Fiber wetting

Fiber wettability can be studied with the Sigma 700/701 force tensiometer and Thetaoptical tensiometer. The wetting behavior of fibers is of interest to researchers in a variety of systems.  Fiber wetting applications include: the spreading of ink on paper, water repellency of fabrics, treatments on human hair, fiber reinforced composite production and many others. As the diameter of a fiber decreases, the ratio of surface to volume increases and the effects of the surface on the characteristics of the fiber become more pronounced. In applications involving small diameter fibers, the influence of the surface of the fiber may dominate most interactions.

How is Fiber Wetting Measured?

Characterization of the wetting of fibers presents special research problems. The primary parameter reported to characterize wetting is contact angle. Typically contact angles are measured using an optical tensiometer, also called goniometer. However, force tensiometry has also been used. The best measurement method depends on the nature of your samples and the experimental constraints.

Optical tensiometry

This method requires that a drop of the test liquid be placed on the surface of the solid tested.  Placement of a drop of liquid on fibers of small diameter may be quite difficult. Strategies used to overcome this problem have included using very small drops or analyzing the shape of a drop of liquid surrounding a vertically oriented fiber. Theta optical tensiometer enables analysis of meniscus curvatures formed between a single fiber and a liquid in order to determine the contact angle. The video below shows how such a meniscus measurement can be performed with Theta optical tensiometer:

Force tensiometry

Another approach for fibers is to use force tensiometry to measure contact angle. The Sigma 700/701 tensiometer from Attension allows you to make rapid and reliable measurement of contact angles on fibers.  This technique, commonly called the Wilhelmy method for contact angles, is described in more detail on the contact angle page. Briefly, a sample of your test solid is hung on the balance of the tensiometer and brought into contact with the test liquid. The advancing and receding contact angles are measured. Such a dynamic contact angle measurement performed with a Sigma 700/701 force tensiometer is shown in the video below:

When the fiber contacts the liquid the change in forces is detected and your Sigma 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 Sigma has tarred 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 = γ_LV  . P . cos θ

where γ_LV is the liquid surface tension, P is the perimeter of the probe and θ 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’. On almost all surfaces the advancing contact angle will exceed the receding contact angle. The difference between these two values, known as the contact angle hysteresis, is a subject of great interest in wetting studies and you are recommended to the references cited at the end of this note for further details.

An example of the type of graph generated in such an experiment is displayed below:

The graph displays data as wetting force versus depth of immersion. In the experiment shown two cycles of immersion/emersion were performed.  The lower slopes of the graph represent the force values as the fiber is immersed into the liquid and are used to calculate the advancing contact angle. The upper slopes of the graph represent the force values as the fiber is emersed from the liquid and are used to calculate the receding contact angle.

Mounting: Attension makes sample holders which make fiber mounting simple. If you do not have these holders the easiest way to mount small fibers may be as follows: make a small throw-away hook from light gauge wire. Attach a length of your fiber to the end of the hook using a fast drying adhesive. This will create the type of rigid anchor for your fiber which is necessary for reproducible data. Be careful not to expose any length of the fiber which will come in contact with your liquid to the adhesive. Cut off any part of the end of the fiber which has been contaminated by contact during this manipulation.

Length:  The fiber sample needs to be able to penetrate the surface of your liquid and maintain a vertical orientation as it moves through the liquid. For this reason you may have to trim very small and limp fibers to less than 5mm length. Although it is useful to assay longer lengths of the sample, lengths as short as 2mm can yield good data. If your fiber is not rigid and/or wets poorly with your liquid, you will notice that it resists penetration of the surface.  In such a case you should attempt to shorten the length of the sample until you achieve penetration.

Fabrics: Analysis of the wetting of fabrics is usually treated differently than the analysis of the fibers of which they are composed. The most frequently used approach to finding contact angles for fabrics it to treat them as porous solids and use the Washburn method of analysis. An alternate approach is to study the fibers which compose the fabric. Studies have shown that the contact angle of the fabric wetting, as assayed by Washburn method equals the contact angle of the fibers of which it is composed. This appears to be true regardless of the weave of the fabric (see Textile Res. J.,62(11)677-685(1992)).

Adhesion: Interest in the wetting characteristics of fibers has increased greatly in recent years. Much of this new interest involves the study of fiber reinforced composites. The successful use of fibers in composite materials depends on the adhesion of the fibers into the matrix material. The quality of the adhesion is in turn dependent on the wetting of the fibers by the matrix. Characterization of this wetting is done by contact angle experiments as described above.

Perimeter of fibers: Fibers of a variety of sizes are easily tested for contact angles. It is not unreasonable to expect that you can get good results for fibers with diameters below 10 micrometers.  As in all cases, the accuracy of your results is directly related to the accuracy of your measurement of the perimeter of the fiber. Measurement of the perimeter of your fiber can be performed manually or by the following approach which is recommended for fibers with irregular shapes.
Perform a contact angle experiment on your fiber using a completely wetting liquid such as hexane. Remembering that:

Wetting force = γ_LV  . P . cos θ

 
and that for completely wetting liquids θ = 0 and cos θ = 1,  lets Wetting force = γ_LV  . P

Using the measured wetting force and known γ_LV, it is simple to solve for perimeter. Please note that the wetting force used for this technique should be the receding wetting force. This force is chosen rather than the advancing force as it is more reasonably representing a situation with a zero contact angle. The software provided with your Sigma performs an automatic calculation of the perimeter of samples along their entire length.