Contact Angles by the Wilhelmy Technique AN #103

Contact angles are used as a measure of the wetting interaction between a liquid and a solid. Two techniques are commonly employed; goniometry and tensiometry. This application note explains how contact angles may be measured using the Sigma 70 precision tensiometer produced by KSV Instruments.

The tensiometric method for measuring contact angles measures the forces that are present when a sample of solid is brought into contact with a test liquid. If the forces of interaction, geometry of the solid and surface tension of the liquid are known the contact angle may be calculated. The user first makes a measurement of the surface tension of the liquid using either a Wilhelmy plate or DuNouy ring. The sample of the solid to be tested is hung on the balance and tared. The liquid is then raised to contact the solid. When the solid contacts the liquid the change in forces is detected and your Sigma70 registers this elevation as zero depth of immersion. As the solid is pushed into the liquid the forces on the balance are recorded. The forces on the balance are

F_total = wetting force + weight of probe - buoyancy

Your Sigma70 has tared the weight of the probe and can remove the effects of the buoyancy force by extrapolating the graph back to zero depth of immersion. The remaining component force is the wetting force which is defined as:

Wetting force = g _LV P cosq

where g _LV is the liquid surface tension, P is the perimeter of the probe and q is the contact angle. Thus at any depth data is received which can be used to calculate contact angle. This contact angle, which is obtained from data generated as the probe advances into the liquid, is called the ‘advancing contact angle’. The sample is immersed to a set depth and the process is reversed. As the probe retreats from the liquid data collected is used to calculate a ‘receding contact angle’. The process will appear as follows:

                          1               2                 3                4

                              Advancing                             Receding

The graph of force/wetted length vs depth of immersion will appear as follows:

1. The sample is above the liquid and the force/ length is zeroed.

2. The sample hits the surface. For the sample as shown, with a contact

angle < 90° , the liquid rises up .... causing a positive force.

3. The sample is immersed, buoyant force increases causing a decrease in

force on the balance. Forces are measured for advancing angle.

4. After having reached the desired depth the sample is pulled out of the

liquid. Forces are measured for receding angle.

The contact angles measured in this way are termed Dynamic Contact Angles. The difference between static advanced/receded contact angles and advancing/receding dynamic contact angles is that in the static case motion is incipient while in the dynamic case motion is actual. Dynamic contact angles may be assayed at various rates of speed. Dynamic contact angles measured at low velocities should be equal to properly measured static angles.

Why don’t more researchers use this technique?

The use of tensiometry to measure contact angles was outlined quite early in the literature of wetting measurements. However, general application of this technique was significantly delayed due to the absence of commercially available tensiometers which had the precision and versatility needed for easy measurement of contact angles. Without a sophisticated tensiometer designed for contact angle measurement the Wilhelmy plate technique is difficult. The Sigma 70 is a precision tensiometer integrated with a lifting mechanism and controlled through a PC which now makes measurements of contact angles fast and accurate.

The two most important considerations when considering a tensiometer are the quality of workmanship and the quality of the software powering it. KSV produces instruments that are unsurpassed in quality. The finest components are integrated into a complete package which is durable, reliable and precise. The balance will handle a maximum load of 2.5grams(more with optional counterweight) and has a maximum resolution of 1m g. The stage movement which controls the wetting speed can go to 40mm/min(667m /sec) at increments of 0.1mm/min(1.7m /sec). The liquid may be temperature controlled.

The software for the Sigma 70 is a Windows-based program which is simple to use yet provides the widest range of control options available. The software controls available include;

* speed of wetting

* speed of dewetting

* detection force recognizing interface

* start depth for subsequent cycles

* maximum wetting depth

* sample interval time

* number of cycles tested

* delay between cycles

* automatic taring option

The software contains other pre-programmed experiments which include;

* surface/interfacial tension by DuNouy ring or Wilhelmy plate

* powder wettability

* absorption profiles

* critical micelle concentration

The use of tensiometry for measurement of contact angle has several advantages over conventional goniometry. At any point on the immersion graph, all points along the perimeter of the solid at that depth contribute to the force measurement recorded. Thus the force used to calculate q at any given depth of immersion is already an averaged value. You may calculate an averaged value for the entire length of the sample or average any part of the immersion graph data to assay changes in contact angle along the length of the sample. Hysteresis, the difference between advancing and receding angles, is very easy to determine using this method. Using multiple cycles you can identify variations in surface structure and notice adsorption phenomena. Analysis of fibers, very problematic for goniometry, is handled easily by your tensiometer. Fibers with diameters below 10 microns should give reliable data. It is also easy to test the contact angle of coatings. Simply coat a solid substrate, for example a microscope slide, and test the coated solid.

The references for this note include several papers comparing the efficacy of tensiometry and goniometry for measuring contact angles.

There are two major limitations for the application of this technique. Firstly the user must have enough of the liquid being tested available so that he can immerse a portion of his solid in it. Secondly the solid in question must be available in samples which meet the following constraints. The sample must be formed or cut in a regular geometry such that it has a constant perimeter over a portion of it’s length. Rods, plates or fibers of known perimeter are ideal. The sample must have the same surface on all sides which contact the liquid. The sample must also be small enough so that it can be hung on the microbalance of your tensiometer.

References:

J.C.Berg, Wettability, Marcel Dekker,N.Y.(1993)

M.E.Schrader,G.Loeb, Modern Approach to Wettability, Plenum Press N.Y.(1992)

A.W.Adamson,Physical Chemisrty of Surfaces, Wiley & Sons,(1976)

S.Wu, Polymer Interface & Adhesion, Marcel Dekker,N.Y.(1982)

J.D.Andrade in Surface & Interfacial Aspects of Biomedical Polymers,Vol 1, Plenum Press,N.Y.(1985)

R.L.Bendure,J.Colloid Interface Sci.,42(1),137-144(1973)

L.W.Schwartz,S.Garoff,Langmuir, 1,219-230,(1985)

J.H.Wang,P.M.Claesson,J.L.Parker,H.Yasuda,Langmuir,10,3887-3897(1994)

R.E.Johnson,R.H.Dettre,J.Colloid Interface Sci, 62(2),205-212(1977)

J.Drelich, Polish J. Chem,71,525-549(1997)

L.M.Lander,L.M.Siewierski,W.J.Brittain,E.A.Vogler,Langmuir,9,2237-2239(1993)

J.E.Seeberg,J.C.Berg, Chem. Eng. Sci., 47(17),4468-4470(1992)

Y.Uyama,H.Inoue,K.Ito,A.Kashida,Y.Ikada, J.Colloid Interface Sci.,141(1), 275-279(1991)

F.Hoecker, J.Karger-Kocsis, J Applied Polymer Sci.,59,139-153(1996)

B.Miller,L.Penn,S.Hedvat, Colloids & Surfaces, 6,49-61(1983)

Application Notes

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