Tikalon Blog by Dev Gualtieri

Wet Adhesion

October 1, 2015

Many families have the tradition of having special meals on weekends when they're not rushed to exit the house for work and school. My family always enjoyed having pancakes on Sunday. The only thing I hate about a pancake breakfast is the sticky syrup bottle. Perhaps I'm not alone in my dislike of having sticky fingers and wanting to wash them as quickly as possible.

Sticky fingers are one example of adhesion. Sticky fingers arise from a chemical mechanism; namely, hydrogen bonding. Hydrogen atoms, while bonded to molecules, can still bond lightly to each other. That's what gives liquid water its cohesion.

Syrup is a solution of monosaccharide sugar molecules in water, and these monosaccharide molecules have a lot of hydrogen atoms available for bonding (see figure). The stickiness arises from the combined bonding of the molecules to themselves and to your fingers.

Quite a few hydrogens for bonding.

D-glucose (Dextrose).

(Created using Inkscape.)

Along with hydrogen bonding, there are two simple, mechanical forms of adhesion. The simplest case is the suction cup that holds my GPS receiver onto the automobile windshield. There are no chemical forces at work there, just the pressure of the air outside the cup pushing against the lack of air inside.

A novel form of mechanical adhesion is found in Velcro™. I wrote about Velcro™ in an earlier article (Insect Velcro, April 22, 2013). As can be seen from the figure from the Velcro™ patent, below, the principle of operation is having a myriad number of hooks linking with eyelets.

Figure 4 of US Patent No. 3,009,235, 'Separable Fastening Device,' by George de Mestral, November 21, 1961

Figure four of US Patent No. 3,009,235, 'Separable Fastening Device,' by George de Mestral, November 21, 1961.

(Via Google Patents.[1])

Another type of adhesion arises from the electrostatic attraction between surfaces. A common example is the attraction of a rubber balloon to objects after rubbing on some fabrics. This fascinated me when I was a child. That, and magnetism, may have been the reasons that I became interested in physics. A form of chemical adhesion different from hydrogen bonding involves a chemical reaction between two surfaces. An example of this known to every mechanic is the bond between a rusted bolt and nut.

A most interesting type of adhesion is cold welding. Clean and atomically-flat surfaces will bond together when placed in contact in a vacuum. As Richard Feynman explained for copper surfaces in his Feynman Lectures (Vol. I, Chap. 12-3, Molecular forces),

"The reason for this unexpected behavior is that when the atoms in contact are all of the same kind, there is no way for the atoms to 'know' that they are in different pieces of copper."[2]

This type of adhesion is used in a metrology device called a gauge block.

The most technologically useful type of adhesion is when the atoms or molecules of solids diffuse into each other each other to form a graded interface linking them. An example of this process is the sintering of metals and ceramics in which a sintering aid lowers the melting point of the material at the surface of its particles to allow them to merge.

As most people have found, wet adhesive tape doesn't stick, and the directions on tubes of household adhesives state that the surfaces to be bonded must be "clean and dry." These adhesives bond to water leaving no active bonding sites to mend your broken geegaws and knick-knacks.

However, certain marine animals such as barnacles and mussels have no problem affixing themselves to underwater objects. A team of scientists from the University of California, Santa Barbara (Santa Barbara, California) has researched such wet adhesion.[3-5] They found that these marine organisms create protein-based ions that suppress the activity of saltwater ions. As a consequence of their experiments, the research team has developed a compound that mimics the activity of mussel adhesive.[3-5]

Dreissena polymorpha (Zebra Mussel).

(United States Geological Survey photograph, via Wikimedia Commons.)

The Santa Barbara team included two graduate student co-lead authors, Greg Maier of the Chemistry and Biochemistry Department, and Michael Rapp from Chemical Engineering; and professors Alison Butler, Jacob Israelachvili, and J. Herbert Waite.[5] The motivation for this research, as explained by Butler, is that "there's real need in a lot of environments, including medicine, to be able to have glues that would work in an aqueous environment."[5]

Mussels accomplish their adhesive magic through overproduction of the catechols, 3,4-dihydroxyphenylalanine (dopa), and lysine, a protein with a cationic amine residue; that is, a functional group attached to the molecule.[3] It was assumed that the adhesive properties arise from a synergistic effect of these two chemicals.[3] The team experimented with siderophores, molecules having both catechol and lysine functionalities.[3]

The lysine acts to displace hydrates cations from a mineral surface, thereby allowing catechol to bind to surface oxides.[3] The siderophores and synthetic analogs have good adhesion to mica, exhibiting binding energies of -15 millijoules per square meter in saline solutions with a pH from 3.5 to 7.5.[3] In creating a synthetic adhesive, the research team decided to modify a small siderophore molecule called cyclic trichrysobactin (CTC).[5] CTC contains both lysine and a compound similar to dopa.[5]

This synthetic adhesive had a wet adhesive strength similar to that of mussel adhesive.[5] Says Butler,

"We just happened to see a visual similarity between compounds in the siderophore CTC and in mussel foot proteins... We developed a better, more stable molecule than the actual CTC... Then we modified it to tease out the importance of the contributions from either lysine or the catechol."[5]

Lysine was important. Says Maier, "Our tests showed that lysine was key, helping to remove salt ions from the surface to allow the glue to get to the underlying surface."[5]

As Rapp summarizes,

"By looking at a different biosystem that has similar characteristics to some of the best-performing mussel glues, we were able to deduce that these two small components work together synergistically to create a favorable environment at surfaces to promote adherence... Our results demonstrate that these two molecular groups not only prime the surface but also work collectively to build better adhesives that stick to surfaces."[5]

UCSB adhesive action (artist impression)

Artist's impression of wet adhesion to mica. A cationic amine, shown in pink, penetrates the hydration layer, evicting potassium ions (gold balls) to prepare the mica surface for hydrogen bonding (green). (UCSB illustration By Peter Allen.)

Wet Adhesion

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