1. How Plastic Wrap Stays on a Plate Without Glue
Place the edge of plastic wrap on the rim of a plate and lightly stroke it with your finger. Even when you let go, the wrap stays stuck, following the shape of the plate. The manufacturer Asahi Kasei Home Products explains that the surface of Saran Wrap is "very smooth, making it hard for gaps to form between the film and the plate, so it adheres firmly." [3] There is no mention of adhesive.
In media explanations, this is attributed to "intermolecular forces" (specifically van der Waals forces). Because the wrap’s surface is so smooth, a force strong enough to make it stick is generated. [1] This means that even without a "connecting substance" like glue, if the wrap gets close enough to the dish, a sticking force is created. So, why can the wrap get so close to the dish?
2. How a Flat, Soft Wrap Fits Closely Against a Dish
Because it is soft, it fits the dish. In the TV program "Chico-chan ni Shirakareru!", the answer to why wrap sticks was "because it is perfectly flat, and because it is soft." [2] A representative from the manufacturer’s technical department also stated that "flat" and "soft" are the two main characteristics of wrap. [2]
When the wrap is soft, it bends perfectly to fit the shape of the dish. This pushes the air that was between the wrap and the dish outwards. When the air between them decreases, the surrounding air pushes the wrap against the dish from the outside. This mechanism is called "reduced pressure adsorption," which is the same reason suction cups stick to things. [1]
Static electricity is also introduced as playing a partial role, but it is not described as the main cause. [2] It is thought that the reason wrap sticks is a combination of several forces, not just one. By the way, how close do you need to get for the forces between molecules to become strong?
3. How Close Wrap and Dish Must Be for Molecular Forces to Work
It weakens suddenly if you move just a little away. Van der Waals force, a type of intermolecular force, acts between atoms and molecules. Encyclopedic explanations state that in a narrow sense, it is inversely proportional to the 7th power of the distance between atoms. [4]
Inverse proportion to the 7th power means that if the distance doubles, the force becomes 1/128th as strong (since 2 to the 7th power is 128). Note that this applies to a pair of atoms; for a broad surface like wrap, the total does not follow this exact rule. This shows that it is a force that only works when things are very close.
The more widely you touch, the stronger it is. This force works more strongly as the contact area increases. [1] If the wrap is flat and soft, it touches the dish widely and closely. Small forces add up over a wide area, allowing it to stay stuck even when you let go.
4. Does the Wrap's Material Change How Well It Sticks?
Home use is mainly two materials. Household wrap is primarily made of polyvinylidene chloride (PVDC) or polyethylene. [6] For commercial use, polyvinyl chloride (PVC) was used in the past, but now multilayer films are used. [6]
Adhesion changes with material and additives. Some polyethylene wraps include additives that act as adhesives to help with sticking. [7] The official explanation for commercial Saran Wrap states that it has better adhesion than wraps made of other materials. [8] This suggests that stickiness is adjusted not just by the material, but also by additives and structure.
5. Does the Dish's Material Change How Well Wrap Sticks?
Wrap adheres easily to dishes like glass, but it adheres poorly to items like wooden ones. [1] The surface of glass is smooth, so the wrap can touch it widely and closely. This fits the conditions for intermolecular forces to work. However, this is an inference based on the conditions explained so far in this article.
The reason wooden dishes stick poorly is shown only in diagrams in the original explanation, so it cannot be definitively stated that it is due to surface roughness. [1] It is certain that the dish side also has conditions that determine stickiness, and that there are still details that cannot be specified precisely.
6. An Experiment: Compare How Wrap Sticks to Different Dishes at Home
Compare by changing dishes. Choose 3 dishes from your home, such as glass, plastic, and wood, which have different materials. Apply the same wrap in the same way, lightly sticking it along the rim. Gently pull the edge of the wrap to compare which dish is hardest to pull off by feel. Also see how the results change when the inside of the dish is wet versus dry to understand the difference in conditions.
To avoid cutting your hand with the blade on the wrap box, use it with an adult. Do not use chipped or easily breakable dishes. [1]
If you want to know more, the manufacturer’s official page explains the wrap’s surface and adhesion in short words. By comparing the manufacturer’s official pages and media explanations found in the sources, you can see "where the manufacturer’s explanation ends and the commentator’s explanation begins."