One day, a few years ago, I ate a bowl of cereal and I haphazardly forgot one piece of cereal in the bowl. I then also neglected to wash the bowl for a number of days, causing the milk to dry out. When I picked up the bowl again, I noticed that the piece of cereal, Cheerios, was stuck firmly to both the spoon and the bowl.
Being interested in the potential of such a Cheerios-based adhesive, I decided to 'glue' a spoon to a window by dipping the Cheerios in milk and clamping it between a spoon and a piece of glass:
Seen from the side it would look like this:
I didn't really have any idea of how it worked at the time, but knowing that both metal and glass are two materials that are often difficult to stick together, it was striking to me that Cheerios, when combined with milk, would be able to act as an adhesive for these two objects.
I never quite figured out how to clearly show that it was in fact the combination of milk and Cheerios that kept the spoon in its unusual place, so it never went very far as an artwork.
However, it still intrigued me from a chemical point of view, as it was an odd combination of materials to be stuck together so easily. Adhesion of such dissimilar materials is often very much influenced by mechanical adhesion. In mechanical adhesion, the (invisible) roughness of a surface is filled up with a material that is liquid at first but then hardens to a solid. These two materials are then not chemically bonded together, yet they can't move as there is no physical space to do so.
It was however unlikely that this is the full story in this particular case, as both glass and metal have relatively smooth surfaces and nothing in milk actively polymerises as it dries. There are therefore very few cavities to fill and no obvious substance to fill them with.
Being interested in surface interactions for another project, it occurred to me that the electrostatic activity on the surface of the metal, combined with the free electron pairs in the silicon dioxide of the glass, could perhaps create non-trivial hydrogen bonds with the sugar molecules in the milk. The Cheerios are in turn largely comprised of long chains of sugars, so that the sugar from the milk can form hydrogen bonds with those and possibly have an intertwining crystallisation structure, providing rigidity.
This combination is partly illustrated in the following diagram, where (1) denotes the crystal lattice of the metal and the free electron pairs on the surface, (2) are hydrogen bonds with the sugar molecules, (3) are the sugar molecules that are left over when the water has evaporated from the milk, (4) are the hydrogen bonds between the sugar from the milk and the polysaccharides from the cereal, and (5) are those polysaccharides.
To test the plausibility of this hypothesis, I devised several experiments where different combinations of materials were tried out iin order to isolate and test a number of variables.
For these experiments, single Cheerios were placed in liquid and left to soak for 30 minutes. The liquids used were semi-skimmed milk, water, or water with an amount of sugar dissolved in it.
The wet Cheerios were then placed on a glass or plastic surface, and a spoon was placed on top. The spoons were balanced so that their own weight pressed upon the Cheerios.
This was then left to dry for ~3 days.
The degree of adhesion was then determined by the experimenter through detaching the materials from each other. This could result in either low, or no, tack (denoted as --), some tack (denoted as +/-) or high tack (denoted as ++).
The results of the various experiments can be found in the following table:
| Materials | Result, Expected |
Result, Observed |
|---|---|---|
| Glass, Spoon (std), Cheerios, Milk | ++ | ++ |
| Glass, Spoon (std), Cheerios, Water | -- | -- |
| Glass, Spoon (std), Cheerios, Sugar Water | ++ | ++ |
| PolyPropylene, Spoon (std), Cheerios, Milk | -- | -- |
| PolyMethyl MethAcrylate, Spoon (std), Cheerios, Milk | ++ | +/- |
| Glass, Spoon (smth.), Cheerios, Milk | +/- | ++ |
| Glass, Spoon (std.), Milk | +/- | +/- |
| Glass, Spoon (std.), Sugar Water | +/- | -- |
| Glass, Spoon (std.), Kitchen Paper, Milk | ++ | -- & ++ |
The expected result was the result based on the theory as outlined above and the observed result is what actually was the case. It's clear to see that the expected and observed results match each other closely.
There were a couple of instances where the observed result differed from the expectation, however, namely in the case of the PMMA substrate, a smooth metal spoon, sugar water in the absence of cereal and the substitution of Cheerios for kitchen paper.
The observation that there was high tack in the combination of Cheerios with both milk and sugar water, while there was no adhesion at all when the Cheerios was only soaked in water, shows that the presence of sugar is very important in the adhesive properties of this combination of materials.
That the Cheerios with milk showed high tack on glass, some tack on PMMA and no tack on polypropylene also indicates that hydrogen bonding is very important to the adhesion to the glass substrate, as was expected.
An experiment done with a spoon that had a very smooth surface also shows that the observed adhesion is chemical or electrostatic, rather than mechanical, in nature. It was expected that a smoother surface would give less adhesion to the spoon, yet no discernible difference was observed between a well-used spoon and a new, smooth, spoon.
Two experiments performed with only milk and sugar water in the absence of Cheerios showed that sugar alone can't act as an effective adhesive for these materials. While the sugar stuck firmly to the glass, likely through hydrogen bonding, it showed virtually no adhesion to the metal spoon. Nevertheless, a thin droplet of milk did have some tack to the metal, so that some other component of the milk must be the substance that binds to the metal. The most likely candidate is calcium, as calcium ions are very large and able to form complexes with a high coordination number, thereby binding various molecules together.
To examine the influence of the Cheerios, an experiment was performed where a wad of kitchen paper, made out similarly long polysaccharides, was soaked in milk.
This gave an interesting result, where this wad strongly adhered to the glass, but showed no tack on the metal surface. This is most likely caused by the greater absorbance of kitchen paper, so that the sugars or ions in the milk where in little contact with the metal as the water evaporated.
In conclusion, when using Cheerios and milk as an adhesive for metal and glass, all four components are important contributors to the overall effect. A major contributor to the adhesive strength is the large amount of sugar found in milk, which is aided by other components, where an abundancy of calcium likely aids in bonding to the metal of the spoon. The combination of milk and Cheerios binds to the glass through hydrogen bonding and to the metal by some other chemical or electrostatic force, where mechanical adhesion only has a limited contribution.
