Behavior of liquid drops on flexible fibers

Dr. Neil Canter, Contributing Editor | TLT Tech Beat July 2012

New research could lead to a better understanding of how a specific lubricant performs on a fiber surface. 

KEY CONCEPTS
• The movement of a drop on multiple flexible fibers is dependent upon six variables.
• The volume of the drop determines whether it will spread on the fiber.
• A model shows whether drops will totally spread, partially spread or not spread, regardless of wetting liquid type or fiber composition.

THE EFFECTIVENESS OF A LUBRICANT is determined by how it interacts with a specific surface. One property that has been covered extensively in this column is the ability of a lubricant to wet or spread on a specific substrate. Another aspect has been work done to develop hydrophilic-resistant surfaces that resist water, which is known to cause premature failure in many types of lubricants, leading to the preparation of superhydrophobic surfaces.

In a previous TLT article, a new material known as SLIPS (slippery liquid- infused porous surfaces) was developed from a perfluorinated lubricant (1). SLIPS has the unique ability to not only reject hydrophilic materials but also to reject hydrophobic substances such as mineral oil. The result is the development of a self-lubricating omniphobic surface.

Lubrication of textile fibers is also an important characteristic that many of us take for granted but which has a powerful influence on whether we will purchase a specific piece of clothing. A previous TLT article discussed a strategy for measuring the friction found on fabrics through the use of a refined friction factor (2).

Much of the work done to evaluate the ability of a liquid to wet a fiber has been on single fibers. Howard Stone, Donald R. Dixon and Elizabeth W. Dixon professor in mechanical and aerospace engineering at Princeton University in Princeton, N.J., says, “Many of the studies on wetting fibers involve looking at the flow of drops on individual rigid fibers that are often arranged in an array.”

There has not been much attention paid to research evaluating how liquid drops move on flexible fibers. Stone says, “Elasticity of the fibers can be very important in many applications such as the strengthening of paper after drying or the shrinkage of porous fiber membranes.”

A new study has now been conducted to determine the variables that impact how drops move on flexible fibers.

DROP SIZE
Stone, in collaboration with his coworkers Camille Duprat and Suzie Protiere, conducted a study to see how tiny drops interact with multiple flexible fibers. The initial work was done by placing silicone oil drops on two glass fibers that are maintained at a fixed distance away from each other and in a horizontal orientation. The fibers were clamped at one end and free to move at the other end.

Duprat says, “In working with more than one fiber, we are dealing with a very complex system that depends on six variables that include (1.) the diameter of the fiber, (2.) the distance between the fibers, (3.) the length of the fibers, (4.) the elasticity of the fibers, (5.) the surface tension of the liquid and (6.) the volume of the drop. Based on the results, we focused on the volume of the drop and the length of the fiber.”

The researchers found that the ability of a drop to spread on the fiber decreases as the volume of the drop also increases. Factored into this is the length of the parallel fibers. The ability of the drop to spread increases as the length of the fiber also increases.

In transitioning from no spreading to full spreading, under the right volume and fiber length parameters, a drop also can partially spread. Duprat explains, “Partial spreading occurs when there is a drop at one edge and a column of liquid spreading along the surface of the fibers.”

When the volume of the drop increases above a certain value, no spreading is observed. Duprat says, “At a specific volume, the force applied by the drop is not strong enough to deform the fibers sufficiently to spread the drop on the fiber. Spreading of the drop takes place when the overall energy of the system declines as compared to the liquid remaining as a drop.”

Figure 1 shows this phenomenon. In the top two pictures, the drop size is small enough so that it spreads on the fibers. Only partial spreading is seen with the larger drops in the third and fourth photos from the top, while little to no spreading is observed with the even larger drop in the bottom photo.


Figure 1. The size of drops determines whether they spread along flexible glass fibers. Below a critical size, the drops spread into columns of liquid (shown in the top two pictures), but large drops remain compact with minimal spreading (shown in the bottom picture). Drops of intermediate volume spread partially (shown in the third and fourth picture from the top). (Courtesy of Princeton University)

Using the large amount of data they developed, the researchers prepared a phase diagram showing the total spreading, partial spreading and no spreading regions. Stone says, “We can now predict the degree a drop can spread on a fiber surface based on the volume of the drop.”

For example, a study was made about how readily silicon oil can spread on bird feathers. For this system, spreading is achieved if the radius of the drop is less than 20 microns. No spreading is seen if the radius of the drop is greater than 140 microns.

Stone says, “Our model will work no matter the type of wetting liquid used or the composition of the fiber. This means that we can accurately predict how well a water drop will spread on a fiber.” Future work will look at what factors affect the ability of a fiber surface to dry, which is, in effect, the opposite of wetting.

This research has the potential to help maximize the wettability of a liquid on any surface. A worthwhile application pertinent to our industry is to examine how the size of a drop of lubricant on a metal surface affects its spreadability. The result may lead to recognizing that drop size may be one of the factors affecting how a specific lubricant performs on a particular metal surface.

Additional information can be obtained from a recent article (3) or by contacting Stone at hastone@princeton.edu.

REFERENCES
1. Canter, N. (2011), “Self-Lubricating Omniphobic Surface,” TLT, 68 (1), pp. 10-11.
2. Canter, N. (2006), “Determining the Friction on Fabrics,” TLT, 62 (6), pp. 14-16.
3. Duprat, C., Protiere, S., Beebe, A. and Stone, H. (2012), “Wetting of Flexible Fibre Arrays,” Nature, 482 (7386), pp. 510-513.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.