### Mistletoe Glue

August 15, 2022

One of the most important items in my laboratory is five-minute epoxy, and this is likely the case for most experimental physicists. Any experiment includes disparate components that need to be combined, and this epoxy is often the best expedient. Physicists in times before the invention of five-minute epoxy made frequent use of sealing wax as an instant glue. I've also used UV-curable photopolymers when five minutes was still too long to wait. Optical physicists have solved this problem in a more elegant fashion through the invention of the optical table.

Sometimes, adhesives are not required, as interatomic forces can be used to hold together objects that are much larger than atoms. Gauge blocks are ceramic or metal blocks that have been polished to extreme flatness and smoothness, and this polishing allows the blocks to be joined together with atomic forces in contact. These blocks are known by machinists as Jo Blocks in honor of their originator, Swedish inventor, Carl Edvard Johansson (1864-1943).

While working at a rifle factory, Johansson modified his wife's sewing machine to do grinding and lapping, and he fabricated blocks in his own home. After promising results, Johansson persuaded his employer to fund further development, and he received a Swedish patent entitled, "Gauge Block Sets for Precision Measurement," in 1901. He started his own company in 1917, moved the company to the United States, and sold the company to Ford in 1923 to form its Johansson division. He was posthumously awarded a gold medal by the Royal Swedish Academy of Engineering Sciences in 1943.

A simple model of atomic force was proposed in 1929 by physicist, John Lennard-Jones (1894-1954).[1-2] The equation for the Lennard-Jones potential models the repulsive force that exists when non-bonding electron orbitals overlap as the inverse twelfth power of distance, and the attractive force, the van der Waals force, as the inverse sixth power of the distance. The resultant curve (in blue) is shown in the following figure.

Lennard-Jones potential with its van der Waals force contribution.

The Lennard-Jones potential as a function of separation for an arbitrary molecule is shown in blue.

The force, which is attractive force below the dotted line, tapers to zero at large distances.

As is true in many cases, Aristotle (384-322 BC) had a similar observation. As he wrote in his Meteorology (11.1069a), "...where there is no contact, there is no coalescence."[3]

(Graph rendered using Gnumeric. Click for larger image.)

While the repulsive force in the Lennard-Jones potential is quite understandable as like-charged electrons repelling each other, the attractive contribution is harder to understand. It arises from electrostatic attraction between induced dipoles at the material surfaces. This attraction has a strength that's proportional to the polarizability of the material molecules.

This force was elucidated by Dutch physicist, Johannes Diderik van der Waals (1837-1923), who was awarded the 1910 Nobel Prize in Physics "for his work on the equation of state for gases and liquids." The van der Waals force is still important more than a hundred years later, since a search for papers on arXiv with "van der Waals" in the title gives 1,551 results at this writing.

Johannes Diderik van der Waals (1837-1923)

Van der Waals won the 1910 Nobel Prize in Physics "for his work on the equation of state for gases and liquids."

The idea of such a force was a part of his 1873 thesis in which he attributed the differences between real gases and an ideal gas to the existence of intermolecular interactions.

Minor planet 32893 van der Waals is named in his honor.

(Image modified for artistic effect, via Wikimedia Commons.)

How does this abstract physics relate to anything in the real world? As a child living in a wooded suburban area of Upstate New York, some of my playtime patina was the result of sticky pine sap and milkweed sap. As it turns out, the van der Waals force is responsible for the stick in such sticky substances that we normally encounter.

In the inaugural issue of PNAS Nexus, an Open access journal of the National Academy of Sciences, scientists from the Max Planck Institute of Colloids and Interfaces (Potsdam, Germany) and McGill University (Montreal, Quebec, Canada) have investigated the adhesive properties of mistletoe viscin, a natural cellulosic adhesive. This substance's skin adhesion makes it a candidate as a wound sealant.[4-5]

Mistletoe viscin is also known as birdlime, since this sticky substance is smeared on tree branches to trap birds.

Mistletoe was used by the Roman mythological hero, Aeneas, as a way to reach the underworld.

In the last few centuries, mistletoe is a Christmas decoration under which lovers are expected to kiss.

(A modified Wikimedia Commons image from a 1902 book, "A story from the Philippines," by Katherine Elizabeth Driscoll, also found at archive.org. The associated text reads, "And once Morris crept up and caught Tween Anne under the mistletoe just as papa, used to catch Mother-dear." Click for larger image.)

The European mistletoe (Viscum album) is a hemiparastic plant species, and it was used as a medicine for various ailments at the time of the ancient Greeks, and possibly earlier.[4] Mistletoe berries can produce a sticky viscin thread up to two meters in length.[5] This fiber-reinforced adhesive has an evolutionary advantage for seed dispersal of this parasitic plant, since it allows the mistletoe seeds to stick to and infest host plants.[4-5] Viscin is comprised of hierarchically organized cellulose microfibrils embedded in a humidity-responsive matrix.[4]

Deficiencies in many synthetic adhesives include irreversible adhesion and lack of adhesion under wet conditions.[4] That's why Matthew Harrington, an associate professor in the Department of Chemistry at McGill University and a member of the mistletoe research team, was intrigued with his first encounter with mistletoe. Says Harrington, "I had never seen mistletoe before living in Germany... So, when my daughter was playing with a berry from a mistletoe we bought from a local Christmas market, and it started sticking to everything, I was intrigued."[5]

A previous study demonstrated that the stiffness of viscin fiber is highly tunable through changes in relative humidity.[4] Fiber stiffness was as large as 20 GPa near 0% relative humidity, and this was reduced to about 300 MPa at relative humidity close to 95%.[4] Above 50% relative humidity, the fibers will flow under strain, but at low relative humidity the ultimate strain value was less than 2%.[4] Those properties allowed simple processing in which wet viscin fibers could be stretched into thin films or assembled into 3-D printing three-dimensional structures (see photograph).[5]

A cube formed from viscin thread.

Premanufactured two-dimensional meshes were fused by rehydration to form the three-dimensional object.

The hygroresponsive behavior of the fibers is fully reversible, and the mechanical properties at a given relative humidity were reproducible.[4] The viscin fibers have humidity-activated self-adhesive properties that allow contact welding into complex two-dimensional and three-dimensional shapes under ambient conditions.[4] Stiff and transparent free-standing films of viscin can be created by biaxial stretching in the hydrated state, followed by drying.[4] The viscin will adhere strongly to synthetic materials, such as metals, plastics, and glass, and also biological tissues, such as skin and cartilage.[4]

Skin adhesion makes viscin a candidate as a wound sealant, and its ability to stick to things reversibly under humid conditions makes it a candidate for many other applications.[4-5] Says Nils Horbelt, a recently graduated doctoral student at the Max Planck Institute and the first author on the paper, "I wore a thin film of viscin on my skin for three days to observe its adhesive qualities and was able to remove it from my fingers afterwards by simply rubbing them together."[5] The abundance of mistletoe plants, and their biodegradability and renewability, are an added benefit.[5]

### References:

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