contact angle measurement 1

[redclouds]

I met a problem when measuring contact angle of water drop on my sample. Since my sample is very hydrophobic, the water drop prefer staying with the disposing needle to standing on my sample surface. If I increase the water volumn until 8ul, the water fall down on the surface due to gravity. but I concerned the big drop may affect the contact angle.

Another question is how to measure the advancing contact angle and receeding contact angle. I know to measure the contact angle when increasing the drop volume and decreasing the volume. But what's the crition?

Hope some professionals can help me out.

 

[nairbo]

Right I'm not exactly upon this but I think the following is true;

The droplet volume has no effect on the contact angle go ahead with the larger volume droplet (try on a less hydrophobic surface, with a small drop then a big one measure contact angles compare+tell me if I'm wrong!)

The advance and receding contact angle are maximum and minimum figures if you like. So just increase and decrease your droplet and record the two extremes you measure.

poohbah.cem.msu.edu/courses/cem419/cem419exp3.pdf

This link might help but it sounds like your doing everything right to me.

Regard Nairbo

 

[joseONE]

The Contact Angle (CA)is not in function of the size drop. The CA must be mesured between interfases. Certainly there is a limit on this interfacial force. Therefore, in order to measure qith the optimal sensiblity the Interfacial tension, you must dispose a drop of liquid over your surface.
Otherway I am interested on your experimental data to try to find a better explanation about your remark.

Thank you

 

[redclouds]

refer to the same question on contact angle measurement, I am confused with the criteria of advancing and receding contact angle measurement. Increasing the drop volume by remaining the needle tip in the drop results in rise in contact angle. But which angle is the advancing contact angle? Likewise, the contact angle decreases when syringe picking up the drop slowly. I noticed that the angle decreases continuously until he drop is completely picked up. then which angle is the receding contact angle?

Any suggestion or comments are welcome.

 

[kenvlach]

It is uncertain whether you are truly observing contact-angle hysteresis because of interference with the syringe. When creating the drop, hold the syringe just high enough that the droplet forms a teardrop shape and releases under gravity. Remove the syringe and measure.

Contact-angle hysteresis.
“Possible causes of contact-angle hysteresis are chemical heterogeneity of the solid surface, surface roughness and the presence of impurities.” -- Properties of Liquids and Solutions, J. N. Murrell and E. A. Boucher, p. 257 (1982).

A book (which I don’t have) that deals extensively with the subject: Second International Symposium on Contact Angle, Wettability and Adhesion held in Newark, NJ, June 21-23, 2000. ISBN 90-6764-370-X (published 2002).
Table of Contents:

http://www.vsppub.com/books/apsci/cbk-ConAngWetAdhVol2.html

(note: there is also a Volume 1, proceedings of the 1992 symposium)

Some websites:

http://www.ksvinc.com/contact_angles.htm

Basic contact angle measurement and instruments; some mention of hysteresis.

“A model for contact angle hysteresis,” J. F. Joanny and P. G. de Gennes, The Journal of Chemical Physics Vol 81(1) pp. 552-562. July 1, 1984.

http://content.aip.org/JCPSA6/v81/i1/552_1.html

(abstract only)

“Impact of nanometer-scale roughness on contact-angle hysteresis and globulin adsorption," Bert Mueller et al., J. Vac. Sci. Technol. B 19(5), pp. 1715-1720 Sept-Oct 2001. (entire article, with 80 references).

http://www.textorgroup.ch/pdf/publications/journals/60/Mueller_B_2001.pdf

“The results show that the advancing contact angle of water monotonically increase by 20° from the flat substrates [cf. Fig. 1(a)] to substrates with maximum pyramid density [Fig. 1(d)] whereby the receding contact angle remains constant within the error bars. Note that the contact angle measurement is rather difficult for values below 15°. This means that the contact-angle hysteresis, which is the difference between the advancing and the receding angles, increases with the surface roughness. From the intersection of the fits for advancing and receding angles versus contact angle hysteresis, &[ignore]Delta[/ignore]; &[ignore]Theta[/ignore];, with the ordinate at &[ignore]Delta[/ignore]; &[ignore]Theta[/ignore]; = 0, one finds the equilibrium contact angle &[ignore]Theta[/ignore];[sub]e[/sub] {ref. 44} (cf. Fig. 3).”

Lots more Internet sites that I didn't explore. Suggest doing your own search for "contact angle" as well as "contact-angle hysteresis."

Please report on measurements without the syringe in the drop. Also note whether you are conducting the experiment in ~saturated air (Relative Humidity near 100%) or non-equilibrium conditions.