Tag Archives: physics

Partial cleaning may make surfaces look dirtier

The reason why incomplete cleaning may increase the visual perception of dirt is by increasing the contrast between the patches of thicker grime and the normal colour by removing a uniform covering of thinner dirt. If something is uniformly grimy, then the colour of the covering dirt may be perceived as the thing’s normal hue. Cleaning may remove approximately the same thickness of dirt from all points on the surface. If some patches initially have a thicker layer, then these remain the colour of the dirt after the cleaning, but other areas may be fully cleaned and revert to the original look of the surface. The human visual system mostly perceives contrast, not the absolute wavelength of the reflected light, as various optical illusions demonstrate. Higher contrast between the thicker patches of grime and the rest of the surface then enhances the perception of dirtiness.

Algae tattooed for doping and oxygen administration

The paper by Qiao et al. (2020) in Science Advances shows that unicellular algae injected near a hypoxic tumour photosynthesise oxygen in the body in response to infrared light with wavelength 660nm that penetrates >4mm into tissues. The oxygen saturation of the tumour rises from 6.2% to 30% in 2 hours after the algae receive a 5-minute laser exposure. The oxygen sensitises the tumour to radiation therapy. No side effects were found from the algae in this or previous research. The performance of the algae stayed the same when these were coated with red blood cell membranes to delay their clearance from the body.

Another application of algae that can produce oxygen in the organism is doping in sports. The algae can be tattooed under skin that is exposed to light containing enough of the wavelengths which the algae use and which can penetrate under the skin. For example, long-distance runners outdoors in warm weather have most of their skin exposed to sunlight, thus have a large surface area suitable for algal oxygen production. The additional oxygen from photosynthesis improves athletic performance. The only question is whether the oxygen generation is quantitatively fast enough to make a difference. In elite sports, every little advantage counts, so athletes are probably willing to use algae tattoos.

The algae are not dangerous even in deep blood vessels and tissues. Eventually the organism clears the algae, but the clearance of foreign particles is slower in the skin than in deep tissues, as evidenced by the persistence of ordinary tattoos. So the algae will last for a daylong competition.

Patients with breathing problems, for example with coronavirus-induced lung inflammation, may also benefit from algae tattooed on a large area of skin which is then illuminated with 660nm light. Such oxygen supplementation reduces the need for mechanical ventilation. Again, the question is the amount of oxygen from a whole-body algal tattoo.

Chemical vapour deposition with sound waves

Creating a standing sound wave in the chamber in which CVD occurs may generate interesting patterns in the deposited film like on Chladni plates. A strong enough compression and strain induced by the wave may even change the crystal structure of the deposit. Maybe even form freestanding filaments through the volume of the chamber, not just a flat deposit on a surface.

Diffraction grating of parallel electron beams

Diffraction gratings with narrow bars and bar spacing are useful for separating short-wavelength electromagnetic radiation (x-rays, gamma rays) into a spectrum, but the narrow bars and gaps are difficult to manufacture. The bars are also fragile and thus need a backing material, which may absorb some of the radiation, leaving less of it to be studied. Instead of manufacturing the grating out of a solid material composed of neutral atoms, an alternative may be to use many parallel electron beams. Electromagnetic waves do scatter off electrons, thus the grating of parallel electron beams should have a similar effect to a solid grating of molecules. My physics knowledge is limited, so this idea may not work for many reasons.

Electron beams can be made with a diameter a few nanometres across, and can be bent with magnets. Thus the grating could be made from a single beam if powerful enough magnets bend it back on itself. Or many parallel beams generated from multiple sources.

The negatively charged electrons repel each other, so the beams tend to bend away from each other. To compensate for this, the beam sources could target the beams to a common focus and let the repulsion forces bend the beams outward. There would exist a point at which the converging and then diverging beams are parallel. The region near that point could be used as the grating. The converging beams should start out sufficiently close to parallel that they would not collide before bending outward again.

Proton or ion beams are also a possibility, but protons and ions have larger diameter than electrons, which tends to create a coarser grating. Also, electron beam technology is more widespread and mature (cathode ray tubes were used in old televisions), thus easier to use off the shelf.

Dark-coloured buildings and cars are silly

Many buildings in Australia, especially new developments, are black, dark grey or brown, or at least the roof is. Many cars are black (other dark colours are less prevalent). The dark colouring increases both cooling and heating costs, because it absorbs and emits solar and infrared radiation faster. In addition, the dark buildings are depressing and ugly. Dark-coloured cars are more difficult to notice, especially in low-visibility conditions, thus have more accidents. White or yellow vehicles are the safest (Lardelli-Claret et al. 2002, Solomon and King 1995).

For cars, the choice of black colour is probably caused by the owner’s desire to seem wealthy by making the car look expensive – limousines in films and popular culture are often black. For buildings, the association in people’s minds between colour and price is weak. If anything, light-coloured houses, reminiscent of Mediterranean villas and the White House, may slightly raise the owner’s status. The reason for dark-coloured roofs may be the cost – tar paper is a cheap material, easy to install. Windows may appear dark due to the one-way glass used. However, for walls, the cheapest material is usually bare concrete, as shown by its choice for purely functional structures (warehouses, barriers, piers, military buildings). For private dwellings, wood or brick may be the cheapest. Neither concrete, wood nor brick is particularly dark in colour, so the choice to build black or brown houses is puzzling. Maybe it is an architectural fad – fashions often trump practicality.

Bending polycarbonate glasses

Polycarbonate glasses (sunglasses, safety goggles) can be bent to better fit one’s head by heating the thermoplastic polymer with a lighter, gas stove or other heat source. Example photos are below. Polycarbonate is not flammable and tolerates repeated heating and cooling. The only problem is that the paint discolours when heated and the plastic buckles and wrinkles when bent, so the lenses should not be heated, lest they become unusable. However, for the handles, a better fit outweighs cosmetic appearance in many applications.

Blind testing of clothes

Inspired by blind taste testing, manufacturers’ claims about clothes could be tested by subjects blinded to what they are wearing. The test would work as follows. People put clothes on by feel with their eyes closed or in a pitch dark room and wear other clothes on top of the item to be tested. Thus the subjects cannot see what they are wearing. They then rate the comfort, warmth, weight, softness and other physical aspects of the garment. This would help consumers select the most practical clothing and keep advertising somewhat more honest than heretofore. For example, many socks are advertised as warm, but based on my experience, many of them do not live up to the hype. I would be willing to pay a small amount for data about past wearers’ experience. Online reviews are notoriously emotional and biased.

Some aspects of clothes can also be measured objectively – warmth is one of these, measured by heat flow through the garment per unit of area. Such data is unfortunately rarely reported. The physical measurements to conduct on clothes require some thought, to make these correspond to the wearing experience. For example, if clothes are thicker in some parts, then their insulation should be measured in multiple places. Some parts of the garment may usually be worn with more layers under or over it than others, which may affect the required warmth of different areas of the clothing item differently. Sweat may change the insulation properties dramatically, e.g. for cotton. Windproofness matters for whether windchill can be felt. All this needs taking into account when converting physical measurements to how the clothes feel.

Food calories are not directly related to obesity

It seems that the change in bodyweight should be a strictly increasing function of calories eaten minus calories spent. The first caveat to this claim is that calories in the food eaten do not equal the calories absorbed, which is what matters for weight gain. People differ in the efficiency of their digestion – that is what bariatric surgery relies on. Also, food labels use calories measured by the burn method (dry the food, burn it, measure the heat output), which ignores for example that coarsely chewed food chunks pass through the digestive tract relatively unchanged, thus contribute few nutrients to the organism. An extreme example is wholegrain flax (linseeds) that remain undigested due to a waxy coating. In nature, seed dispersal by birds relies on the indigestibility of the seeds.

Even if calories absorbed could be accurately measured, the type of food eaten would still matter for weight gain due to imperfect willpower. Some foods are more addictive than others, notably those rich in refined carbohydrates – an easy example is drinks that are essentially sugar-water. Consuming a given amount of calories from high-glycaemic-load sucrose makes people eat more sooner on average than ingesting the same calories from slowly digested whole grains or unsaturated fat. Similarly, ignoring willpower limitations is why abstinence-based programs to prevent sexually transmitted infections, which naively might be expected to be 100% effective, are in fact ineffective (Underhill et al 2007, doi:10.1136/bmj.39245.446586.BE).

There are various tricks to circumvent limited willpower to win the game against one’s future tempted self. To reduce temptation, food should be out of sight outside mealtimes (in cupboards, drawers, fridge) and unhealthy snacks should not be bought at all. Even seeing dishes and cutlery may trigger cravings, in which case these too should be placed out of sight when not in use.

Avoiding grocery shopping while hungry is an old piece of advice, which may be taken further by having someone else buy your food. Two people can even agree to grocery-shop for each other according to shopping lists exchanged beforehand. Online ordering may be a solution, but of course the merchants want customers to buy more, so advertise tempting foods with photos on their website. These ads can be blocked with some effort. A more sophisticated solution is to have one’s own user interface (front end) interact with the merchant’s website – scrape the data on inventory and prices, send commands to buy.

Chair material should be breathable

Strangely enough, fancy office chairs are often upholstered in leather or other non-breathable material. After an hour or so, sitting in them gets uncomfortable because the moisture does not evaporate from the skin that is separated from the chair only by some cloth. However, people usually sit in office chairs for hours at a time, and I doubt that the chairs are designed to deliberately provide some discomfort to encourage users to occasionally stand up for health reasons. Especially in a hot climate when the air conditioning may break down or just be too weak, a breathable chair would make much more sense. Making a mesh chair is simple: stretch a breathable fabric on the chair frame. The small holes in the fabric let moisture evaporate from the surface. A mesh chair is probably cheaper to manufacture than a leather-upholstered one.

A reason for using leather may be to signal wealth by using a material associated with expensiveness.

Modern synthetic meshes are as durable as leather for practical purposes, because sitting in a chair is not a high-wear use.

Slippery sidewalk paving

In Singapore, the streets are well planned and maintained, smooth and clean like everything else. However, the sidewalks have one illogical aspect: the pavement is smooth stone, which gets very slippery when wet. Singapore is tropical and humid, with frequent rain. When initially paving the sidewalks, it would be easy and probably cheaper to use rougher covering (asphalt for example) that would not get slippery in the rain. After the smooth pavement has been laid down, changing it is of course costly, and if the locals have adjusted to the slipperiness, then switching the sidewalk cover may not be worth it.

The University of Queensland has a similar problem with the sidewalk in front of its main building. The sidewalk is coarse, like yellow asphalt with 1 cm stones in it instead of sand. One would expect such a coarse surface to provide good grip in all conditions, but unfortunately the looks are deceiving. When that sidewalk gets wet, it becomes slippery like polished glass. Again, it would be cheaper and more practical to pave that sidewalk with asphalt.

A broader point generalising the above observations is that things should be field-tested in realistic conditions before putting them to widespread use. For example, the sidewalk stones should be walked and biked on under all local weather conditions before paving a street with them.

Perhaps the smooth stones in Singapore are meant to make street cleaning easier. Still, there are materials that do not become slippery when wet and are smooth enough for mechanised cleaning and cheap enough to use as pavement.