Glycerol is a sweet, highly viscous fluid that’s very good at absorbing moisture from the ambient air. That’s why a drop of pure glycerol in laboratory conditions quickly develops convection cells – even when upside-down, as shown above. This is not the picture of Bénard-Marangoni convection we’re used to. There’s no temperature or density change involved; in fact, there’s no buoyancy involved at all! This convection is driven entirely by surface tension. As glycerol at the surface absorbs moisture, its surface tension decreases. This generates flow from the center of a cell toward its exterior, where the surface tension is higher. Conservation of mass, also known as continuity, requires that fresh, undiluted glycerol get pulled up in the wake of this flow. It, too, absorbs moisture and the process continues. (Image credit: S. Shin et al., pdf)
You’ve probably noticed that cereal clumps together in your breakfast bowl, but you may not have given much thought as to why. This tendency for objects at an interface to attract is known as the Cheerios effect, although it happens in more than just cereal, as Joe Hanson from It’s Okay to Be Smart explains. The effect is a combination of buoyancy, gravity, and surface tension acting in concert.
When air, a liquid, and a solid meet, they form a meniscus, the curvature of which depends on characteristics of their interaction. Light, buoyant cereal and the walls of your bowl both have upward-curving menisci. Denser objects, like the tacks shown below, stay at the surface only because surface tension holds them up. Their meniscus curves downward.
Objects with a similar meniscus curvature will attract. For cereal approaching a wall, the light Cheerio is buoyant enough that there’s an upward force on it, but it’s constrained to stay at the interface. It cannot rise, but that buoyancy is enough to let it climb the meniscus at the wall. The two tacks attract one another for similar reasons, except this time their weight helps them fall into one another. Check out the full video to see more examples of this effect in nature! (Video and image credit: It’s Okay to Be Smart; research credit: D. Vella and L. Mahadevan, pdf)
Building microfluidic circuits is generally a multi-day process, requiring a clean room and specialized manufacturing equipment. A new study suggests a quicker alternative using fluid walls to define the circuit instead of solid ones. The authors refer to their technique as “Freestyle Fluidics”. As seen above, the shape of the circuit is printed in the operating fluid, then covered by a layer of immiscible, transparent fluid. This outer layer help prevent evaporation. Underneath, the circuit holds its shape due to interfacial forces pinning it in place. Those same forces can be used to passively drive flow in the circuit, as shown in the lower animation, where fluid is pumped from one droplet to the other by pressure differences due to curvature. Changing the width of connecting channels can also direct flow in the circuits. This technique offers better biocompatibility than conventional microfluidic circuits, and the authors hope that this, along with simplified manufacturing, will help the technique spread. (Image and research credit: E. Walsh et al., source)
Surface tension is the result of an imbalance between intermolecular forces near an interface. Imagine a water molecule far from the surface; it is surrounded on all sides by other water molecules and feels each of those pulling on it. Since all the nearby molecules are water, the tugs from every direction balance and there is no net force. Now imagine that water molecule near the air interface. Instead of being influenced on all sides by water, our molecule now feels water in some directions and air molecules in another. The water molecules tug harder on it than air, leaving a net force that pulls along the interface. This is surface tension, and, for a liquid-gas interface, it behaves somewhat like an elastic sheet. Surface tension is even strong enough to let a jet of soap solution bounce repeatedly off a soap film. Each bounce deforms the interface, like a trampoline dimpling when someone jumps on it, but surface tension keeps the interface taut enough for the jet to skip off without breaking it. (Image credit: C. Kalelkar and S. Phansalkar, source)
Have you ever wondered just how detergents are able to get grease and oil off a surface? This simple example demonstrates one method. In the top image, a drop of oil sits attached to a solid surface; both are immersed in water. An eyedropper injects a surfactant chemical near the oil drop. This lowers the surface tension of the surrounding water and allows the mixture to better wet the solid. That eats away at the oil drop’s contact with the surface. It takes awhile – the middle animation is drastically sped up – but the oil droplet maintains less and less contact with the surface as the surfactant works. Eventually, in the bottom image, most of the oil drop detaches from the surface and floats away. (Image credits: C. Kalelkar and A. Sahni, source)
Somewhere between the 24th and 28th week of the pregnancy, surfactant
– sometimes called “lung detergent” – starts being produced in the
amniotic fluid. As the pregnancy continues, more surfactant is produced.
That is why the closer to term, 38 to 40 weeks, the better a baby is
able to breathe outside the womb.
What is a surfactant ?
Surfactants are essentially
chemicals that reduce the surface tension of the fluid. (or) It is a
chemical that reduces the stiffness of a balloon.
If you reduce
the stiffness of the balloon, I think from personal experience you can
understand the breathing in becomes a lot easier!
Why is the surfactant important in the lungs?
Surfactant coats the inside of the lungs and keeps the alveoli, or air sacs, open by keeping them at the right pressure.
RDS ( Respiratory Distress Syndrome )
Babies that are born before prematurely do not have much of this surfactant in their lungs and as a result can suffer from something known as RDS – Respiratory Distress Syndrome.
A – Alveoli of a baby with RDS B – Healthy alveoli
With less surfactants in their lungs, some of the alveoli collapse due to the excess pressure.
If you remember this post on ‘Smaller Bubbles, Higher Pressure’, then you might know that the balloon with the smaller radius has the higher pressure and therefore collapses.
This is exactly what happens in the lungs of babies born prematurely as well, many of their alveolus collapse.
With less alveoli available, the infant has to work hard to breathe. He or she might not be able to breathe in enough oxygen to support the
body’s organs. The lack of oxygen can damage the baby’s brain and other
organs if proper treatment isn’t given.
Babies who have RDS are given surfactant until their lungs are able to start making the substance on their own.
That’s how simple physics blends with biology to yield elaborate ecstatic phenomenon that gears life. Have a great day!