(a) What is surface tension? How is it measured experimentally? How does it depe
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Question
(a) What is surface tension? How is it measured experimentally? How does it depend on temperature?
(b) From the results in Table II and III, what are the main conclusions? How does fracture depend on stress and defect size? Which equation is verified from these experiments?
Please Use the experimental verification of Griffth’s original paper to answer the questions above.
Please give the explanation for each sub-questions! Thanks.
172 MR. A. GRIFFITH ON t. Experimental Verification of the Theory. In order to test formula (13 it was necessary to select an isotropic material which obeyed HooKE's law somewhat closely at all stresses, and whose surface tension a ordinary temperatures be For could estimated. these reasons glass was preferred to the metals in common use. A comparatively hard English glass, having the following properties, was employed Composition Sio,, 69.2 per cent. Kao, 12.0 per cent Nazo, 0.9 per cent. ALO, 11.8 per cent. Cao, 4-5 per cent. Mno, 0.9 per cent. Specific gravity--2.40. YOUNG's modulus-9.01 x 10 lbs. per sq. inch. PorssON's ratio 0.251. Tensile strength- 24,900 lbs. per sq. inch. The three last-mamed quantities were determined by the usual tension and torsion tests on round rods or fibres about 0.04-inch diameter and 3 inches long between tl gauge points. The fibres had enlarged spherical ends which were fixed into holders with sealing wax. A slight load was applied while the wax was still soft, to ensure freedom from bending. The possible error of the extension measurements was about t 0.3 per cent., and HooKE's law was obeyed to this order of accuracy. No elastic after working was observed with this glass, though more accurate measurements would doubtless have indicated its existence. The problem of estimating the surface tension of glass, in the solid state, evidently requires special consideration. Direct determinations a to be impracticable, ppeared and ultimately an indirect method was decided on, in which the surface tension was found at a number of high temperatures and the value at ordinary temperatures deduced by extrapolation. On the accepted theory of matter, intermolecular forces in solids and liquids consist mainly of two parts, namely, an attraction which increases rapidly as the distance between the molecules diminishes, balanced by a repulsion (the intrinsic pressure), which is due to the thermal vibrations of the molecules. It is reasonable to assume that the attraction, at constant volume, is sensibly independent of the temperature; this amounts merely to supposing that the attraction exerted by a molecule does not depend on its state of motion. On this view, the temperature variation, at constant volume, of the intermolecular forces is determined entirely by the change in thermal energy. Hence, it may be inferred, on the accepted theory of surface tension,t that the surface tension of a material, at constant volume, is equal to a constant diminished by a quantity proportional to the thermal energy of the substance. In the case solids, nearly the same result should hold at constant pressure, as the temperature- volume change is small Supplied in the form of test-tubes by Messrs. J. J. Griffin, Kingsway, London. t PoYNTING and THONSON, Properties of Matter,' eh. xvExplanation / Answer
Surface: this is referring to the molecules of some liquid on the surface.
Tension: it says that these molecules are in tension
Surface tension is a collection of cohesive forces between the molecules of a liquid, at the surface of the liquid, thereby binding them together. But these aren't so strong and could handle forces up to an extent of 0.1N approx.
This is an important property of fluids which can explain the phenomenon of capillarity in tubes, shape of a drop, concavity or convexity of liquid surfaces in different tubes of different materials
However, molecules on the surface do not have neighbors on upper side. This results in net downward attractive force and stronger attraction between molecules on the surface. This forms so called 'film' on the surface of water. This film forms a barrier and requires additional force to penetrate due to strong inter-molecular bonding on the surface.
Surface tension is defined as force required to break a unit length of liquid surface OR Work required to create an additional unit surface of liquid (Forming surface requires energy as the molecules on surface try come down due to net downward force).As a direct consequence, liquid droplets are spherical. Because molecules on surface try to 'shrink together' due to net downward force and sphere becomes the only configuration possible with least surface area.
HOW TO MEASURE IT:
The most common way is through use of a tensiometer. A geometry is submerged into the liquid and slowly drawn upwards until it contacts the surface of the liquid. The rate of withdrawal is then slowed until the geometry breaks completely free from the surface of the liquid. The most common geometries are either rings (sometimes called dunouy rings), rectangular slides (sometimes called Wilhelmy plates) or rods.
For a pure liquid in equilibrium with its vapour:
F- Surface tension.
F0 -Surface tension at 0C0C
T -Temperature (Celcius)
Tc- Critical temperature. (Celcius)
n a number 1..2
F= F0(1T/Tc)n
There is a drop off to zero as the temperature increases to the critical point. For water that point is 650 C. Surface tension is dependent on temperature. For that reason, when a value is given for the surface tension of an interface, temperature must be explicitly stated. The general trend is that surface tension decreases with the increase of temperature, reaching a value of 0 at the critical temperature.
The real fracture strength of a material is always lower than the theoretical value because most materials contain small cracks or contaminants (especially foreign particles) that concentrate stress. Fatigue cracks always start at stress raisers, so removing such defects increases the fatigue strength.
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