PHYSICAL PROPERTIES OF LIQUIDS

Measurement of surface tension byStalagmometric (drop) method

Comenius University Faculty of Medicine in BratislavaInstitute of Medical Physics, Biophysics, Informatics and

Telemedicine

• Surface phenomena, capillary phenomena, intermolecular forces in the liquids.

• Surface tension and its physical significance, surface energy, surface tension units.

• The dependence of surface tension on the concentration and temperature.

• Measurement of surface tension by stalagmometer –principle of the drop method.

• The meaning of the surface tension in medicine.

Key concepts

• Liquid molecules are similar to the molecules of gases or solids in a continuous disordered motion.

• They are maintained in about the same distance apart.They are not tied to one place and slide over each other.

• Fluids are liquids, their shape is determined by the shape of the container. They are incompressible.

• The molecules affect each other by the repulsive forces, if they are too close together. By longer distance between the molecules act attractive forces.

• Result of forces acting between the molecules is a flexible thin layer on the surface of the liquid.

• It property describes a physical quantity the surface .

What force is applied to a moleculeby other molecules inside the liquid?(around it are other molecules, fromany direction act the same attractiveforces)

The resulting force is zero.

What force is applied to a molecule near to the liquid surface?

(at the surface act molecular attractive forces from below

molecules)

The resulting force is directed from the liquid surface to down.

Intermolecular forces in the liquidand on its surface

From mentioned phenomena follows the effort of fluid to reduce it surface as much as possible.

Molecules of the liquid are drawn inwardly causing the liquid surface acts as a thin

elastic layer.WHAT ARE THE CONSEQUENCES OF THIS

PHENOMENON?

Modified by: http://www.slideserve.com/dusty/vlastnosti-kapalin-povrchov-nap-t

VELMOVSKÁ, K., LAPITKOVÁ, V.: e-fyzika.ddp.fmph.uniba.sk/dokumenty/Ucebnica_na_stranku/05_VS_ucebnica_Mo

Water strider Hydrometra stagnorum can walk

slowly over the surface of the water

The shape of small droplets on Earth approaches the spherical

shape

Fluid surface can hold paper clip

Surface tension ( )

Constant which is determined by the force F acting on thefluid perpendicular to the surface layer per unit length of arow of molecules .

(N.m-1)lF

Surface tension can be also defined as energy,respectively work W necessary to increase self-generatedliquid surface of a unit area A:

(J.m-2)

- depends on the physico-chemical properties of themedium (temperature, degree of contamination, theelectric field).

AW

Liquid Surface tension -1] at t=20 °C

Water 73Ethanol 22Methanol 22,7Aniline 40,5Glycerol 62,5Formic acid 37,8Acetic acid 28Mercury 491

SPN, 230 s. ISBN 80-08-03335-5.

Examples of surface tension values of some liquids

Source: Reger, D. L., Goode, S. R., Ball, D. W.: Chemistry. http://academia.cengage.com/chemistry/reger

Surface tension can be measured by direct method (a) and undirect methods (b, c):

a. tear- – using torsion balance is measured the force needed to wire tearing off from the liquid surface.

b. by help of capillary elevation and depression

ph – hydrostatic pressure, pc – – surface tension, – liquid density, g – gravity constant, R – radius of the capillary

c. - using stalagmometer and drops weighing or counting according to Traube´s method modification.

rhg .2..

Ph = Pc

rpc

.2 hgph ..

Traube´s stalagmometer

A constant volume of liquid is given by volume of the reservoir of stalagmometer VS (volume isdefined by the upper and lower mark).The number of drops depends on the surface tension of the liquid, and inversely proportional to the size of the droplet.

During dripping drop gravity force must overcome force of surface tension acting on the circumference of the end of the capillary.

F= .l = 2. .r.

G = m.g

G > F

Tasks

• Repeatedly (5 times) measure the number of drops which are formed at a constant flow of VS of the liquid with known surface tension from the end of the thick-walled capillary of stalagmometer.

• Repeat it for liquid whose surface tension is unknown.

• The measured values put in the table.

• Calculate the average number of drops and standard deviation for the two fluids.

• In the table find the density of both liquids and the surface tension of the known liquid corresponding to room temperature.

• Tabular and measured values substitute to the formula for calculating the mean and standard deviation of the surface tension of an unknown liquid.

Recommended tools and devices

Traube´s stalagmometer, syringe, rubber tube, laboratory stand, thermometer, distilled water, 20 % citric acid solution, physiological solution, glycerine, mull in the Petri dish, beaker waste, physical tables, tray with filter paper.

Object of measurement

Distilled water or other liquid of known surface tension, sample of liquid of unknown surface tension (20 % citric acid solution, physiological solution, glycerine).

Procedure

• Dry and clean stalagmometer fasten vertically to the laboratory stand, on itupper part we load the rubber tube with the syringe

• The reservoir VS we fill up to the top mark with liquid of known surface tension (usually distilled water).

• Suction we finish at the appropriate time by strangulation of the tube and disconnecting the syringe.

• Then we count how many drops shall be created until the liquid meniscus falls from the top to the bottom marks.

• We measure the number of drops at least five times.

• The same procedure we repeat when measure the number of drops of unknown surface tension liquid.

• Stalagmometer has to be protect against mechanical shocks during measurement.

Tab. Measured number of drops

Surface tension of distilled water arises due to the polar nature of the water molecule

and the temperature drops significantly.

At 25°C has a value of 72 mN.m-1.

Modified by: NAVE, C. R. 2005. HyperPhysics. [online]. [cit. 2013-05-01].

Surface tension of blood in the sample 71 healthy subjects

(24 men and 47 women) measured by drop methodat 22 °C has a value of:

= 55,89 mN.m-1, SD=3,57 mN.m-1.

ROSINA, J. et al. Temperature Dependence of Blood Surface Tension. -

Obr. Temperature dependence of blood surface tension

Modified by: NAVE, C. R. 2005. HyperPhysics. [online]. [cit. 2013-05-01].

The existence of surface tension at the phase boundaries of living organisms has the biological significance:

• Capillary elevation and depression (in aqueous solutions is observed mostly capillary elevation).

• Importance in transport processes on the membrane surface and breathing.

• Adsorption - adsorbent binds adsorbed material by weak Van der Waals forces to the surface.

• The surface active substances reduce the surface tension of liquids (surfactants).

Examples of application in medicine:

• Some drugs influence environment in the digestive tract by reducing the surface tension of the fluid contained in the stomach and intestine, thereby facilitate passage of gas, eliminatethe discomfort associated with it.

• Some supplements act in the human body at all mucosal surfaces and reduce the surface tension, thus support detoxification and excretory functions of the body.

• Influenza vaccine protect against infection by preventing adsorption of the virus to the target cell receptor sites.

• Drug delivery in the form of drops (cough drops, nasal drops, eyedrops, infusion). The dose of drugs with the same active substance but with different additives can be varied as a result of different size of droplets (i.e. different surface tension).

• The surface tension in the alveoli plays an important role inrespiration. The cause of negative pleural pressure by quietbreathing in addition to the relative rigidity of the rib cage is theretractive force of the lungs (the tendency to decrease volume)due to the elasticity of the lung tissue and the surface tension ofthe liquid covering the alveoli (5-10 mN.m-1).

Retraction forces cause an inspiratory decreasing of alveoli,which is described by Laplace's law:

where p is distension pressure, is surface tension and r is theradius of the alveoli.

An important factor that reduces the surface tension of the alveoli,is the presence of a lipoprotein substance called surfactant.Surfactant deficiency can cause the collapse of the alveoli, therebyseverely disordered breathing.

rp .2

• Surface tension of waterSoap aqueous solution has a lower surface tension than purewater. This fact allows it easier to get to the surface of thehands and to wash them thoroughly.

Washing and cleaning solutions containing detergents havealso lower surface tension and better cleaning properties.

Demonstration video:

https://www.youtube.com/watch?v=1Z_JqcHjJss

Discussion

1. Physical principle of stalagmometric method of surface tension measurement.

2. Evaluation of the measurement conditions, the impact of microclimate factors, possible sources of errors in the measurementprocedure.

3. Results and their comparison to the table values.

4. Physical interpretation of results.

Conclusion

1. The meaning of the surface tension in the physiological processes in vivo.

2. Importance of surface tension and surfactants for hygiene and working environments.

Recommended literature

• Oxford dictionaries. - -2015]. Available on the Internet:

http://www.oxforddictionaries.com/

• Hyperphysics - -2015]. Available on the Internet:

http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html.

• Basics of Medical Physics and Biophysics for Electronic

Education of Health Professionals. - -2015]. Available on

the Internet:

http://staryweb.fmed.uniba.sk/fileadmin/user_upload/editors

/ustavy/fyzika/Literatura/Literatura_2013/Basics_of_Biophysic

s.pdf

STALAGMOMETRY

H2O ± 3) kg.m-3

citric acid ± 3) kg.m-3

alcohol = (804 ± 3) kg.m-3

physiol = (1005.3 ± 3) kg.m-3

H2O = (72.5 ± 3) mN.m-1