Drop friction

Dr. Neil Canter, Contributing Editor | TLT Tech Beat April 2010

Researchers explore how friction affects the movement of liquids on a solid surface.

KEY CONCEPTS
• The effect of friction on the movement of liquid drops on a solid surface had not been previously examined.
• The Centrifugal Adhesion Balance measures the changes in friction for drops situated at any particular angle on a surface.
• The lateral forces needed to move pendant drops are higher than those for sessile drops. This differs from what occurs in moving a solid object on a surface.

A better understanding of friction at the macroscale and the nanoscale has been the subject of much research and a number of articles in TLT. We all know that a solid object placed on a surface can be moved once frictional forces are overcome.

The force needed to push the object is proportional to the object’s weight or load. A heavier object has a greater contact area with the surface than a lighter one and, as a result, has more friction to overcome in order to move. This is the basis for Amontons’ laws of friction.

In a previous TLT article, work was done to show that friction at the nanoscale is consistent with what is seen at the macroscale (1). Both effects are dependent upon the roughness of the surface. In the case of the nanoscale, roughness is defined by the number of atoms that interact between two surfaces.

An important aspect of friction that has not been explored is how friction affects the movement of liquid drops on a solid surface. As most of the lubricants we work with are liquids, this should be an important consideration in selecting a product for a specific application.

Rafael Tadmor, associate professor of chemical engineering at Lamar University in Beaumont, Texas, started looking into this issue after seeing a difference in performance between drops based on oleic acid compared to stearic acid. He says, “We were studying different coatings used for the stabilization of ferric oxide colloidal dispersions, which form magnetic fields utilized to seal computer hard drives. An oleic acid coating results in a stable dispersion in hexadecane, but a stearic acid coating cannot form a stable dispersion in the same medium.” 

“Placing a hexadecane drop onto a mica surface treated with the salt derivatives of two surfactants showed very different results: The drop wetted and spread on the oleic acid-derivated surface but formed a well-defined drop on the stearic-derivated surface. This showed that the oleic acid is solvated by the hexadecane while the stearic acid drop is not.” 

The structure of oleic acid and stearic acid only differ in that the former has a double bond between the ninth and tenth carbon. Yet, they produced markedly different wetting properties. This result prompted Tadmor to use drops as a sensitive tool for surface characterization and eventually to take a closer look at the frictional behavior of liquid drops on surfaces.

CENTRIFUGAL ADHESION BALANCE
Tadmor and his research group built an instrument called the Centrifugal Adhesion Balance (CAB) that can generate any combination of normal and lateral forces to assess the ability of an object (a drop in this study) to remain on a surface. An image of the apparatus is shown in Figure 1.


Figure 1. The Centrifugal Adhesion Balance is used to measure the lateral forces needed to move a drop at any particular angle to a surface. (Courtesy of Lamar University)

The cylinder at the right end of the CAB is a closed chamber that contains a camera and a light source. In between the two is a surface on which a drop can be applied. To the left of the chamber is a control box that takes data from the camera and sends it wirelessly to a nearby computer. Further to the left of the control box is a counterbalance.

Underneath the table holding the CAB is a DC motor that is used to rotate the cylindrical chamber containing the drop.

By tilting the chamber, the normal and lateral components of the centrifugal force as well as those of gravitation force are varied. By controlling both the tilt and the centrifugal acceleration, any combination of normal and lateral force can be obtained.

The researchers measured the change in friction for hexadecane on various surfaces. Drops are situated initially at angles of 0 degrees and 180 degrees. This corresponds to a sessile drop and a pendant drop, respectively. The former is a drop that rests on a surface, while the latter is a drop literally hanging from the surface.

The drop is moved to a particular angle and allowed to rest for a specific time. Then the CAB arm is slowly rotated with a slowly increasing angular velocity until the drop starts to slide along the surface.

Tadmor says, “We found that the lateral forces needed to move pendant drops are higher than those for sessile drops. This means that less force is required to move a drop with a greater contact area on the surface, which differs from what we know about a solid object on a surface.”

In other words, greater friction is seen for a pendant drop than a sessile drop.

While seemingly counterintuitive, this result is consistent with current thinking about how drops move on surfaces. In the case of a pendant drop, the gravitational force works in the same direction as the pulling of the solid surface by the drop’s surface tension. This factor leads to a molecular orientation that creates a stronger intermolecular interaction between the drop and the surface. On the other hand, when a drop is resting in a sessile fashion, the gravitational and surface tension forces act in opposite directions, resulting in a weaker drop-surface intermolecular interaction.

Some experimentation was tried with water drops, which seem to exhibit the same behavior but it was more difficult work with this material. Tadmor says, “We worked with water, but factors such as evaporation made it very difficult to maintain fixed conditions. The high surface tension of the water also served to destroy some of our surfaces.” 

Future work will be conducted in different media, including the use of silane grafted onto a silica surface. Additional details about this research can be found in a recent paper (2) or by contacting Tadmor at rafael.tadmor@lamar.edu. 

REFERENCES
1. Canter, N. (2009), “Understanding Friction Laws at the Nanoscale and their Relation to the Macroscale,” TLT, 65 (7) , pp. 10–11.
2. Tadmor, R., Bahadur, P., Leh, A., N’guessan. H., Jaini, R. and Dang, L. (2009), “Measurement of Lateral Adhesion Forces at the Interfaces between a Liquid Drop and a Substrate,” Physical Review Letters, 103 (26), 266101.


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.