Hello? This is Science Speaking
This spring, I decided to try my hand at answering science questions on Reddit’s r/askscience message board. Do I have what it takes to be a science communicator?
This spring, I decided to try my hand at answering science questions on Reddit’s r/askscience message board. Do I have what it takes to be a science communicator?
There are two main things that change in space: pressure and temperature decrease.
Pressure:
Kinetic sand is sand with a bit of lubricating agent. So long as the agent does not evaporate from the loss of pressure, the sand will act exactly the same as it does on Earth (except for the microgravity).
[Here is a video of kinetic sand in a vacuum pump.]
Temperature:
The lubricating agent is a silicon polymer called PDMS, and it exhibits properties similar to a plastic. Plastics have a property called "glass-transition temperature" which is where a substance vitrifies– this means it turns from a hard, brittle plastic into a rubbery and flowy plastic. On Earth, PDMS is always flowy. This is why it is used as a lubricant in kinetic sand and industrial applications, especially because it doesn't evaporate. Since space is -270 degrees Celsius, it is less than PDMS's glass transition temperature of −150 degrees Celsius and it would harden into a more brittle plastic substance.
[Polydimethylsiloxane (PDMS), Springer]
Sounds enable communication, from I-hear-you-therefore-I-can-eat-you to an inspirational we-have-nothing-to-fear-but-fear-itself speech. None of this communication is possible without hearing, so I think hearing will serve as a good framework to answer this question. There are 1.1 million known species in the insecta and arachnida classes, so to grasp hearing, I think it is good to start with an evolutionary perspective.
Firstly, ears are a thing for those with spines. All vertebrates have ears which evolved from the basilar papilla organ which originated in jawless fish during the Cambrian Explosion. Variations on this have become the ears in everything you know. It seems like ears are obvious evolutionarily. Everything from lampreys to frogs to snakes to monkeys to pigeons have them. But, really, those things broke off from the same tree. So what I think you are asking is: Is hearing a product of convergent evolution? Was a sound organ invented by someone other than the jawless fish?
The answer is, unsurprisingly, yes! Since understanding the vibrations around you give you an evolutionary advantage, of course they exist in other places. At the same time, since sound organs aren't in the base model for invertebrates, they evolved very differently in different creatures. The grasshopper you mentioned is a good example.
There are six insect orders which have evolved sound organs: grasshoppers, butterflies, cicadas, cockroaches, flies, and heteroptera (a class of true bugs). These all have variations on the way they hear. The two types of these organs are hair (mechanoceptors called scolophore) and eardrums (tympanal organs). Tympanal organs vary from the advanced sound organ of the cicada to the basic butterfly. Cockroaches and grasshoppers have cercal organs (technically they are different orders, but the cercal organ evolved in the common ancestor of the dictyoptera and orthoptera classes). Flies and mosquitos have a bulb that surrounds the base of their antenna that feels the vibrations of the antenna with scolophores. Heteroptera have a wonderful combination of both tympanic and scolophore sound organs. In addition to all of these, there is good evidence that ants/bees/wasps and beetles can hear, but their sound organs have not been found.
For arachnida, the question becomes whether or not you would consider feeling a really good subwoofer to be hearing. Some spiders have striated organs in their legs called lyriform organs that allow them to pick up vibrations out of the ground or leaves or etc. It is known that these noises are used in mating in wolf spiders, and new research shows that some orb spiders use their webs to catch sound. I wouldn't say that spiders hear, but they definitely make sounds!
Even within vertebrates frequency varies a lot (https://en.wikipedia.org/wiki/File:Animal_hearing_frequency_range.svg), with a number of different sense organs, some insecta and arachnida go above and some go below the ranges of human hearing! If you would like to try listening to some though, I've included some helpful links.
Further listening:
Insects
https://www.npr.org/2015/08/27/432934935/good-vibrations-key-to-insect-communication
Archnida
Sources:
General
https://www.britannica.com/science/sound-reception/Organs-of-sound-reception-in-invertebrates
Insects
Spiders
Cartilage is a bit like water balloons in a rubber mold. These cells (chondrocytes) push against the cartilaginous matrix they create around themselves to become cushions in your joints and not-quite-bone-hard tissue elsewhere. The most interesting thing about cartilage is that it has no nerves or blood vessels. This makes it a great cushion because you can't feel it hurt, but this also means that it takes forever to heal; nutrients have to dissolve into it. This can lead to a lot of pain in the joints that rely on its cushioning properties. As you age, your cartilage often fails to keep up with an injury, causing joint pain and reduced function. [1]
There are three places to find cartilage for a transplant: yourself, another human, an animal, or a lab. A procedure using each of these is called an autograft, an allograft, a xenograft, or a tissue engineering treatment respectively. Arthritis is very complicated because it is an autoimmune disease, and generic cartilage transplants for joint repair (osteochondral transplants) are better covered by the literature, so it makes more sense to ask this question without the complications of arthritis.
Autografts are great, but they are generally used in restorative work, whereas allografts are used in reparative work. Autografts repairable lesions have a limit of 2 centimeters, but there are advantages to these surgeries: there is no risk of pathogens, the donor is the recipient, and there is only one surgery. Allografts are more expensive because they are harder to do; you only have 28 days before a stored piece of cartilage goes bad. Nevertheless, they allow for more repair and more advanced procedures. [2] Both of these operations are very safe and common. [3]
Xenografts are generally only used when one is really desperate, human donors are always better. In the case of cartilage, there has never been a successful xenograft. Pig cartilage is a much better candidate for transplant than shark cartilage. In 1997, it was shown that pig cartilage is rejected by monkey immune systems, and research was halted for a while. [4] Recently, however, isolation of the triggers of human immune response from pig transplants shows a promising theoretical basis for gene therapy. [5] This might take a while to develop considering the death of the recipient of the first pig-to-human heart transplant in March. [6] As for sharks, they remain the largest source of collagen for many dietary suppliments for joints, but they will hopefully be replaced by fish farms in order to reduce human's environmental impact. [7]
There is some research into lab-built matrixes using collagen. The idea is they provide a structure for the chondrocytes to "climb" in order to facilitate repair. These are not as effective as autologous transplants. [8] [9] [10]
If you ask me, the most interesting thing going on in the frontiers of cartilage research is in the genetics of fish and sharks. Research in fish gives me hope that gene therapy can help people to grow their own cartilage, possibly through an increase in chondrocyte progenitor cells around the outside of the cartilage. [11]
[1] https://www.physio-pedia.com/Cartilage
[2] https://www.aaos.org/aaosnow/2019/oct/clinical/clinical01/
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6240981/
[4] https://pubmed.ncbi.nlm.nih.gov/9075831/
[5] https://www.ecmjournal.org/papers/vol032/pdf/v032a02.pdf
[6] https://www.webmd.com/heart-disease/news/20220309/man-with-pig-heart-dies
[7] https://www.sciencedirect.com/science/article/pii/S2665927122000594
[8] https://pubmed.ncbi.nlm.nih.gov/25691368/
[9] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6617307/
Your question is secretly not just a chemistry question, but a physics question too. To understand the impact of different material properties in tire gripping, you have to understand what tire gripping is. I will answer your question in three parts: how do wheels roll, what makes tires a special type of wheel, and what impact do the chemical properties of rubber have on tire rolling.
Wheels (imagine a wooden wagon wheel)
An object stays in motion right? A hockey puck with zero friction will slide indefinitely. But if there is friction, the puck will slow and slow and stop. How can you make something keep moving when friction exists? Make something that levitates. Can't do that either. So make something that has a single line of contact with the ground. Now you've got a sledge, it's only a little bit better; there is friction from the line after all. The Sumerians discovered a cheat code a long time ago, you can move the sledge sideways if you pick it up and set it back down on the other side. You can move it the way it naturally doesn't want to. That's the wheel. The wheel goes in two ways: forward and in a circle. The translational (forward) velocity is going forward in two important places: the center and the contact point. The angular (in a circle) velocity of the wheel is going backwards at the point of contact, but has a net zero effect at the center. This means the angular motion cancels out the forward motion at the contact point, adding to zero overall motion. Since there is no motion at the point of contact, there is no friction to oppose that motion. If there's technically no movement, there is no friction. Overall, there is forward motion, and there is angular motion that cancels itself out–on flat ground.
A car on a hill has three options. First, it can roll down the hill, second, it can slide down the hill, or third, it can not move at all. If the car locks the wheels, it will slip down the hill or it will stay in place. Options two and three have nothing to do with rolling. If it is slipping or staying, it might as well be a block. If the car unlocks its wheels, however, magic happens. The car tries to slip like a block would, but the hill fights back with static friction. An unlocked wheel is able to start moving because it changes location instead of switching to dynamic friction. The static friction is pulls uphill anyway. Remember how before the tire was rolling against the forward motion on flat ground? Well, now the static friction is inducing that force. Since the wheel has a cheat code and doesn't care about friction, the friction creates rotational motion, like tearing a tablecloth from under a bowl, leaving it rocking. The force pulling up on the contact point allows the wheel to start to roll, making it go down the hill at its center. [1] In a car, this force comes from the engine. This is why you need traction when you are accelerating: getting up to speed or going around a turn. [2]
Tires
Tires are smushed into the ground, so instead of kissing the earth, the wheel squashes into a square patch area. This leads to complications. Rubber's imperfectness as a spring causes the tire to lose energy every time it expands. [3] This energy-wasting factor is called hysteresis. The patch area factor leads to something called slip. Slip is when the car acts like a sliding block and experiences regular ol' dynamic friction. In addition, the patch area of tires also sticks to the ground with chemical bonds. This is a form of static friction.
So, there are a few factors to consider in tire rubber:
The first leads to wear by heat, the second leads to regular wear, the third leads to wear by scraping.
Car tires derive most of their ability to exert force from molecular adhesion. As the flat spot of the tire touches down, it grabs the ground. If you hit the gas really hard and use dynamic friction to accelerate, you are "burning rubber." This is bad for the clutch, transmission, and, of course, the tires. Since this isn't the expectation for most car use, this isn't the most important factor in longevity; nevertheless, it does have an impact. Hysteresis is important because it decides what I call 'wheelness' – the more hysteresis, the less wheelness. More hysteresis then more surface area then more stickiness. Less hysteresis then less surface area then less wheelness. The more wheelness you have, the better gas milage you get on the highway (closer to the ideal physics wheel, closer to the ideal physics situation). The less wheelness you have, the better acceleration you get (more surface area for stickiness). So really, you want to have the minimum hysteresis possible where you can still accelerate. Tires are a matter of engineering and require lots of testing.
Physical Properties
So, we want to know a couple of things about tires:
These three values are found experimentally at different temperatures and the curves are compared for determination. [5] Isoprene is used in caster wheels, a high hysteresis application. Tires are a low hysteresis application, so they use butadiene for that, but butyl internally because it is gas impermeable.
But what determines these characteristics?
Chemical Fundamentals
The reasons for the stickiness are quite technical, but they can be summarized as: the electrons in the long rubber polymers cluster together randomly and create temporary intermolecular charges.
External and internal adhesion is determined by Van der Waals' forces, of these, induction forces and London Forces are important. Induction forces are induced by the polar molecules in the road onto the rubber, and London forces emanate from the large size of the molecules themselves. [7] Within the rubber "the Van der Waals' forces of attraction responsible for holding the molecules together in a precise crystalline formation are strong enough to give rise to very appreciable distortions of bond positions." [8] As for compression set, molecules containing double bonds are likely to be more elastic. [8] Isoprene and butadiene both have equal concentration of double bonds in their repeating chain types, it is effects of their chain shapes in general that cause the differences. This is driven by conformation, which is energy minimization (repelling itself until in a crystalline structure eg. s-trans for 1,3-butadiene) and steric hindrance (repelling nearby versions of itself so they aren't able to minimize their energy). [9]
.-'--`-._
'-O---O--'
Comments:
[1] A force which produces rotational motion is called a torque. This friction is a torque. The relationship between the rotational acceleration and the torque is always the same for the same wheel. This is really cool because it means there is a proportionality constant for any given wheel. This constant is called inertia. It is not an actual thing, it is just a number which you can divide the torque by to get the rotational acceleration.
[2] You need friction to turn. This is because your turned wheels are pulled backward by static friction (ground torque) while your back wheels are pulled backward in a different direction. This leads to forward motion in the opposite direction. You probably intuitively think that the force of acceleration of the car is in the direction of the turned wheels, but a physics person told you that it is actually straight towards the center of the circle. This is annoying because you are right. The front wheels are pulling you the way they are pointing. But, the back wheels are pulling the car the way it was going. The net effect is that the car is being pulled inward. As the driver though, you are hyper aware of the front tires because those are the ones you control. For more information see: https://physics.stackexchange.com/questions/421707/when-a-cars-non-driving-wheels-are-turned-what-is-the-frictional-force-vector
[3] Also, because compression requires higher forces than expansion, the load is not perfectly centered in the patch, it is moved forward in the patch. See the video on tires
[4] The bad news is that this, rolling resistance, is experimentally calculated https://physics.stackexchange.com/questions/597132/rubbers-resistance-to-compression-along-one-axis.
[5] See pages 2-14/2-15 https://www.parker.com/literature/Section%20II.pdf
[6] If polarity were induced in rubber, they would become brittle (and therefore have increased "wheelness") https://www.hindawi.com/journals/jpol/2013/279529/
[7] Theoretical Models for Surface Forces and Adhesion and Their Measurement Using Atomic Force Microscopy (Levite, Bueno et. al.)
[8] https://www.jstor.org/stable/97779
[9] https://www.sas.upenn.edu/~badoyle/Conformation.html For the specifics of the shapes, read https://www.researchgate.net/figure/Isoprene-in-the-planar-s-trans-a-and-s-cis-b-conformations-The-gauche-s-cis_fig2_307831339 and https://doi.org/10.1021%2Fjp8095709
Physics Resources & Visualizations
Wheels https://www.youtube.com/watch?v=hxa6jAYA980
Tires https://www.youtube.com/watch?v=_S2lyaMgBQ8
Slip https://www.youtube.com/watch?v=tsnYqCRWTbE
All this is making me think about the vulcanization process...but we'll have to save that for another time.
