Technology & Engineering
Surface Tension
Surface tension is the property of a liquid that causes its surface to behave like a thin, elastic sheet. It is due to the cohesive forces between the liquid molecules, which pull the molecules at the surface inward. This results in the formation of droplets and allows certain insects, like water striders, to walk on water.
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12 Key excerpts on "Surface Tension"
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Research World(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter-10 Surface Tension Surface Tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects (e.g. water striders) and even reptiles (basilisk) to run on the water surface. This property is caused by cohesion of like molecules, and is responsible for many of the behaviors of liquids. Surface Tension has the dimension of force per unit length, or of energy per unit area. The two are equivalent—but when referring to energy per unit of area, people use the term surface energy—which is a more general term in the sense that it applies also to solids and not just liquids. In materials science, Surface Tension is used for either surface stress or surface free energy. Cause Diagram of the forces on two molecules of liquid ________________________ WORLD TECHNOLOGIES ________________________ Surface Tension prevents the paper clip from submerging. The cohesive forces among the liquid molecules are responsible for this phenomenon of Surface Tension. In the bulk of the liquid, each molecule is pulled equally in every direction by neighboring liquid molecules, resulting in a net force of zero. The molecules at the surface do not have other molecules on all sides of them and therefore are pulled inwards. This creates some internal pressure and forces liquid surfaces to contract to the minimal area. Surface Tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer. In the absence of other forces, including gravity, drops of virtually all liquids would be perfectly spherical. The spherical shape minimizes the necessary wall tension of the surface layer according to Laplace's law.
eBook - PDFPhysics of Continuous Matter
Exotic and Everyday Phenomena in the Macroscopic World
- B. Lautrup(Author)
- 2011(Publication Date)
- CRC Press(Publisher)
5 Surface Tension At the interface between two materials, physical properties change rapidly over distances comparable to the molecular separation scale. Since a molecule at the interface is exposed to a different environment than inside the material, it will also have a different binding energy. In the continuum limit where the transition layer becomes a mathematical surface separating one material from the other, the difference in molecular binding energy manifests itself as a macroscopic surface energy density . And where energy is found, forces are not far away. Molecules sitting at a free liquid surface against vacuum or gas have weaker binding than molecules in the bulk. The missing (negative) binding energy can therefore be viewed as a positive energy added to the surface itself. Since a larger area of the surface contains larger surface energy, external forces must perform positive work against internal surface forces to increase the total area of the surface. Mathematically, these internal surface forces are represented by Surface Tension , defined as the normal force per unit of length. This is quite analogous to bulk tension (negative pressure), defined as the normal force per unit of area. In homogeneous matter such as water, Surface Tension does not depend on how much the surface is already stretched. Certain impurities, called surfactants , have dramatic influence on Surface Tension because they agglomerate on the surface and form an elastic skin that resists stretching with increasing force. Best known among these are soaps and detergents from which one can blow such beautiful bubbles. A lipid bilayer also surrounds every living cell and separates the internal biochemistry from the environment. Although Surface Tension is present at all interfaces, it is most important for small fluid bodies (at the human scale).
eBook - ePubLiquid Marbles
Formation, Characterization, and Applications
- Andrew T. Tyowua(Author)
- 2018(Publication Date)
- CRC Press(Publisher)
Table 1.1 . A liquid surface is basically a liquid-air interface. The cohesive forces between the molecules of the liquids are greater than the adhesive forces between the air and the liquid molecules. The result is a force imbalance with a net inward force in the liquid bulk phase. This causes the liquid surface to stretch as though it were covered with an elastic membrane. This is the origin of Surface Tension in liquids. Surface Tension is responsible for many observed physical phenomena. For example, due to Surface Tension, liquid drops minimize their surface area by taking a spherical shape (geometry of least surface area). The rising of liquids in thin capillaries once submerged in them is also a consequence of Surface Tension. It is also because of Surface Tension that insects are able to walk on the surface of water.TABLE 1.1Surface Tension of Some Liquids at 25°CLiquid γla /mN m−1 at 25°CHexane 17.89 Ethanol 21.97 Methanol 22.07 Cyclohexane 24.65 Acetone 24.02 Chloroform 26.67 Acetic acid 27.10 Toluene 27.93 Benzene 28.22 Hexadecane 27.05 Formamide 57.02 Water 71.99 Source: Jasper, J.J., J. Phys. Chem. Ref. Data 1, 841–1009, 1972.1.2.2 CAPILLARITY AND WICKINGA liquid will either rise against gravity or fall in a thin capillary once submerged (vertically) in it, and the phenomenon is known as capillarity. The rise or fall is, however, dependent on whether the cohesive forces between the liquid molecules are higher than the adhesive forces between the liquid molecules and those of the capillary. For liquids that rise in thin capillaries, the adhesive forces are higher than the cohesive forces. The converse is true for liquids that fall in capillaries. This is the reason water rises in glass capillaries while mercury falls in them. When “rising” or “falling” stops in capillaries, the liquid-air interface in them curves inwardly (concave, e.g. water in glass capillaries) or outwardly (convex, e.g
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Research World(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter- 9 Surface Tension Surface Tension prevents a coin from sinking: the coin is indisputably denser than water, so it cannot be floating due to buoyancy alone. Surface Tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even ________________________ WORLD TECHNOLOGIES ________________________ though they are denser than water, and in the ability of some insects (e.g. water striders) and even reptiles (basilisk) to run on the water surface. This property is caused by cohesion of like molecules, and is responsible for many of the behaviors of liquids. Surface Tension has the dimension of force per unit length, or of energy per unit area. The two are equivalent—but when referring to energy per unit of area, people use the term surface energy—which is a more general term in the sense that it applies also to solids and not just liquids. In materials science, Surface Tension is used for either surface stress or surface free energy. Cause Diagram of the forces on two molecules of liquid ________________________ WORLD TECHNOLOGIES ________________________ Surface Tension prevents the paper clip from submerging. The cohesive forces among the liquid molecules are responsible for this phenomenon of Surface Tension. In the bulk of the liquid, each molecule is pulled equally in every direction by neighboring liquid molecules, resulting in a net force of zero. The molecules at the surface do not have other molecules on all sides of them and therefore are pulled inwards. This creates some internal pressure and forces liquid surfaces to contract to the minima Surface Tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer.
eBook - PDF- John Ward-Smith(Author)
- 2018(Publication Date)
- CRC Press(Publisher)
Although commonly adopted in the past, the use of this term is now discouraged as imprecise. A more meaningful term for a fluid of zero viscosity is inviscid fluid . 1.7 Surface Tension Surface Tension arises from the forces between the molecules of a liquid and the forces (generally of a different magnitude) between the liquid molecules and those of any adjacent substance. The symbol for Surface Tension is γ and it has the dimensions [MT -2 ]. Water in contact with air has a Surface Tension of about 0.073 N · m -1 at usual ambient temperatures; most organic liquids have values between 0.020 and 0.030 N · m -1 and mercury about 0.48 N · m -1 , the liquid in each case being in contact with air. For all liquids the Surface Tension decreases as the temperature rises. The Surface Tension of water may be considerably reduced by the addition of small quantities of organic solutes such as soap and detergents. Salts such as sodium chloride in solution raise the Surface Tension of water. That tension which exists in the surface separating two immiscible liquids is usually known as interfacial tension. As a consequence of Surface Tension effects, a drop of liquid, free from all other forces, takes on a spherical form. The molecules of a liquid are bound to one another by forces of molecular attraction, and it is these forces that give rise to cohesion , that is, the tendency of the liquid to remain as one assemblage of particles rather than to behave as a gas and fill the entire space within which it is confined. Forces between the molecules of a fluid and the molecules of a solid boundary surface give rise to adhesion between the fluid and the boundary. If the forces of adhesion between the molecules of a particular liquid and a particular solid are greater than the forces of cohesion among the liquid molecules themselves, the liquid molecules tend to crowd towards the solid surface, and the area of contact between liquid and solid tends to increase.
eBook - PDFPhysicochemical Hydrodynamics
An Introduction
- Ronald F. Probstein, Howard Brenner(Authors)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
9 Surface Tension 9.1 Physics of Surface Tension The effects of surface and interfacial tensions give rise to so many commonplace phenomena observed in liquid behavior that we often take for granted the complex physical-chemical interactions involved, not all of which are under-stood even today. Among the many familiar examples of Surface Tension effects are the formation of soap bubbles that float gently upward until they break, or the thin capillary tube in which a liquid will rise to a height greater than the pool in which it is placed. There is also the breakup into drops of a stream of water flowing out of a faucet, the physics of which is the basis of the ink jet printer or gel encapsulation processes to encase everything from monoclonal antibodies to perfume. Or there is the phenomenon of a liquid drop remaining stationary when placed on a solid surface, as well as the opposite situation of the spreading of a drop of water when placed on a clean glass surface. The examples, both observed and applied, that result from interfacial effects between liquids, gases, and solids are indeed numerous. Interfacial phenomena attracted considerable scientific attention from the 18th century onward. At first the attempts to characterize the different be-haviors were mechanical, where the liquids were described as being stretched at their interfacial surface like a membrane, with a state of tension existing there. Today we know that the liquid state itself is composed of molecules in motion that are kept relatively close to each other by attractive van der Waals forces. However, a principal method of analysis of problems of interfacial effects rests upon the assumption that the liquid can be described by a continuum mean-field approximation or mean molecular field, wherein it is assumed possible to define an element of the liquid that is small compared to the range of the intermolecu-lar force but large enough to contain a sufficient number of molecules.
- Christos Ritzoulis(Author)
- 2013(Publication Date)
- CRC Press(Publisher)
Line tension is observed at the edges of geo-metric shapes, for example, at the point of contact of a droplet with a surface. The shape of a droplet on a surface is spherical due to the attempt of the droplet to minimize the free surface area and because of the symmetri-cal distribution of forces. This is due to, among other things, the capillary phenomenon, as we will see later. The corresponding point tension has been described and studied theoretically. 4.2 Interface tension As mentioned in the previous section, Surface Tension is the additional free energy per unit surface that results from the inability of the surface molecules to form interactions with other molecules in the direction of the empty space above the surface. If we bring two substances into contact in order to form two separate phases that are separated by an interface , then maybe the molecules of the two phases that are found at the interface can exert some forces between them. These forces may be weaker than those 80 Introduction to the physical chemistry of foods exerted between molecules of the same phase, but they can reduce some-what the free surface energy. Thus, if we bring into contact two substances A and B with Surface Tensions γ A and γ B , respectively, then the interface tension γ AB will be γ AB = γ A + γ B − 2 σ AB (4.7) where σ AB is the energy gain per surface unit area due to attractive interac-tions between the molecules of A and B. This equation shows that the free energy per unit area γ AB is large when the Surface Tensions are large and the interactive forces between the phases are small. As each of the two populations of surface molecules A and B tends to retire to its respective bulk phase A or B, the two surfaces tend to dis-sociate from each other. The sum γ A + γ B quantifies the energy per unit area relevant to this dissociation. These two surfaces are mutually insol-uble and will form a clear interface between them.
eBook - PDF- Yip-Wah Chung(Author)
- 2011(Publication Date)
- CRC Press(Publisher)
For a liquid, the force is isotropic and acts within the plane of the surface. This is what causes liquid droplets to adopt spherical shapes, because that is the minimum surface area configuration for a given volume. The ten-sile force arises because, compared with atoms in the bulk, the atoms at the surface are lacking bonds with neighbors outside of the volume of the liquid. Because of the tensile force, one can think of the surface as acting like a stretched elastic membrane, an effect that is readily observed, for example, when light objects that repel water rest on the water surface, enabling the water strider insect (Figure 4.4) to skim, instead of having to swim! The Surface Tension leads to a real force that can indeed act on other objects and can be measured. Figure 4.5 illustrates a thin liquid film suspended within a C-shaped frame and a movable slider. The slider has a length L . The liquid has both an upper and a lower surface in contact with the slider, as shown in the side view free body diagram. Each surface exerts a force per unit length γ on the wire, so the total force is 2 γ L acting to the left. A force F applied equal in magnitude and opposite 56 Micro-and Nanoscale Phenomena in Tribology in direction must be applied to counteract this force. Here we see illustrated a criti-cal consequence of Surface Tension when a liquid–vapor interface makes contact with another object: the surface produces a force per unit length, where the force is directed in the plane of the surface normal to the line of contact with the object, and the length is measured along the line of contact. This force enables the Surface Tension of a liquid to be measured experimentally—for example, by the Whilhelmy plate method [6]. FIGURE 4.4 A water strider. The membrane-like nature of water due to its Surface Tension is clearly seen. The behavior is assisted by the hydrophobic nature of the insect’s tentacles.
eBook - PDFSurface Chemistry
Theory and Applications
- J. J. Bikerman(Author)
- 2013(Publication Date)
- Academic Press(Publisher)
CHAPTER I Liquid-Gas §1. Surfaces are conveniently classified according to the state of ag-gregation of the bodies which they separate. Thus, there are liquid-gas, liquid-liquid, solid-gas, solid-liquid and solid-solid surfaces. They are also known as interfaces between liquid and gas, liquid and liquid and so on. Apparently, there are surfaces between immiscible compressed gases above their critical temperatures, but the properties of these surfaces have not yet been investigated. Surface Tension §2. The most striking property of liquid-gas surfaces is their Surface Tension. In innumerable instances liquids behave as if they were covered with a contractile membrane. The method of Surface Tension measurement described in §7 is most easily understood if such an elastic skin is imagined. Surface Tension acts along the surface and tends to make its area as small as possible. If on a liquid surface a line, n cm. long, is considered, the sur-face tension on the right hand side of the line pulls the line to the right with the force /, and an equal force pulls it on the left hand side to the left. The ratio f:n is the magnitude of Surface Tension. Its dimension is force:length, i.e., in the metric system, dyne:cm. or g.sec 2 . Fortunately, Surface Tension is expressed in metric units also by those who for other quantities use less logical systems. Another way to describe the same capillary phenomena is to assume the existence of a surface energy. When the surface is expanded by an experi-menter, he must expend some work. This work remains stored in the sur-face and can be used again when the surface is allowed to contract. The energetic approach to surface phenomena is nearly always the safest but is sometimes too cumbersome. Examples for it are given in §38 and several other instances throughout this book. When an area, A, of new surface is created, the work, W, must be done and the specific surface energy (more exactly, free surface energy, see §35) is the ratio W:A.
eBook - PDFAir and Water
The Biology and Physics of Life's Media
- Mark Denny(Author)
- 2020(Publication Date)
- Princeton University Press(Publisher)
In each case we would have obtained the same answer: the force acting on a segment of the perimeter is equal to twice the product of the segment's length and the surface energy. Thus, the overall force exerted by the Surface Tension in the water film is equal to twice the total perimeter of the interface times 7. And finally, we make explicit a fact that has been implicit in this experiment: the direction of the force imposed by a Surface Tension lies tangent to the air-water interface. In this case, we simply mean that the force acting on the wire perimeter acts inward in the plane of the water film. In many cases, however, the interface between water and air is curved. At any point along the curve, the tension can act only in the interface itself. In this respect, and this respect alone, Surface Tension behaves in a manner similar to that of a thin rubber membrane. Atmospheric Pressure B Tension Net Tensile / Force Fig. 12.7 Surface Tension reduces the pressure of water in a glass tube (A), allowing atmospheric pressure to force water upward (B). 12.2 Capillarity We are now in a position to apply the physics of Surface Tension to a problem of biological interest. How does water get to the tops of tall trees? We begin by exploring the simple situation shown in figure 12.7. A rigid-walled tube of radius r pierces the surface of a bowl of water. The tube is open at both ends, allowing water to enter from below, and an air-water interface is created at the top of the water in the tube. The existence of this interface tells us that a Surface Tension must also exist. Our task is to quantify the effect of this tension on the column of water in the tube. We first note that the water meets the walls of the tube with a contact angle 6 C (fig. 12.7B). Because Surface Tension acts tangential to the interface, we know that the tension has a vertical component, 7 cos 0 c . If 9 C < 90° (TT/2 radians) this tension pulls up on the column of water, if 9 C > 90°, it pulls down.
- Norihiro Matubayasi(Author)
- 2013(Publication Date)
- CRC Press(Publisher)
3 Introduction to Thermodynamic Consideration of Fluid/Fluid Interface 1.3 PHENOMENA RESULTING FROM Surface Tension The shape of a liquid drop is easily deformed by an external force, but forms a spherical drop unless acted upon by the force This change arises naturally from the lowering of the magnitude of the surface area, A , since all physical properties except the surface area are kept unchanged throughout the change in the shape Thus the larger the Surface Tension γ of the liquid, the larger the potential of the surface, γ ∆ A When this liquid droplet contacts with a solid surface, it either spreads on the surface of higher surface potential or rolls away on the surface of lower potential At the beginning of the nineteenth century, Thomas Young described these phenomena by introducing concepts of Surface Tension, wettability, and contact angle (Young 1805) These three key words will well illustrate various phenomena caused by the bound-ary If our face does not get wet, we could not wash our face But even if it is wet, we could not wipe our face when the wettability of the towel is less than that of our face If the legs of water striders get wet, they can not enjoy skating because surface ten-sion pulls them into the ponds Fish could not breathe without wetting their gills The heterogeneous natural world is an ensemble of boundaries, and many phenomena in nature are relevant to wetting The contact angle is a measure of the wetting, and it varies depending on the magnitude of the Surface Tension of boundaries between adjacent phases Moreover, the Surface Tension varies depending on the temperature, pressure, and concentration of solute species dissolved in the bulk solutions Now let us consider how Young’s equation illustrates the shape of a droplet on a flat solid surface There are three boundaries: gas/liquid (G/L), liquid/solid (L/S), and gas/solid (G/S)
eBook - PDF- Girma Biresaw, K.L. Mittal(Authors)
- 2008(Publication Date)
- CRC Press(Publisher)
Since the surface atoms now sit in a higher average charge-density environment than the optimal value, the response of the surface atoms would be to increase the interatomic distance in order to achieve a new balance, which at the same time exerts a compressive stress on the surface. Surface energy is most commonly quantified using contact angle goniometry. Wetting is quantitatively defined with reference to a liquid droplet resting on a solid surface, as shown in fig. 7.5. The tensions at the three-phase contact point are indi-cated such that S/V represents the solid/vapor interface, L/V is the liquid/vapor 0.20 0.15 0.10 0.05 ∆ σ (Nm –1 ) ∆ σ sat (Nm –1 ) ∆z (nm) 0 1000 800 600 400 Time (s) (a) (b) 200 4 0 8 12 16 0 –100 0 100 0.25 0.20 0.15 0.10 0.05 0 n = 12 n = 8 n n = 4 Ref. Experiment Fit 200 FIGURE 7.4 (a) Deflection, ∆ z , and change in surface stress, ∆ σ , of a gold-coated AFM microcantilever are plotted as a function of time after exposure to vapors of alkanethiols with different chain lengths. (b) Adsorption-induced surface stress at saturation coverage (∆ σ sat ) is plotted as a function of alkyl chain length for n = 4, 6, 8, 12, and 14. (From Berger, R., et al. 1997. Science 276: 2021. Reprinted with permission from AAAS.) Surface Forces, Surface Energy, and Adhesion of SAMs 147 interface, and S/L is the liquid/solid interface. Thomas Young described surface energy as a result of the interplay between the forces of cohesion and the forces of adhesion, which, in turn, dictates whether wetting would occur [35, 36]. If wetting occurs, the droplet will spread out. In most cases, the droplet beads to some extent, and the surface energy of the system can be determined by calculating the contact angle formed where the droplet makes contact with the solid.
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