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86-406: A gel is a complex solid but fluid substance with liquid-like properties. Gel may also refer to: Gel Gels are mostly liquid by mass , yet they behave like solids because of a three-dimensional cross-linked network within the liquid. It is the cross-linking within the fluid that gives a gel its structure (hardness) and contributes to the adhesive stick ( tack ). In this way, gels are

172-423: A change in pressure at one point in a liquid is transmitted undiminished to every other part of the liquid and very little energy is lost in the form of compression. However, the negligible compressibility does lead to other phenomena. The banging of pipes, called water hammer , occurs when a valve is suddenly closed, creating a huge pressure-spike at the valve that travels backward through the system at just under

258-544: A degree of flexibility very similar to natural tissue, due to their significant water content. As responsive " smart materials ," hydrogels can encapsulate chemical systems which upon stimulation by external factors such as a change of pH may cause specific compounds such as glucose to be liberated to the environment, in most cases by a gel-sol transition to the liquid state. Chemomechanical polymers are mostly also hydrogels, which upon stimulation change their volume and can serve as actuators or sensors . The first appearance of

344-655: A dispersion of molecules of a liquid within a solid medium. The word gel was coined by 19th-century Scottish chemist Thomas Graham by clipping from gelatine . The process of forming a gel is called gelation . Gels consist of a solid three-dimensional network that spans the volume of a liquid medium and ensnares it through surface tension effects. This internal network structure may result from physical bonds such as polymer chain entanglements (see polymers ) (physical gels) or chemical bonds such as disulfide bonds (see thiomers ) (chemical gels), as well as crystallites or other junctions that remain intact within

430-550: A drug release matrix. A hydrogel is a network of polymer chains that are hydrophilic, sometimes found as a colloidal gel in which water is the dispersion medium. A three-dimensional solid results from the hydrophilic polymer chains being held together by cross-links. Because of the inherent cross-links, the structural integrity of the hydrogel network does not dissolve from the high concentration of water. Hydrogels are highly absorbent (they can contain over 90% water) natural or synthetic polymeric networks. Hydrogels also possess

516-422: A fairly constant temperature, making a liquid suitable for blanching , boiling , or frying . Even higher rates of heat transfer can be achieved by condensing a gas into a liquid. At the liquid's boiling point, all of the heat energy is used to cause the phase change from a liquid to a gas, without an accompanying increase in temperature, and is stored as chemical potential energy . When the gas condenses back into

602-408: A free energy difference density. The form of f gel ( λ 1 , λ 2 , λ 3 ) {\displaystyle f_{\text{gel}}(\lambda _{1},\lambda _{2},\lambda _{3})} naturally assumes two contributions of radically different physical origins, one associated with the elastic deformation of the polymer network, and

688-417: A gas, a liquid maintains a fairly constant density and does not disperse to fill every space of a container. Although liquid water is abundant on Earth, this state of matter is actually the least common in the known universe, because liquids require a relatively narrow temperature/pressure range to exist. Most known matter in the universe is either gas (as interstellar clouds ) or plasma (as stars ). Liquid

774-403: A given rate, such as when it is being sheared at finite velocity. A specific example is a liquid flowing through a pipe: in this case the liquid undergoes shear deformation since it flows more slowly near the walls of the pipe than near the center. As a result, it exhibits viscous resistance to flow. In order to maintain flow, an external force must be applied, such as a pressure difference between

860-407: A liquid suitable for applications such as hydraulics . Liquid particles are bound firmly but not rigidly. They are able to move around one another freely, resulting in a limited degree of particle mobility. As the temperature increases, the increased vibrations of the molecules causes distances between the molecules to increase. When a liquid reaches its boiling point , the cohesive forces that bind

946-433: A liquid this excess heat-energy is released at a constant temperature. This phenomenon is used in processes such as steaming . Since liquids often have different boiling points, mixtures or solutions of liquids or gases can typically be separated by distillation , using heat, cold, vacuum , pressure, or other means. Distillation can be found in everything from the production of alcoholic beverages , to oil refineries , to

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1032-451: A mixture of otherwise immiscible liquids can be stabilized to form an emulsion , where one liquid is dispersed throughout the other as microscopic droplets. Usually this requires the presence of a surfactant in order to stabilize the droplets. A familiar example of an emulsion is mayonnaise , which consists of a mixture of water and oil that is stabilized by lecithin , a substance found in egg yolks . The microscopic structure of liquids

1118-442: A polymer solution of concentration ϕ 0 {\displaystyle \phi _{0}} and volume V 0 {\displaystyle V_{0}} is mixed with a pure solvent of volume ( λ 1 λ 2 λ 3 − 1 ) V 0 {\displaystyle (\lambda _{1}\lambda _{2}\lambda _{3}-1)V_{0}} to become

1204-471: A relatively high concentration of H + {\displaystyle {\text{H}}^{+}} and salt cations inside the gel. But because the concentration of H + {\displaystyle {\text{H}}^{+}} is locally higher, it suppresses the further ionization of the acid sites. This phenomenon is the prediction of the classical Donnan theory. However, with electrostatic interactions, there are further complications to

1290-482: A rotating liquid forms a circular paraboloid and can therefore be used as a telescope . These are known as liquid-mirror telescopes . They are significantly cheaper than conventional telescopes, but can only point straight upward ( zenith telescope ). A common choice for the liquid is mercury. Quantities of liquids are measured in units of volume . These include the SI unit cubic metre (m ) and its divisions, in particular

1376-400: A solid, and much higher than that of a gas. Therefore, liquid and solid are both termed condensed matter . On the other hand, as liquids and gases share the ability to flow, they are both called fluids. A liquid is made up of tiny vibrating particles of matter, such as atoms, held together by intermolecular bonds . Like a gas, a liquid is able to flow and take the shape of a container. Unlike

1462-490: A solution with polymer concentration ϕ {\displaystyle \phi } and volume λ 1 λ 2 λ 3 V 0 {\displaystyle \lambda _{1}\lambda _{2}\lambda _{3}V_{0}} . The free energy density change in this mixing step is given as where on the right-hand side, the first term is the Flory–Huggins energy density of

1548-472: Is a fixed amount of energy associated with forming a surface of a given area. This quantity is a material property called the surface tension , in units of energy per unit area (SI units: J / m ). Liquids with strong intermolecular forces tend to have large surface tensions. A practical implication of surface tension is that liquids tend to minimize their surface area, forming spherical drops and bubbles unless other constraints are present. Surface tension

1634-470: Is a solid formed from a gel by drying with unhindered shrinkage. Xerogels usually retain high porosity (15–50%) and enormous surface area (150–900 m /g), along with very small pore size (1–10 nm). When solvent removal occurs under supercritical conditions, the network does not shrink and a highly porous, low-density material known as an aerogel is produced. Heat treatment of a xerogel at elevated temperature produces viscous sintering (shrinkage of

1720-440: Is complex and historically has been the subject of intense research and debate. A few of the key ideas are explained below. Microscopically, liquids consist of a dense, disordered packing of molecules. This contrasts with the other two common phases of matter, gases and solids. Although gases are disordered, the molecules are well-separated in space and interact primarily through molecule-molecule collisions. Conversely, although

1806-436: Is considered to be a promising candidate for these applications as it is a liquid near room temperature, has low toxicity, and evaporates slowly. Liquids are sometimes used in measuring devices. A thermometer often uses the thermal expansion of liquids, such as mercury , combined with their ability to flow to indicate temperature. A manometer uses the weight of the liquid to indicate air pressure . The free surface of

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1892-420: Is extruded around it. Liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a nearly constant volume independent of pressure. It is one of the four fundamental states of matter (the others being solid , gas , and plasma ), and is the only state with a definite volume but no fixed shape. The density of a liquid is usually close to that of

1978-408: Is given by where: For a body of water open to the air, p 0 {\displaystyle p_{0}} would be the atmospheric pressure . Static liquids in uniform gravitational fields also exhibit the phenomenon of buoyancy , where objects immersed in the liquid experience a net force due to the pressure variation with depth. The magnitude of the force is equal to the weight of

2064-400: Is high both elastically and electrostatically and hence suppress ionization. Even though this ionization suppression is qualitatively similar to that of Donnan prediction, it is absent without electrostatic consideration and present irrespective of ion partitioning. The combination of both effects as well as gel elasticity determines the volume of the gel at equilibrium. Due to the complexity of

2150-437: Is important since machinery often operate over a range of temperatures (see also viscosity index ). The viscous behavior of a liquid can be either Newtonian or non-Newtonian . A Newtonian liquid exhibits a linear strain/stress curve, meaning its viscosity is independent of time, shear rate, or shear-rate history. Examples of Newtonian liquids include water, glycerin , motor oil , honey , or mercury. A non-Newtonian liquid

2236-421: Is initially formed by the assembly of particles into a space-spanning network, leading to a phase arrest. In the aging phase, the particles slowly rearrange to form thicker strands, increasing the elasticity of the material. Gels can also be collapsed and separated by external fields such as gravity. Colloidal gels show linear response rheology at low amplitudes. These materials have been explored as candidates for

2322-437: Is just a polymer network, without solvent). This is so because the free energy penalty to stretch an ideal polymer segment N {\displaystyle N} monomers of size b {\displaystyle b} between crosslinks to an end-to-end distance R {\displaystyle R} is approximately given by This is the origin of both gel and rubber elasticity . But one key difference

2408-776: Is monomer volume, N {\displaystyle N} is polymer strand length and χ {\displaystyle \chi } is the Flory-Huggins energy parameter. Because in a network, the polymer length is effectively infinite, we can take the limit N → ∞ {\displaystyle N\to \infty } and f ( ϕ ) {\displaystyle f(\phi )} reduces to Substitution of this expression into f mix ( λ 1 , λ 2 , λ 3 ) {\displaystyle f_{\text{mix}}(\lambda _{1},\lambda _{2},\lambda _{3})} and addition of

2494-473: Is no equilibrium at this transition under constant pressure, so unless supercooling occurs, the liquid will eventually completely crystallize. However, this is only true under constant pressure, so that (for example) water and ice in a closed, strong container might reach an equilibrium where both phases coexist. For the opposite transition from solid to liquid, see melting . The phase diagram explains why liquids do not exist in space or any other vacuum. Since

2580-805: Is of vital importance in chemistry and biology, and it is necessary for all known forms of life. Inorganic liquids include water, magma , inorganic nonaqueous solvents and many acids . Important everyday liquids include aqueous solutions like household bleach , other mixtures of different substances such as mineral oil and gasoline, emulsions like vinaigrette or mayonnaise , suspensions like blood, and colloids like paint and milk . Many gases can be liquefied by cooling, producing liquids such as liquid oxygen , liquid nitrogen , liquid hydrogen and liquid helium . Not all gases can be liquified at atmospheric pressure, however. Carbon dioxide , for example, can only be liquified at pressures above 5.1 atm . Some materials cannot be classified within

2666-425: Is one of the four primary states of matter , with the others being solid, gas and plasma . A liquid is a fluid . Unlike a solid, the molecules in a liquid have a much greater freedom to move. The forces that bind the molecules together in a solid are only temporary in a liquid, allowing a liquid to flow while a solid remains rigid. A liquid, like a gas, displays the properties of a fluid. A liquid can flow, assume

Gel (disambiguation) - Misplaced Pages Continue

2752-435: Is one where the viscosity is not independent of these factors and either thickens (increases in viscosity) or thins (decreases in viscosity) under shear. Examples of non-Newtonian liquids include ketchup , custard , or starch solutions. The speed of sound in a liquid is given by c = K / ρ {\displaystyle c={\sqrt {K/\rho }}} where K {\displaystyle K}

2838-450: Is only partially by the classical Donnan theory. As a starting point we can neglect the electrostatic interactions among ions. Then at equilibrium, some of the weak acid sites in the gel would dissociate to form A − {\displaystyle {\text{A}}^{-}} that electrostatically attracts positive charged H + {\displaystyle {\text{H}}^{+}} and salt cations leading to

2924-441: Is polymer volume fraction. Suppose the initial gel has a polymer volume fraction of ϕ 0 {\displaystyle \phi _{0}} , the polymer volume fraction after swelling would be ϕ = ϕ 0 / λ 1 λ 2 λ 3 {\displaystyle \phi =\phi _{0}/\lambda _{1}\lambda _{2}\lambda _{3}} since

3010-447: Is responsible for a range of other phenomena as well, including surface waves , capillary action , wetting , and ripples . In liquids under nanoscale confinement , surface effects can play a dominating role since – compared with a macroscopic sample of liquid – a much greater fraction of molecules are located near a surface. The surface tension of a liquid directly affects its wettability . Most common liquids have tensions ranging in

3096-421: Is stretch by a factor of λ {\displaystyle \lambda } in all three directions. Under the affine network approximation, the mean-square end-to-end distance in the gel increases from initial R 0 2 {\displaystyle R_{0}^{2}} to ( λ R 0 ) 2 {\displaystyle (\lambda R_{0})^{2}} and

3182-474: Is stretched by factors λ 1 {\displaystyle \lambda _{1}} , λ 2 {\displaystyle \lambda _{2}} and λ 3 {\displaystyle \lambda _{3}} in the three orthogonal directions during swelling after being immersed in a solvent phase of initial volume V s 0 {\displaystyle V_{s0}} . The final deformed volume of gel

3268-411: Is that gel contains an additional solvent phase and hence is capable of having significant volume changes under deformation by taking in and out solvent. For example, a gel could swell to several times its initial volume after being immersed in a solvent after equilibrium is reached. This is the phenomenon of gel swelling. On the contrary, if we take the swollen gel out and allow the solvent to evaporate,

3354-425: Is the bulk modulus of the liquid and ρ {\displaystyle \rho } the density. As an example, water has a bulk modulus of about 2.2  GPa and a density of 1000 kg/m , which gives c = 1.5 km/s. At a temperature below the boiling point , any matter in liquid form will evaporate until reaching equilibrium with the reverse process of condensation of its vapor. At this point

3440-424: Is then λ 1 λ 2 λ 3 V 0 {\displaystyle \lambda _{1}\lambda _{2}\lambda _{3}V_{0}} and the total volume of the system is V 0 + V s 0 {\displaystyle V_{0}+V_{s0}} , that is assumed constant during the swelling process for simplicity of treatment. The swollen state of

3526-413: Is used to fill the plastic tubes containing the fibers. The main purpose of the gel is to prevent water intrusion if the buffer tube is breached, but the gel also buffers the fibers against mechanical damage when the tube is bent around corners during installation, or flexed. Additionally, the gel acts as a processing aid when the cable is being constructed, keeping the fibers central whilst the tube material

Gel (disambiguation) - Misplaced Pages Continue

3612-1247: The cryogenic distillation of gases such as argon , oxygen , nitrogen , neon , or xenon by liquefaction (cooling them below their individual boiling points). Liquid is the primary component of hydraulic systems, which take advantage of Pascal's law to provide fluid power . Devices such as pumps and waterwheels have been used to change liquid motion into mechanical work since ancient times. Oils are forced through hydraulic pumps , which transmit this force to hydraulic cylinders . Hydraulics can be found in many applications, such as automotive brakes and transmissions , heavy equipment , and airplane control systems. Various hydraulic presses are used extensively in repair and manufacturing, for lifting, pressing, clamping and forming. Liquid metals have several properties that are useful in sensing and actuation , particularly their electrical conductivity and ability to transmit forces (incompressibility). As freely flowing substances, liquid metals retain these bulk properties even under extreme deformation. For this reason, they have been proposed for use in soft robots and wearable healthcare devices , which must be able to operate under repeated deformation. The metal gallium

3698-447: The elastic properties and firmness of the organogel. Often, these systems are based on self-assembly of the structurant molecules. (An example of formation of an undesired thermoreversible network is the occurrence of wax crystallization in petroleum . ) Organogels have potential for use in a number of applications, such as in pharmaceuticals , cosmetics, art conservation, and food. A xerogel / ˈ z ɪər oʊ ˌ dʒ ɛ l /

3784-709: The operating temperature range of the component. Oils are often used in engines, gear boxes , metalworking , and hydraulic systems for their good lubrication properties. Many liquids are used as solvents , to dissolve other liquids or solids. Solutions are found in a wide variety of applications, including paints , sealants , and adhesives . Naphtha and acetone are used frequently in industry to clean oil, grease, and tar from parts and machinery. Body fluids are water-based solutions. Surfactants are commonly found in soaps and detergents . Solvents like alcohol are often used as antimicrobials . They are found in cosmetics, inks , and liquid dye lasers . They are used in

3870-470: The cavities left by the bubbles with tremendous localized force, eroding any adjacent solid surface. In a gravitational field , liquids exert pressure on the sides of a container as well as on anything within the liquid itself. This pressure is transmitted in all directions and increases with depth. If a liquid is at rest in a uniform gravitational field, the pressure p {\displaystyle p} at depth z {\displaystyle z}

3956-454: The charged nature of H + {\displaystyle {\text{H}}^{+}} and A − {\displaystyle {\text{A}}^{-}} , electrostatic interactions with other ions in the systems. This is effectively a reacting system governed by acid-base equilibrium modulated by electrostatic effects, and is relevant in drug delivery , sea water desalination and dialysis technologies. Due to

4042-459: The classical three states of matter. For example, liquid crystals (used in liquid-crystal displays ) possess both solid-like and liquid-like properties, and belong to their own state of matter distinct from either liquid or solid. Liquids are useful as lubricants due to their ability to form a thin, freely flowing layer between solid materials. Lubricants such as oil are chosen for viscosity and flow characteristics that are suitable throughout

4128-405: The coupled acid-base equilibrium, electrostatics and network elasticity, only recently has such system been correctly recreated in computer simulations . Some species secrete gels that are effective in parasite control. For example, the long-finned pilot whale secretes an enzymatic gel that rests on the outer surface of this animal and helps prevent other organisms from establishing colonies on

4214-781: The cubic decimeter, more commonly called the litre (1 dm = 1 L = 0.001 m ), and the cubic centimetre, also called millilitre (1 cm = 1 mL = 0.001 L = 10 m ). The volume of a quantity of liquid is fixed by its temperature and pressure . Liquids generally expand when heated, and contract when cooled. Water between 0 °C and 4 °C is a notable exception. On the other hand, liquids have little compressibility . Water, for example, will compress by only 46.4 parts per million for every unit increase in atmospheric pressure (bar). At around 4000 bar (400 megapascals or 58,000 psi ) of pressure at room temperature water experiences only an 11% decrease in volume. Incompressibility makes liquids suitable for transmitting hydraulic power , because

4300-515: The elastic energy of one stand can be written as where R ref {\displaystyle R_{\text{ref}}} is the mean-square fluctuation in end-to-end distance of one strand. The modulus of the gel is then this single-strand elastic energy multiplied by strand number density ν = ϕ / N b 3 {\displaystyle \nu =\phi /Nb^{3}} to give This modulus can then be equated to osmotic pressure (through differentiation of

4386-439: The elastic nature of the gel, the dispersion of A − {\displaystyle {\text{A}}^{-}} in the system is constrained and hence, there will be a partitioning of salts ions and H + {\displaystyle {\text{H}}^{+}} inside and outside the gel, which is intimately coupled to the polyelectrolyte degree of ionization. This ion partitioning inside and outside

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4472-443: The ends of the pipe. The viscosity of liquids decreases with increasing temperature. Precise control of viscosity is important in many applications, particularly the lubrication industry. One way to achieve such control is by blending two or more liquids of differing viscosities in precise ratios. In addition, various additives exist which can modulate the temperature-dependence of the viscosity of lubricating oils. This capability

4558-423: The enormous variation seen in other mechanical properties, such as viscosity. The free surface of a liquid is disturbed by gravity ( flatness ) and waves ( surface roughness ). An important physical property characterizing the flow of liquids is viscosity . Intuitively, viscosity describes the resistance of a liquid to flow. More technically, viscosity measures the resistance of a liquid to deformation at

4644-445: The extending fluid. Virtually any fluid can be used as an extender including water ( hydrogels ), oil, and air ( aerogel ). Both by weight and volume, gels are mostly fluid in composition and thus exhibit densities similar to those of their constituent liquids. Edible jelly is a common example of a hydrogel and has approximately the density of water. Polyionic polymers are polymers with an ionic functional group. The ionic charges prevent

4730-416: The final swollen gel, the second is associated with the initial gel and the third is of the pure solvent prior to mixing. Substitution of ϕ = ϕ 0 / λ 1 λ 2 λ 3 {\displaystyle \phi =\phi _{0}/\lambda _{1}\lambda _{2}\lambda _{3}} leads to Note that the second term is independent of

4816-624: The food industry, in processes such as the extraction of vegetable oil . Liquids tend to have better thermal conductivity than gases, and the ability to flow makes a liquid suitable for removing excess heat from mechanical components. The heat can be removed by channeling the liquid through a heat exchanger , such as a radiator , or the heat can be removed with the liquid during evaporation . Water or glycol coolants are used to keep engines from overheating. The coolants used in nuclear reactors include water or liquid metals, such as sodium or bismuth . Liquid propellant films are used to cool

4902-633: The formation of tightly coiled polymer chains. This allows them to contribute more to viscosity in their stretched state, because the stretched-out polymer takes up more space. This is also the reason gel hardens. See polyelectrolyte for more information. A colloidal gel consists of a percolated network of particles in a fluid medium, providing mechanical properties , in particular the emergence of elastic behaviour. The particles can show attractive interactions through osmotic depletion or through polymeric links. Colloidal gels have three phases in their lifespan: gelation, aging and collapse. The gel

4988-404: The free energy) to give the same equation as we found above. Consider a hydrogel made of polyelectrolytes decorated with weak acid groups that can ionize according to the reaction is immersed in a salt solution of physiological concentration. The degree of ionization of the polyelectrolytes is then controlled by the pH {\displaystyle {\text{pH}}} and due to

5074-415: The gel is analogous to the partitioning of ions across a semipemerable membrane in classical Donnan theory, but a membrane is not needed here because the gel volume constraint imposed by network elasticity effectively acts its role, in preventing the macroions to pass through the fictitious membrane while allowing ions to pass. The coupling between the ion partitioning and polyelectrolyte ionization degree

5160-627: The gel is now completely characterized by stretch factors λ 1 {\displaystyle \lambda _{1}} , λ 2 {\displaystyle \lambda _{2}} and λ 3 {\displaystyle \lambda _{3}} and hence it is of interest to derive the deformation free energy as a function of them, denoted as f gel ( λ 1 , λ 2 , λ 3 ) {\displaystyle f_{\text{gel}}(\lambda _{1},\lambda _{2},\lambda _{3})} . For analogy to

5246-478: The gel would shrink to roughly its original size. This gel volume change can alternatively be introduced by applying external forces. If a uniaxial compressive stress is applied to a gel, some solvent contained in the gel would be squeezed out and the gel shrinks in the applied-stress direction. To study the gel mechanical state in equilibrium, a good starting point is to consider a cubic gel of volume V 0 {\displaystyle V_{0}} that

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5332-475: The historical treatment of rubber elasticity and mixing free energy, f gel ( λ 1 , λ 2 , λ 3 ) {\displaystyle f_{\text{gel}}(\lambda _{1},\lambda _{2},\lambda _{3})} is most often defined as the free energy difference after and before the swelling normalized by the initial gel volume V 0 {\displaystyle V_{0}} , that is,

5418-418: The hydrogel structure to obtain nanocomposites with tailored functionality. Nanocomposite hydrogels can be engineered to possess superior physical, chemical, electrical, thermal, and biological properties. Many gels display thixotropy – they become fluid when agitated, but resolidify when resting. In general, gels are apparently solid, jelly-like materials. It is a type of non-Newtonian fluid . By replacing

5504-477: The individual elements are solid under the same conditions (see eutectic mixture ). An example is the sodium-potassium metal alloy NaK . Other metal alloys that are liquid at room temperature include galinstan , which is a gallium-indium-tin alloy that melts at −19 °C (−2 °F), as well as some amalgams (alloys involving mercury). Pure substances that are liquid under normal conditions include water, ethanol and many other organic solvents. Liquid water

5590-488: The initial state. On the other hand, the mixing term f mix ( λ 1 , λ 2 , λ 3 ) {\displaystyle f_{\text{mix}}(\lambda _{1},\lambda _{2},\lambda _{3})} is usually treated by the Flory-Huggins free energy of concentrated polymer solutions f ( ϕ ) {\displaystyle f(\phi )} , where ϕ {\displaystyle \phi }

5676-416: The liquid displaced by the object, and the direction of the force depends on the average density of the immersed object. If the density is smaller than that of the liquid, the buoyant force points upward and the object floats, whereas if the density is larger , the buoyant force points downward and the object sinks. This is known as Archimedes' principle . Unless the volume of a liquid exactly matches

5762-485: The liquid with gas it is possible to prepare aerogels , materials with exceptional properties including very low density, high specific surface areas , and excellent thermal insulation properties. A gel is in essence the mixture of a polymer network and a solvent phase. Upon stretching, the network crosslinks are moved further apart from each other. Due to the polymer strands between crosslinks acting as entropic springs , gels demonstrate elasticity like rubber (which

5848-763: The molecules closely together break, and the liquid changes to its gaseous state (unless superheating occurs). If the temperature is decreased, the distances between the molecules become smaller. When the liquid reaches its freezing point the molecules will usually lock into a very specific order, called crystallizing, and the bonds between them become more rigid, changing the liquid into its solid state (unless supercooling occurs). Only two elements are liquid at standard conditions for temperature and pressure : mercury and bromine . Four more elements have melting points slightly above room temperature : francium , caesium , gallium and rubidium . In addition, certain mixtures of elements are liquid at room temperature, even if

5934-430: The network contribution leads to This provides the starting point to examining the swelling equilibrium of a gel network immersed in solvent. It can be shown that gel swelling is the competition between two forces, one is the osmotic pressure of the polymer solution that favors the take in of solvent and expansion, the other is the restoring force of the polymer network elasticity that favors shrinkage. At equilibrium,

6020-427: The number of monomers remains the same while the gel volume has increased by a factor of λ 1 λ 2 λ 3 {\displaystyle \lambda _{1}\lambda _{2}\lambda _{3}} . As the polymer volume fraction decreases from ϕ 0 {\displaystyle \phi _{0}} to ϕ {\displaystyle \phi } ,

6106-448: The other with the mixing of the network with the solvent. Hence, we write We now consider the two contributions separately. The polymer elastic deformation term is independent of the solvent phase and has the same expression as a rubber, as derived in the Kuhn's theory of rubber elasticity : where G 0 {\displaystyle G_{0}} denotes the shear modulus of

6192-426: The picture. Consider the case of two adjacent, initially uncharged acid sites HA {\displaystyle {\text{HA}}} are both dissociated to form A − {\displaystyle {\text{A}}^{-}} . Since the two sites are both negatively charged, there will be a charge-charge repulsion along the backbone of the polymer than tends to stretch the chain. This energy cost

6278-518: The pressure is essentially zero (except on surfaces or interiors of planets and moons) water and other liquids exposed to space will either immediately boil or freeze depending on the temperature. In regions of space near the Earth, water will freeze if the sun is not shining directly on it and vaporize (sublime) as soon as it is in sunlight. If water exists as ice on the Moon, it can only exist in shadowed holes where

6364-506: The shape of a container, and, if placed in a sealed container, will distribute applied pressure evenly to every surface in the container. If liquid is placed in a bag, it can be squeezed into any shape. Unlike a gas, a liquid is nearly incompressible, meaning that it occupies nearly a constant volume over a wide range of pressures; it does not generally expand to fill available space in a container but forms its own surface, and it may not always mix readily with another liquid. These properties make

6450-460: The speed of sound. Another phenomenon caused by liquid's incompressibility is cavitation . Because liquids have little elasticity they can literally be pulled apart in areas of high turbulence or dramatic change in direction, such as the trailing edge of a boat propeller or a sharp corner in a pipe. A liquid in an area of low pressure (vacuum) vaporizes and forms bubbles, which then collapse as they enter high pressure areas. This causes liquid to fill

6536-467: The stretching factors λ 1 {\displaystyle \lambda _{1}} , λ 2 {\displaystyle \lambda _{2}} and λ 3 {\displaystyle \lambda _{3}} and hence can be dropped in subsequent analysis. Now we make use of the Flory-Huggins free energy for a polymer-solvent solution that reads where v c {\displaystyle v_{c}}

6622-424: The subject discusses the use of hydrogels for nucleus pulposus replacement, cartilage replacement, and synthetic tissue models. Many substances can form gels when a suitable thickener or gelling agent is added to their formula. This approach is common in the manufacture of a wide range of products, from foods to paints and adhesives. In fiber optic communications, a soft gel resembling hair gel in viscosity

6708-626: The sun never shines and where the surrounding rock does not heat it up too much. At some point near the orbit of Saturn, the light from the Sun is too faint to sublime ice to water vapor. This is evident from the longevity of the ice that composes Saturn's rings. Liquids can form solutions with gases, solids, and other liquids. Two liquids are said to be miscible if they can form a solution in any proportion; otherwise they are immiscible. As an example, water and ethanol (drinking alcohol) are miscible whereas water and gasoline are immiscible. In some cases

6794-537: The surface of these whales' bodies. Hydrogels existing naturally in the body include mucus , the vitreous humor of the eye, cartilage , tendons and blood clots . Their viscoelastic nature results in the soft tissue component of the body, disparate from the mineral-based hard tissue of the skeletal system. Researchers are actively developing synthetically derived tissue replacement technologies derived from hydrogels, for both temporary implants (degradable) and permanent implants (non-degradable). A review article on

6880-450: The tens of mJ/m , so droplets of oil, water, or glue can easily merge and adhere to other surfaces, whereas liquid metals such as mercury may have tensions ranging in the hundreds of mJ/m , thus droplets do not combine easily and surfaces may only wet under specific conditions. The surface tensions of common liquids occupy a relatively narrow range of values when exposed to changing conditions such as temperature, which contrasts strongly with

6966-436: The term 'hydrogel' in the literature was in 1894. An organogel is a non-crystalline , non-glassy thermoreversible ( thermoplastic ) solid material composed of a liquid organic phase entrapped in a three-dimensionally cross-linked network. The liquid can be, for example, an organic solvent , mineral oil , or vegetable oil . The solubility and particle dimensions of the structurant are important characteristics for

7052-732: The thrust chambers of rockets . In machining , water and oils are used to remove the excess heat generated, which can quickly ruin both the work piece and the tooling. During perspiration , sweat removes heat from the human body by evaporating. In the heating, ventilation, and air-conditioning industry (HVAC), liquids such as water are used to transfer heat from one area to another. Liquids are often used in cooking due to their excellent heat-transfer capabilities. In addition to thermal conduction, liquids transmit energy by convection. In particular, because warmer fluids expand and rise while cooler areas contract and sink, liquids with low kinematic viscosity tend to transfer heat through convection at

7138-493: The two effects exactly cancel each other in principle and the associated λ 1 {\displaystyle \lambda _{1}} , λ 2 {\displaystyle \lambda _{2}} and λ 3 {\displaystyle \lambda _{3}} define the equilibrium gel volume. In solving the force balance equation, graphical solutions are often preferred. In an alternative, scaling approach, suppose an isotropic gel

7224-426: The vapor will condense at the same rate as the liquid evaporates. Thus, a liquid cannot exist permanently if the evaporated liquid is continually removed. A liquid at or above its boiling point will normally boil, though superheating can prevent this in certain circumstances. At a temperature below the freezing point, a liquid will tend to crystallize , changing to its solid form. Unlike the transition to gas, there

7310-405: The volume of its container, one or more surfaces are observed. The presence of a surface introduces new phenomena which are not present in a bulk liquid. This is because a molecule at a surface possesses bonds with other liquid molecules only on the inner side of the surface, which implies a net force pulling surface molecules inward. Equivalently, this force can be described in terms of energy: there

7396-645: The xerogel due to a small amount of viscous flow) which results in a denser and more robust solid, the density and porosity achieved depend on the sintering conditions. Nanocomposite hydrogels or hybrid hydrogels, are highly hydrated polymeric networks, either physically or covalently crosslinked with each other and/or with nanoparticles or nanostructures. Nanocomposite hydrogels can mimic native tissue properties, structure and microenvironment due to their hydrated and interconnected porous structure. A wide range of nanoparticles, such as carbon-based, polymeric, ceramic, and metallic nanomaterials can be incorporated within

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