There are a lot of soft things out there: cats, infants, expertly-laundered sweaters. If there was some kind of omniscient softness guide, ranking every item in the universe in order of softness, these three items would for sure land towards the top. Of course, such a guide would be very difficult to assemble: as softness is at least partly subjective, you’d need teams of volunteer softness-assessors to handle each item, and some fair/statistically sound way of averaging all their reactions. And at some point, one of these softness-assessors would surely ask why exactly they’re doing any of this in the first place, lowering morale and jeopardising the whole project. Which is why, for this week’s Giz Asks, we’re keeping things simple: four materials scientists, representing no one but themselves, weighing in on their choice for the absolute softest thing.
Assistant Professor, Materials Science and Biomedical Engineering, University of Delaware
At first, it seems like finding the “softest thing [that we can feel]” would be straightforward. You could go to the hardware store and buy an assortment of “soft” materials. Maybe you would start with rubber, move onto marshmallows and foams. The problem with this approach is that it cannot tease apart what people are doing to determine the softness of an object. In fact, you can make two objects made from the same material feel harder or softer depending on its shape. As an example, human hair is often considered soft, but it is made from keratin, the same material as in our fingernails and the horns of a rhinoceros. Designing something that feels soft is related to what the object is made from, but there are some subtleties.
Recently, instead of perceiving a single material property (most people might guess “elastic modulus”), we showed that humans use a combination of two physical cues to determine the perceived softness of an object. These cues are determined in part by material properties and in part by the shape of the object. When you touch an object, two things happen to your finger. First, your finger pushes, or indents, into the object and the object also pushes into your finger. Second, the object spreads across your finger with a certain contact area. This combination of indentation and contact area on your finger forms the physical basis for perception of softness. So, any object which generates a large indentation depth or contact area will feel “soft.” The perception of softness is also not solely determined by the physics of materials. It is also informed by how our brains process information. Even though our fingers are soft, we found that people already account for the softness of their own fingers. In other words, our models were more accurate when we pretended that the finger was as rigid as a rock.
We are excited at the possibilities and possible illusions that these basic studies in softness revealed. We also think that there are careful and methodical ways to find our tactile limits, such as what is the softest and hardest object we can feel. These showed that there are multiple routes to controlling the perceived softness of an object, which could be useful in future virtual reality applications. It also reveals how our brain is processing and synthesizing tactile information.
“When you touch an object, two things happen to your finger. First, your finger pushes, or indents, into the object and the object also pushes into your finger. Second, the object spreads across your finger with a certain contact area. This combination of indentation and contact area on your finger forms the physical basis for perception of softness. So, any object which generates a large indentation depth or contact area will feel ‘soft.’”
Professor, Materials Science and Engineering, Carnegie Mellon University
When we talk about softness, we usually start with the term ‘compliant’ – which is when you push on a piece of material and it gives easily. In other words, you can change its shape without needing much force. That’s the engineer’s definition of soft. But there’s more to it than that, because ‘soft’ is as much a sensation as it is an engineering concept – what’s also important is how the material, the substance, feels to you. There’s a way in which something can feel soft that’s not completely independent of how easily it gives, or how easily it changes shape. Another dimension of softness is how the material deals with heat, or how good an insulator it is. And that might take different people different ways. But for the most part, softness is usually associated with substances that are relatively good insulators – blankets, for instance, tend to feel soft, partly because of the way they respond mechanically, but also partly because they don’t transmit or conduct heat very easily. Whereas most gels feel a bit clammy, which we don’t exactly associate with softness, even though they have a lot of ‘give.’
So I think something like an eiderdown duvet would be one of the softer things around, and that’s partly because it has structure at multiple scales, and this is, I suspect, a multi-scale problem – meaning that the softest materials, or the materials that give you the impression of being soft, often have multiple scales to them. You can make a carpet out of something that’s quite compliant, but unless you’ve got a material like soft wool on top which can also give way easily, then it’s not going to feel particularly soft. But the multi-scale structure of an eiderdown duvet allows it to impart some sensation to your skin while at the same time giving way extremely easily, and as it doesn’t conduct heat very easily there’s a certain element of warmth in it that we associate with softness.
“I think something like an eiderdown duvet would be one of the softer things around, and that’s partly because it has structure at multiple scales, and this is, I suspect, a multi-scale problem – meaning that the softest materials, or the materials that give you the impression of being soft, often have multiple scales to them.”
Data scientist, astronomer, and science communicator
I was asked what the softest material in the world is, but that is too limiting. I’d like to answer, taking some liberties, a different question: what is the softest planet? We now know of over 4,000 planets orbiting around other stars. These “exoplanets” represent only the small fraction that we have been able to observe. Many more, perhaps hundreds of billions just in our own Milky Way galaxy, await our discovery. What we have learned from finding them is there is a staggering diversity of worlds. We have found worlds made of rock like the Earth, worlds with two suns, and worlds likely entirely covered in lava. We’ve also found a handful of bizarre, “soft” worlds, a type of exoplanet called a “super-puff.”
The light any exoplanet reflects or gives off is usually too feeble for us to see compared to the blinding light of the star it orbits. Instead, we “see” it when it orbits in front of the star from our point of view, blocking some of the star’s light in what is called a “transit”. By looking at how much of the star’s light is blocked, we learn that there is a planet there and also how big across it is. Through a different method, we can find their mass. As it orbits, it gravitationally tugs the star toward it, which we see as motion in the star toward Earth, and then away, in a regular pattern caused by the planet’s orbit and revealed in slight changes in the color of the star’s light due to the doppler shift. The strength of the tug gives us the mass of the planet. The mass combined with the size gives us density, and those planets with extremely low densities, close to one tenth that of water, are called super puffs. They appear to be almost the size of Jupiter, but these planets only weigh 1% what Jupiter does.
The Earth’s mossy hills and sandy beaches can be soft in their way, but the strange barely-there softness of super-puff planets come from their extremely low densities. A simple experiment that a child can do tells you something about the density of everyday objects – does it sink, or does it 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 could imagine it’s in the “floats in water” category of things. These odd planets have densities even less than that of ice, more like the density of styrofoam or cotton candy!
Just as gas bubbles rise in water, a planet made of mostly hydrogen and helium gas has a much lower density than a rocky world. Saturn’s density is a little higher than the gas that makes it up because the crushing gravity of the upper layers of gas compress the ones deeper down. So finding a planet with a density close to cotton candy? Astronomers were shocked: that’s just unimaginably weird. How does it not get compressed to higher densities under its own weight?
The mystery of these planets is yet to be solved, but there are many theories. Perhaps the planet’s atmosphere is heated by the star, which fluffs it up, and gas can escape from the planet’s gravity into space over time. Perhaps dust flowing out of the upper layers of the planet’s atmosphere inflates our estimates for the size. Or perhaps our assumptions about the densities are wrong, and what we’re really seeing are planets with opaque rings at an angle which blocks extra light from the host star, inflating our estimate of the size. Luckily, astronomers have plans to search for more super puffs, test theories for how they form and evolve, and learn just how soft super-puffs really are.
“We’ve also found a handful of bizarre, ‘soft’ worlds, a type of exoplanet called a ‘super-puff.’”
Associate Professor, and Director, Sensory Evaluation Center, The Pennsylvania State University
Softness is fundamentally a perception (a percept). This implies it occurs in the brain, which in turn means that it can only be measured with a human assessor. With any human sensory system, there are limits of detection, both at the high end and low end, and outside these limits, we cannot tell (perceive) a difference, even if a machine or instrument might. Consider the hardness of cut glass and a cut diamond – when we touch them with our fingertip, they each depress our skin (and the mechanoreceptors in our skin) by roughly the same amount, so we cannot tell them apart, even if 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 substantially in the context of use – this ice cream is softer than that ice cream means something very different than this t-shirt is softer than that t-shirt. In the first case, softness is some combination of meltability and force required to scoop while in the second, it may refer to friction experienced by the hand as we drag it over the fabric. In yet another context, softness may be some function of compression force and bending force. One branch of experimental psychology (psychophysics) has been working for 150+ years on how best to measure sensations quantitatively.
These kinds of nuances can also be hugely importance for consumer perception and product acceptability. For example, when I see a new hoodie in the store, the first thing I do is stick my hand on the inside to see how soft the sweatshirt is. If it isn’t extremely soft and cosy, I keep looking, no matter how good it might look. Accordingly, understanding and measuring complex sensations can have important economic consequences. Many of the foods and consumer goods you use each day have been optimised by sensory and consumer scientists using classical and applied psychophysics to quantify the sensations they provide.
“Of course, measuring things with people can be quite tricky, because when people use a word like soft, its meaning can vary substantially in the context of use – this ice cream is softer than that ice cream means something very different than this t-shirt is softer than that t-shirt.”
Professor, Mechanical Engineering and Materials Science, Duke University
First, what is softness? Watson, of Crick and Watson fame, was reported to have once 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 level of hardness? From an engineering perspective, hardness is measured by how a material responds to an applied force or stress. There are several standard hardness scales, like the Mohs hardness scale, which consists, according to this definition, “of ten minerals arranged in increasing order of hardness such that each mineral will scratch the one on the scale below it, but will not scratch the one above it.”
Could you do the same for softness, where one material moves or bends another material?
Could you take, say, a feather, and see if it will move a wool fiber?
There are a lot of very soft material in every-day life, like a wool fiber, cotton balls, jello, feathers, or flimsy magnetic tape. But what about a functional material that actually supports life on the planet?
One of the softest functional materials known is the membrane that surrounds every cell on that planet. In my research of the past 40 years, we deal with phospholipid membranes. These are, in fact, the membranes that surround every cell on the planet. They are made of lipid bilayers and are only two-molecules thick (thin). Using special techniques that employ a glass micropipette that can be manipulated in a microscope chamber containing aqueous solution, and to which we can apply very gentle positive (blow) and aspiration (suction) pressures, we have actually measured their softness. Softness, in this case, is what it takes 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 into a glass of tepid-warm water and gently stir; what do you feel? Now, push 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 bit harder than a gas! Right… it’s that soft!