conference concerned with naming units after appropriate persons, he proposed that the unit of rate of liquid flow should be called the falstaff.
James Riddick Partington, “The Nernst Memorial Lecture”, Journal of the Chemical Society, Part 3, 2855, 1953.Introduction
Foaming is less commonly encountered in non-aqueous fluids than in water-based media, but it is generally accepted that similar physico-chemical principles are applicable, such as the adsorption to the bubble interface. This enables some analogies to be drawn with aqueous systems but there are problems which need to be resolved, since it was originally thought that the degree of dissociation of solubilized chemical additives was limited in non-aqueous systems and free ions were almost absent. The physical properties of the liquid also play a more important role in determining the stability of the non-aqueous foam, and if one considers the many different types of non-aqueous liquids, for example liquid metals, liquid polymers, crude oils, drilling fluids, lubricants and solvents (base cleaners), then it is clear that wide variations in physical properties need to be considered. With these liquids, distinct differences in, for example, viscosity, conductivity, dielectric constant and density are evident.
This makes it difficult to generalize the prediction of foaming behavior in non-aqueous systems, but usually some type of surface activity is needed as in aqueous systems. For example, in the generation of aluminum foams, silicon carbide particles are added which are captured by the bubbles and act to stabilize the gas/liquid metal interface (1). In mixed hydrocarbon media, lyotropic liquid crystals which are surface-active self-organized assemblies play a role in foaming performance (2, 3, 4). It has also been shown that the addition of inorganic electrolytes to organic liquids inhibits bubble coalescence by reducing drainage, but it is only recently that an improved understanding of bubble stability and interfacial drainage in non-aqueous liquids has been achieved. In these systems, it has been shown that drainage rates may be influenced by the particular arrangement of ions in the interfacial region (5, 6). For non-aqueous liquids with high bulk viscosity such as molten polymers, thick oils and metal foams, the drainage can be relatively slow, and this will reduce the foam decay rate to some degree.
Contents
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2 - The nature and properties of foaming surfactants
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Summary
Shampoo doesn't have to foam, but we add foaming chemicals because people expect it each time they wash their hair.
Quotescondex.comThe formation of self-assemblies from pre-micellar surfactant species
The adsorption of amphiphilic surfactant molecules at the bubble interface is not the only important phenomenon occurring during foam formation. Another extremely important process also occurs in bulk solution at high surfactant concentrations. This involves a spontaneous self-assembly process in which higher molecular structural aggregates or units of surfactant are formed from lower molecular weight pre-micellar species (monomer, dimer and trimer units). In the simplest case, this corresponds to the formation of a spherical micelle, and the transition concentration (of monomer) at which this occurs is called the “critical micelle concentration” (CMC). Marked changes in foaming behavior, as well as changes in electrical conductivity, surface tension, turbidity and uptake of organic dyes, occur in bulk solution above the CMC, but the molecular concentration of the surfactant in the water remains constant, with the surplus molecules forming additional micelles. Fig. 2.1 depicts the successive steps involved in the growth of the micelle from monomeric species, with monomers initially aggregating to form dimeric and trimeric species. As these complexes grow in size, an increasing proportion of the interface of the added monomer molecules achieves contact with the micellar hydrocarbon segments until the maximum degree of hydrocarbon/hydrocarbon interaction is reached.
For many simple long-chain linear amphiphilic surfactants, this results in the formation of a perfectly spherical complex which produces the maximum surfactant packing density. In this case, the micellar structure is complete, but difficulties may occur with some types of charged surfactants due to the repulsive charge on the head groups, and these interactions must be counterbalanced with the structure, which will result in different types of molecular arrangements within the micelle. A more detailed theoretical examination of the origins of the free energy changes which occur on eliminating the hydrocarbon/water interaction is described in considerable detail in an early classic text The Hydrophobic Effect by Tanford (1).
Several different models have been used to describe the overall process of micellization, and these have been well documented in the literature (2).
Frontmatter
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4 - Processes in foaming
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Summary
Evolution thus is merely contingent on certain processes articulated by Darwin: variation and selection.
Ernst Mayr, What is Evolution, Science Masters Series/Basic Books, Oct 2001.Overview of processes
The evolution of foams occurs through a series of rapid non-equilibrium processes which can be observed by sparging gas through a glass sinter into a column of water. As the air bubbles ascend, their velocities are principally determined by their sizes, the difference in the viscosities of the liquid and gas phases and the properties of the gas/liquid interface. However, as the bubbles grow in size, they may collide and in cases where only weak foaming agents are present in solution, compaction and coalescence can occur. There are several other processes which play an important role in determining the characteristics of the bubbles and the structure of the foam as the bubbles accumulate at the interface. For example, the drainage process or the downward flow of liquid coupled with liquid flow into the Plateau borders can cause thinning of the liquid films. Also, repulsive interactions across the thin film lamellae resulting from strongly adsorbed chemical surfactants can slow down drainage or even prevent bubble coalescence. During the ascent and mixing of bubbles, another important process known as disproportionation occurs. This involves the diffusion of gas from smaller to larger bubbles, and the driving force for this process is the Laplace pressure (the pressure difference between bubbles of different sizes). Although the term “disproportionation” is commonly used by chemists to describe inter-bubble gas diffusion within foams, it is often referred to as Oswald ripening, which was originally used to define the evaporation–condensation mechanism in two-phase separation of binary alloys. The term “coarsening” is often used but coarsening is also frequently considered to be a combination of inter-bubble gas diffusion and coalescence. This confusion in terminology is due to the fact that researchers engaged in foams come from a variety of disciplines, and each has its own terminology. An overview of some of the processes that occur during sparging are outlined in Fig. 4.1.
Molecular processes such as the adsorption and the mobility of chemical surfactant molecules at the air/water interface and also the depletion of surfactant from solution can occur at high gas flow rates can also influence the stability of the bubbles.
11 - Bubble size measurements and foam test methods
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Summary
“…When you can measure what you are speaking about, and express it in numbers, you know something about it, but when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind….”
Lord Kelvin, Electrical Units of Measurements, 3 May, 1883. (zapatopi.net/kelvin/quotes).Introduction
Several non-intrusive analytical techniques have been used to provide information on bubble size distributions, gas fraction, texture and general characteristics of foams. Some of these are relatively simple optical methods, but with the application of image and video analysis, the changes in bubble size and texture of 2D foams can be fairly easily studied during ageing. This information can be useful, but since the rate of deterioration of a wet foam depends on the kinetics of coalescence, drainage and gas diffusion, it is sometimes difficult to separate these events and to resolve the main destabilization mechanism. Bisperink and coworkers (1) reviewed how the variation in 2D bubble size distribution in aging foams may be related to drainage, coalescence and the disproportionation process. Although microscopy can be conveniently used to study 2D foams, more complex techniques based on tomography have been applied to probe the interior and to measure bubble size distribution on 3D foams. The processing of the data usually involves three steps: image acquisition, image processing and data extraction. 3D imaging techniques such as nuclear magnetic resonance imaging (NMRI) or X-ray computerized tomography can be used to develop a scale model in a computer memory. Based on these models the relationship between physical properties and the structure of solid foams can be established. UV, NMR and ultrasonic reflection methods have also been used, but overall it is still difficult to characterize bubble size and liquid fraction in real 3D foams, and hence most investigations are carried on quasi-2D foams.
Although wet foams are unstable, the kinetics of breakdown can range from seconds to weeks, and this has resulted in the development of many different types of test methods for measuring foam stability. Overall, stability tests may be broadly classified as (a) dynamic tests, which measure the height or volume of the foam in a state of dynamic equilibrium between formation and decay, and (b) static tests, in which the rate of foam formation is zero (the gas flow into the liquid is eliminated) and the foam is allowed to collapse.
7 - The stability/instability of bubbles and foams
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Summary
The paradox is easily explained. Profit-seeking people will take more financial risk when they believe the coast is clear. By taking bigger chances, however, they unwittingly make the world unsafe all over again.
Hyman Minsky, US Economist, Paying the Price for the Fed's Success, The New York Times, www.nytimes.com/200801/27/opinion/27grant.htmlOverview
All foams are thermodynamically unstable due to their high interfacial free energy, the decrease of which causes foam decay. It is well known that there are several different types of mechanisms involved in the stabilization and decay of foams, which has caused a considerable amount of confusion. In the literature there are many conflicting explanations frequently caused by experimental anomalies and the incomplete interpretation of foaming experiments. Another aspect to consider is that the lifetime of a foam can pass through several different stages, and each stage may involve a different type of mechanism. To explain the overall stability in terms of one mechanism is almost impossible, and the interplay of different mechanisms needs to be taken into consideration. During generation, bubbles expand and contract and are subjected to severe vibrations and dynamic disturbances causing distortion of the adsorption layer. During this process, the liquid films separating the bubbles are relatively thick and subject to stretching, and viscous elastic forces play a crucial role. Possibly the most important mechanisms for the survival of a wet foam during this stage involves the surface elasticity theories of Gibbs and Marangoni. Gravitational forces also cause fairly rapid drainage to occur during this preliminary stage, but this can be retarded by a high bulk viscosity. On entering a secondary stage, capillary forces come into play causing suction and thinning of the lamellae, and this occurs at a lower rate. In addition, disproportionation may occur causing the diffusion of gas between bubbles. As all these processes occur under dynamic conditions, the equilibrium adsorption coverage is rarely reached.
The process of gas diffusion owes its origins to the difference in pressure, surface tension and curvature of the bubbles, but the gas diffusion to the atmosphere also needs to be considered. In addition to diffusive disproportionation theories to explain the changes in size distribution in bubbles, alternate processes have been considered which involve the effect of interfacial rheology on the shrinkage of bubbles.
3 - Soap bubbles and thin films
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Summary
But all bubbles have a way of bursting or being inflated in the end.
Barry Gibbs, http://www.brainyquote.com/quotes/barrygibb199084.htmlIntroduction and early studies
Surfactants such as soap (sodium and potassium salts of fatty acids) stabilize thin liquid films, which are the basic structural units of foams and act as cell walls encapsulating the gas. The films are stabilized by surfactant monolayers of polar head groups (hydrophilic) solubilized in the water phase and aliphatic hydrocarbon tails (hydrophobic) protruding into the vapor phase. Isolated soap films can be easily produced by dipping a wire loop or rectangular frame into a soap solution and raising the frame slowly, causing the liquid to drain vertically. As the thickness of the film decreases from the top to the bottom of the frame, a pattern of interference fringes rapidly develops. This phenomenon can be spectacular, since each color band flows downward and a swirling motion is frequently observed due to rapid, complex fluid motion. Finally, toward the end of the thinning process, the film reaches a thickness that is less than the wavelength of light, and at this point, black spots appear, which spread rapidly, and eventually the entire film appears black in reflected light.
Foams, bubbles and foam films have been the subject of intense scientific investigations over a period of several hundred years. In the 17th century, both Robert Hooke (1) and later Isaac Newton (2) became captivated by these systems and carried out many fundamental experiments involving film drainage in bubbles in which the brilliant interference colors were carefully observed and reported in the Proceedings of the Royal Society. In Fig. 3.1, a typical spectrum generated by the drainage of thin foam films is illustrated.
It was also noted by scientists such as Newton and Hooke that, as the colored interference bands transformed to thin black films, the thinning ceased and eventually the films collapsed. This can be explained by the formation of a metastable state, resulting from the interaction of intermolecular forces. At first sight, the appearance of black spotswas extremely puzzling. Hooke first thought that these black spots were black holes in the soap bubbles, and Isaac Newton, in his second paper on light and color, described (as illustrated below) the occurrence of several different states of thickness of thin black films on bubble surfaces (as indicated by different shades of black and also the coalescence of the spots).
Acknowledgments
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12 - Bubble and foam chemistry - new areas of foam research
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Summary
You never change things by fighting the existing reality. To change something, build a new model that makes the existing model obsolete.
R. Buckminster Fuller, www.azquotes.comAntibubbles
Antibubbles, which have sometimes been referred to as negative bubbles or inverted bubbles, have been known for at least 70 years. They were first reported in 1931/2 by Katalinic (1) and Hughes and Hughes (2) and originally considered as a scientific curiosity. Essentially, the structure of an antibubble consists of a thin shell or core of air which is separated by liquids, with the surfactant adsorbed at the inner and outer interfaces of the shell. The polar heads of the chemical surfactant are immersed in the aqueous medium, while the nonpolar tails face and extend into the gas. This is the exact opposite situation to a bubble where the thin shell of liquid separates the air inside and outside the bubble. The shell of air on the antibubble is about 10 nm thick and can generate interference colors. Antibubbles are formed at surfactant concentrations below and above the CMC, and several different types of surfactants have been used in their preparation. Dishwashing detergents were often used in the generation process, but more recently they have been prepared using different types of conventional long-chain nonionic, anion and protein (beer) surfactants (3). It is also interesting to note that antibubbles can be generated and stabilized by partially hydrophobic particles (4). Most studies in antibubbles have been mainly concerned with generation rather than stability. In Fig. 12.1, the structure of a common bubble is compared with chemically stabilized antibubbles and particle-stabilized antibubbles.
In recent years, several interesting applications have been put forward. For example, it has been suggested that they can be used as antifoams or new types of lubricants analogous to ball bearings or filtration systems in which the web of passageways would be permeable to gas molecules (5). Since antibubbles provide twice the surface area of ordinary bubbles of the same size, another potential application could be the control of chemical reactions. It has also been shown that it is possible to replace air in the antibubble inner shell with other liquids or dyes, suggesting that they could be used as a drug delivery system.
8 - Particle-stabilized foams
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These motions were such as to satisfy me, after frequently repeated observation, that they arose neither from currents in the fluid, nor from its gradual evaporation, but belonged to the particle itself.
“Microscopical Observations on the Particles Contained in the Pollen of Plants,” by Robert Brown, Philosophical Magazine, NS 4, 162–3, 1828.
Introduction
Partially hydrophobic small particles have the potential to act as stabilizing agents in many foaming processes, and they behave fairly similar in some ways to chemical surfactant molecules in that they can adsorb (attach) at the bubble interface. However, particles show several distinct differences from chemical surfactants. For example, macro-sized particles are considerably larger than the molecular dimensions exhibited by chemical surfactants. They also behave differently, in that particles cannot aggregate at the interface and are unable to buildup self-assembles and cannot solubilize in the bulk solution. Unlike chemical surfactants, it is difficult to generate bubbles or foams solely without particles, but partially hydrophobic particles can be good foam stabilizers at moderate concentrations (about 1 w%). In fact, if the particles exhibit moderate hydrophobicity, then the foams can be extremely stable (with lifetimes of the order of years). However, generally it is more convenient to add other surface-active components to the particles, such as polymers, dispersants or chemical surfactants, to ensure a higher degree of foamability and foam stability. The different features exhibited by surfactant- and particle-stabilized systems are illustrated in Fig. 8.1.
There are only a few foaming processes operating solely with particles, for example, molten metal foams (where ceramic particles are used) and sometimes hydrophobic particles (such as graphite) in the froth flotation process. Excluding the past two decades, the literature on foams stabilized by particles is sparse, but there has been a revival of attention, which was mostly because of the success achieved with particle-stabilized emulsions (Pickering emulsions). For a considerable period of time it has been well known that particles acted as stabilizers in many established industrial foaming processes, such as froth flotation of mineral particles (1), deinking flotation (2) and food processing (3). However, little effort was made to understand the mechanisms until recent years; today a considerable amount of insight has been achieved in understanding the basics.
1 - Basic principles and concepts
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Summary
Thoughts without content are empty, intuitions without concepts are blind.
Immanual Kant, Critique of Pure Reason, B25, 1781.Introduction
Foams can exist in the wet, dry or solid state and can be seen almost everywhere, in the home, in the surrounding natural environment and in numerous technological applications. In fact they are prevalent, and it is almost impossible to pass through an entire day without having contact with some type of liquid or solid foam. They have several interesting properties which enable them to fill an extremely wide range of uses; for example, they possess important mechanical, rheological and frictional characteristics which enable them to behave similar to solids, liquids or gases. Under low shear, wet (bubbly) foams exhibit elastic properties similar to solid bodies, but at high shear, they flow and deform in a similar manner to liquids. On the application of pressure or temperature to wet foams, the volume changes proportionately, and this behavior resembles that of gases. Interestingly, it is the elastic and frictional properties of wet foams which lead to their application in personal hygiene products such as body lotions, foaming creams and shaving foams. While shaving, foam is applied to the skin and the layer on the blade travels smoothly over the surface, reducing the possibilities of nicking and scratching. Another example is their use as firefighting foams, where properties such as low density, reasonably good mechanical resistance and heat stability are required in order to be effective in extinguishing gasoline fires. Essentially, they act by covering the flames with a thick semi-rigid foam blanket. The low density allows the water in the foam to float even though it is generally denser than the burning oils. The chemical composition and mechanical properties of these types of foams can be varied to optimize the firefighting utility.
Foams are also found in many food items, either in finished products or incorporated during some stage in food processing. They primarily provide texture to cappuccino, bread, whipped cream, ice-cream topping, bread, cakes, aerated desserts, etc. Surprisingly, several novel types of food foams have been recently produced from cod, mushroom and potatoes, using specially designed whipping siphons powered by pressurized gas with lecithin or gelatin as alternative foaming agents to replace egg and creams (1).