HOMEWORK Application of Environment Spatial Information System HW – Surface Tension Minkasheva Alena Thermal Fluid Engineering Lab. Department of Mechanical Engineering Kangwon National University

Contents Definition of Surface Tension Surface Tension Illustration Surface Tension Equation Two forces Example: Surface Tension on a Needle Measuring Surface Tension Surface Tension and Surface Wetting Liquids in Contact with a Solid Surface Contact Angle Capillary Action Surface Tension with Temperature Examples of Surface Tension Surface Tension - Final Remarks

Definition of Surface Tension Surface tension is ability of the surface of a liquid to act like a thin, flexible film Surface tension is the amount of work required to extend a liquid surface Droplets take on a spherical shape since a sphere has the smallest surface area for a given volume

Surface Tension Illustration Interior A water molecule at the surface lacks neighboring water molecules (toward the interface) A water molecule deep within the liquid is pulled equally from all sides Net force is not zero! Net force = zero This unequal attraction causes the water at the air-water interface to act as a cohesive surface and the liquid to shrink to the smallest possible surface area

Surface Tension Equation The surface tension is defined as the ratio of the magnitude of the surface tension force to the length along which the force acts: represented by the symbol or σ or T SI units are N/m Can express as J/m2 - in terms of energy, any equilibrium configuration of an object is one in which the energy is a minimum

Two forces Two forces now come into play - the energy it would take to overcome this repulsion and the force of gravity. g If the force of gravity is strong enough, it will prevail and the object will sink If the gravitational force is less than the surface tension then the object will float on the surface of the water Of course, the critical feature here is the amount of force per unit area -- put a needle into water end-on instead sideways and the needle will immediately sink. Air force is negligeble! g

Example: Surface Tension on a Needle Surface tension allows the needle to float, even though the density of the steel in the needle is much higher than the density of the water The needle actually rests in a small depression in the liquid surface The vertical components of the force balance the weight

Measuring Surface Tension The force is measured just as the ring breaks free from the film The 2L is due to the force being exerted on the inside and outside of the ring

Surface Tension and Surface Wetting The force of gravity that provides the energy required to spread the drops via collapse of the drop, two other forces play important roles: 1. Adhesive forces are intermolecular forces between unlike molecules 2. Cohesive forces are intermolecular forces between like molecules The shape of the surface depends upon the relative size of the cohesive and adhesive forces The liquid spreads, because adhesive forces are comparable in strength to cohesive forces The liquid “beads up”

Liquids in Contact with a Solid Surface The adhesive forces (liquid-glass) are greater than the cohesive forces (liquid-liquid) The liquid clings to the walls of the container The liquid “wets” the surface Cohesive forces (liquid-glass) are greater than the adhesive forces The liquid curves downward The liquid does not “wet” the surface

Contact Angle Φ > 90° and cohesive forces are greater than adhesive forces Φ < 90° and adhesive forces are greater than cohesive forces

Capillary Action 1 Capillary action is the result of surface tension and adhesive forces The liquid rises in the tube when adhesive forces are greater than cohesive forces At the point of contact between the liquid and the solid, the upward forces are as shown in the diagram Here, the cohesive forces are greater than the adhesive forces The level of the fluid in the tube will be below the surface of the surrounding fluid

Water rises because of capillary action 2 The height at which the fluid is drawn above or depressed below the surface of the surrounding liquid (density ρ) is given by: - surface tension φ - contact angle r - tube radius Water rises because of capillary action

Surface Tension with Temperature Water molecule representative low T high T Weaker intermolecular forces Increase of surface area Lower Surface Tension Variation of water surface tension with temperature

Examples of Surface Tension Walking on water: small insects can walk on water because their weight is not enough to penetrate the surface. Floating a needle: even though several times as dense as water, a needle carefully placed on the surface of water can float. If agitated to break up the surface tension, then needle will quickly sink. Soaps and detergents: help the cleaning of clothes by lowering water's surface tension to more readily soak into soiled areas. Washing with hot water: hot water's surface tension is lower and it is a better “wetting agent”. Tent materials: can be rainproofed so that the surface tension of water will bridge the pores in the finely woven material. Touching the material will break the surface film & rain will drip through. Clinical test: normal urine has a surface tension, g, of about 66 dynes/cm but if bile is present, it drops to about 55. In the test, powdered sulfur is sprinkled on the urine surface; it floats on normal urine, but sinks if g is lowered by the bile. Capillary action

Surface Tension - Final Remarks Systems always search to minimize energy ⇒ minimize surface/interface with highest energy Surface tension defined as: • the force along a line of unit length perpendicular to the surface • the energy required to increase the surface area of a liquid by a unit area represented by the symbol or σ or T measured in Newtons per meter (N/m) or J/m2

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