There are many soft things: cats, babies, expertly laundered sweaters. If there were some kind of omniscient softness guide, with each item in the universe ranked in order of softness, these three items would definitely land to the top. Of course, such a guide is very difficult to assemble: since softness is at least partially subjective, you need teams of volunteer softness assessors to cover each item, and an honest / statistically sound way of averaging all of their responses. And at some point, one of these softness assessors would certainly wonder why they actually do this in the first place, lower morale, and endanger the whole project. Therefore for this week Giz asks, we keep it simple: four materials scientists, who represent none other than themselves, weigh their choice for the absolute softest.
Assistant Professor, Materials Science and Biomedical Engineering, University of Delaware
At first it seems like you find the ‘softest thing’ [that we can feel]Would be easy. You can go to the hardware store and buy an assortment of ‘soft’ materials. Maybe you start with rubber, then continue with marshmallows and foams. The problem with this approach is that it cannot tell what people are doing to determine the softness of an object. In fact, depending on the shape, two objects made of the same material can be made to feel harder or softer. For example, human hair is often considered soft, but it is made of keratin, the same material used in our fingernails and rhinoceros horns. Designing something that feels soft depends on what the object is made of, but there are some subtleties.
Recently, instead of observing a single material property (most people may recommend “elastic modulus”), we recommend that people use a combination of two physical signals to determine the perceived softness of an object. These indications are partly determined by material properties and partly by the shape of the object. When you touch an object, two things happen with your finger. First, your finger pushes or pushes into the object, and the object also pushes into your finger. Second, the object spreads on your finger with a certain contact area. This combination of dent and the contact area on your finger forms the physical basis for the perception of softness. So any object that generates a large impression depth or contact surface feels “soft”. The perception of softness is not only determined by the physics of materials. It is also informed by how our brains process information. Although our fingers are soft, we found that people are already responsible for the softness of their own fingers. In other words, our models were more accurate when we pretended the finger was as stiff as a stone.
We are excited about the possibilities and possible illusions that these basic studies revealed in softness. We also think that there are careful and methodical ways to find our tangible boundaries, such as what is the softest and most difficult object we can feel. This showed that there are multiple pathways to controlling the perceived softness of an object, which could be useful in future virtual reality applications. It also shows how our brains process and synthesize tactile information.
Professor, Materials Science and Engineering, Carnegie Mellon University
When we talk about softness, we usually start with the term ‘compliant’ – that’s when you push a piece of material and it gives easy. In other words, you can change shape without using much force. That is the engineer’s definition of soft. But there is more to it, because “soft” is both a sensation and a technical concept – what is also important is how the material, the substance, feels to you. There is a way in which something can feel soft that is not completely independent of how easy it gives or how easily it changes shape. Another dimension of softness is how the material handles heat, or how well it is an insulator. And that can use different people in different ways. But for the most part, softness is usually associated with fabrics that are relatively good insulators – for example, blankets feel soft, partly because of the way they react mechanically, but also partly because they don’t allow heat to pass or conduct easily. While most gels feel a little clammy which we don’t exactly associate with softness even though they have a lot of ‘give’.
So I think something like a down duvet would be one of the softer things, and that’s partly because it has multi-shell structure, and this is, I suspect, a multi-shell problem – meaning the softest materials or the materials that give you the impression of being soft often have multiple scales. You can make a rug out of something that will last quite a bit, but unless you have a material like soft wool on top that can easily budge, it won’t feel particularly soft. But the multi-layered structure of a down duvet ensures that it gives your skin some sensation while at the same time giving way extremely easily, and because it doesn’t conduct heat very easily, there is a certain element of heat that we associate with softness.
Data scientist, astronomer and science communicator
I was asked what the softest material in the world is, but that is too limiting. I would like to answer some freedoms, another question: what is the softest planet? We now know more than 4,000 planets orbiting other stars. These “exoplanets” represent only the small fraction that we have been able to observe. Much more, perhaps hundreds of billions in our own Milky Way Galaxy, await our discovery. What we learned by finding them is that there is a dizzying diversity of worlds. We have found worlds made of rocks like the Earth, worlds with two suns, and worlds probably completely covered with lava. We’ve also found a handful of bizarre, “soft” worlds, a type of exoplanet called “super puff.”
The light that an exoplanet reflects or gives off is usually too dim for us to see compared to the blinding light from the star orbiting it. Instead, we ‘see’ it when it turns in front of the star from our point of view, blocking some of the star’s light from being called a ‘passage’. By looking at how much of the star’s light is blocked, we learn that there is a planet and how big it is. We can find their mass by another method. As it rotates, it pulls the star gravitatively toward it, which we see as movement in the star toward Earth, and then away, in a regular pattern caused by the planet’s orbit and revealed in small changes in the color of it. light from the star due to the Doppler shift. The power of the tugboat gives us the mass of the planet. The mass combined with its size gives us density, and those planets with extremely low densities, almost a tenth that of water, are called super clouds. They look almost as big as Jupiter, but these planets only weigh 1% what Jupiter does.
Earth’s mossy hills and sandy beaches may be soft in their own way, but the strange, barely-present softness of super-cloud planets is due to their extremely low density. A simple experiment a child can do tells you about the density of everyday objects: does it sink or float? Rock is much denser than water, so it would sink, ice floats. In our own solar system, Saturn has a density less than water, so you can imagine it to be in the ‘floating in water’ category. These strange planets have a density even less than that of ice, more like the density of styrofoam or cotton candy!
Just as gas bubbles rise in water, a planet composed mainly of hydrogen and helium gas has a much lower density than a rocky world. Saturn’s density is slightly higher than the gas it forms, because the crushing gravity of the top layers of gas pushes it down deeper. So find a planet with a density close to cotton candy? Astronomers were shocked: it’s just incredibly weird. How is it not compressed to higher densities under its own weight?
The mystery of these planets has yet to be solved, but there are many theories. Perhaps the planet’s atmosphere is heated by the star, causing it to swell, and gas can escape from the planet’s gravity into space over time. Perhaps dust flowing from the top layers of the planet’s atmosphere is blowing up our size estimates. Or maybe our assumptions about the densities are wrong, and what we really see are planets with opaque rings at an angle that blocks extra light from the host star, blowing up our size estimate. Fortunately, astronomers have plans to look for more super clouds, test theories of how they form and evolve, and learn how soft super clouds actually are.
Associate professor and director of Sensory Evaluation Center, The Pennsylvania State University
Softness is essentially a perception (a percept). This means that it occurs in the brain, which means that it can only be measured with a human assessor. With any human sensory system, there are detection limits, both high and low, and beyond these limits we cannot see (perceive) any difference, even if a machine or instrument could. Consider the hardness of cut glass and a cut diamond – when we touch them with our fingertips, they each squeeze our skin (and the mechanoreceptors in our skin) about the same amount, so we can’t distinguish them even as a lab instrument could.
Of course, measuring things with people can be quite tricky because when people use a word like soft, its meaning can vary significantly in the context of usage – this ice cream is softer than that ice means something completely different than this t-shirt softer than that t-shirt. In the first case, softness is a combination of meltability and strength required to create, while in the second it may refer to friction experienced by the hand as we drag it across the fabric. In yet another context, softness can be a function of compression force and bending force. A branch of experimental psychology (psychophysics) has been working for more than 150 years on the way in which sensations can best be measured quantitatively.
These kinds of nuances can also be extremely important for the perception of the consumer and the acceptance of the product. For example, when I see a new hoodie in the store, I first put my hand inside to see how soft the sweatshirt is. If it’s not extremely soft and cozy, I keep looking no matter how good it looks. Therefore, understanding and measuring complex sensations can have important economic consequences. Many of the foods and consumer goods you consume every day are optimized by sensory and consumer scientists who use classical and applied psychophysics to quantify the sensations they provide.
Professor, Mechanical Engineering and Materials Science, Duke University
First, what is softness? Known for Crick and Watson, Watson is said to have said, “If you want to understand depression, study happiness.” So I say, “if you want to understand softness, study hardness.”
So: is softness just a lower hardness? From a technical point of view, hardness is measured by how a material reacts to an applied force or tension. There are several standard hardness scales, such as the Mohs hardness scale, which according to this definition consists of “ten minerals arranged in increasing hardness so that each mineral will scratch the mineral on the scale below, but not the one above.”
Could you do the same for softness, where one material moves or another material bends?
For example, can you take a spring and see if it will move a wool fiber?
There is a lot of very soft material in everyday life, such as a wool fiber, cotton balls, jello, feathers or thin magnetic tape. But what about a functional material that actually supports life on Earth?
One of the softest functional materials known is the membrane that surrounds every cell on that planet. In my research over the past 40 years, we deal with phospholipid membranes. These are basically the membranes that surround every cell on Earth. They are made of lipid bilayers and are only two molecules thick (thin). Using special techniques using a glass micropipette that can be manipulated in a microscope chamber containing an aqueous solution and on which we can apply very gentle positive (bladder) and aspiration (suction) pressures, we actually measured their softness. Softness in this case is what is needed to stretch and bend the membrane.
This softness has values between that of a liquid and a gas. How soft is a liquid? How soft is a gas?
Dip your finger in a glass of lukewarm water and stir gently; what do you feel? Now put your hands out in front of you; what do you feel?
So this two-molecule-thin lipid bilayer material is softer than a liquid and a little harder than a gas! Right … it’s that soft!
“One of the softest functional materials we know is the membrane that surrounds every cell on that planet.”