Have you ever tested any method? there are some methods for contact angle measurements on fibers. drop shape on single fiber, or any of wicking methods are well-known for this. if you have a surface made up of these fibers you may use sessile drop shape method which is quite easy than former ones.

Its difficult. Wettability, hydrophobicity, and water repellency are terms used interchangeably but mean totally different things. What do you actually want to know?

Can you make a flat surface of the polymer by spin coating or something? This would enable you to measure advancing and receding contact angles. The same contact angles should be observed on the surface of the fibres. Advancing CAs determine if water will spread on the fibre (wettability) whereas CAs hysteresis determines how easily liquids can slide across the surface (water-repellency). But this is only valid if the water drop is smaller than the fibre. If the drop is larger, like in most real life applications this becomes even more complicated due to the curvature of the fibres.

As stated by the others: conatct angle measurements are going to be difficult. Doing these measurements on a flat material instead might also not work since many fibers have sizings on their surface which actually determine the fiber's surface properties.

Using the Wilhelmy balance technique (http://en.wikipedia.org/wiki/Wilhelmy_plate) is probably the way to go. However, with small diameter fibers it can be difficult as well. In that case the Washburn method (http://en.wikipedia.org/wiki/Washburn%27s_equation) can help.

In all case you will have to keep in mind the sizings which might dissolve in the test liquid.

We worked with the Washburn method but we obtained extremely large numbers

Radmila finding large contact angle using washburn is quite normal!! this method gives you the "Advance Contact Angle" contact angle for glass is around 30 in sessile drop but we found it 40-45 using washburn. can I ask how did you measured capillary radius for the material you used? this is quite important!

Quantitative wettability measurements on fibers or yarn - or any textile substrate for that matter -are more difficult than one would expect.

Basically you need to distinguish, whether you analyze single fibers or yarn, first. In BOTH cases I strongly discourage you to measure contact angles using a sessile drop method. On a single fiber, spreading will be more complex than on a smooth, flat surface (cf. De Gennes). Also, you have to deal with processing residuals, which cannot always be removed, and - depending on the material itself - with porosity, swelling, and surface roughness. A droplet positioned on a yarn will obviously spread according to surface free energy, but also be affected by capillary forces as well as surface texture.

Washburn might be helpful, if you analyze yarn, but one will NEVER be certain about pore size distribution. There is no well-define pore size, and even pore size distribution is too easily affected by sample handling.

Basically the only way to really measure a contact angle on a fiber is Wilhelmy using a SINGLE fiber, but one needs to make sure that the fiber cannot "swim" on the liquid and bends.I refer to Claudio Della Volpe, who made some useful analyses in this direction.

Finally, I one considers fabrics and just needs to compare wettabilities of, say, surface modified samples, drop penetration tests are quite reliable and helpful. 

The sessile drop method is difficult to measure the wettability of fiber,  because the diameter of the fiber is too small.

Try measurement of static and dynamic contact angles (Wilhelmy-method) of individual fibres. In case the filaments are so fine that the measurement of individual fibres is complicated you can use the Washburn-method instead. This methode is based on measurement of fibre bundels placed into a tube. With this method only the advancing contact angle can be determined.

I strongly agree with the fact that , surface of the fiber is playing a major role and that is why the goniometer or the optical tensiometer doesn't work properly , but using Wilhelmy plate is the best solution the method is based on net force measurement not on the meniscus profile , so that you can manipulate the travel of the plate into the lower phase the machine will show u both the adv. and receding dynamic contact angles and if you go with very slow plate speed both values are close to the equilibrium contact angle .

bonjour

chère Radmila mon anglais est très faible (il me faut beaucoup de temps pour rédiger mais j’arrive)　:voici ma réponse en Français et je reste à ta disposition pour autres compléments

 je suis d’accord en partie avec la réponse de Amer al-shareef (Salam) sur la méthode de mesure car la mesure de l’angle de contact sur la base du profil de la goutte est presque impossible dans le cas des fibres de faibles diamètres - on peut toujours mesurer l’angle de contact sur un fibre hydrophobe par la méthode de la goutte pendante: voir [1] = exemple du cheveu , eau et sebum [1]　-

le cas du fibre est délicat car il est toujours associer à d’autres fibres (soit de la même famille ou bien un mélange de fibre de différente hydrophobie)

dans le cas du textile la maille joue un rôle prédominant dans cette affaire - selon la maille on peut obtenir une surface de textile (de fibres hydrophobe) qui laisse échapper l’eau ou bien la vapeur d’eau - ce cas est très intéressant pour l’industrie des textiles car on peut obtenir une surface hydrophobe qui n’adhère pas à la peau par exemple (faible coefficient de friction) [2,3] mais qui va laisser échapper l’eau (ou sa vapeur)

en résumé la maille du textile joue un rôle important sur la balance hydrophobe/hydrophile du fibre

exercice　: tisser les mêmes fibres avec des mailles différentes et déposer de une goutte d’eau sur chaque modèle - vous allez voir que pour une maille donnée le textile laisse échapper l’eau (vapeur d’eau) de façon étonnante - ce cas est fort intéressant puisque il donne lieu à un textile agréable au touché = faible coefficient de frottement [hydrophobe = faible coefficient de friction [2] + il fait évacuer la sueur = évite la grattage et n’abîme pas la peau.

on peut optimiser les choses on mesurant la "　critical surface tension　" du textile selon le principe de Zisman ou bien calculer son % d’’hydrophobie/hydrophile [3]

avis　: un autre sujet très intéressant à discuter est textile, cosmétologie et pansement

référence　:

1) Elkhyat et al. Influence of the physicochemical properties of the surface in the significance of the friction. Skin Res Technol 2000; 6: 156-7.

2) Elkhyat et al. Influence of the hydrophobic and hydrophilic characteristics of sliding and slider surfaces on friction coefficient: in vivo human skin friction comparison. Skin Research and Technology 2004; 10 : 1–7.

3) A. Elkhyat et al. A new method to express in vivo human skin hydrophobia . J. Int Cos Sci　2001; 23: 1-6.

Hello did you find a solution to measure contact angle? I am with the same problem.

Thank you

catarina

Thank you very much for answers. I think that the best solution in this moment is using wettability kinetic test and modified   Washburn’s equation.  I hope that we will published soon paper about it.