Resolving the nagging problem of misty glass
— Photo: V. Ganesan
Temperature drop: Hot, humid air hits the relatively cooler surface of the glass and condenses into water droplets. .
Good ideas in science often come from some of the common inconveniences we face in daily life. The use of sponge to absorb water, and release it when needed is one. Band-aid strips, Velcro fasteners, Post-it slip of paper are some more.
Each one of them is based on a simple principle in science, and is an application of a method to find an easy and affordable solution to a nagging inconvenience. And the solution is so simple that you are left to wonder “why did I not think of it!”
Fogging, or misting up, of the windshield in a car is one such nag. Hot, humid air hits the relatively cooler surface of the windshield.
Water in the air condenses to form droplets that scatter the light. Result, you cannot see well through the windshield. Even your eyeglasses fog up as you exit the car.
Strangely enough, there is as of now, no easy and lasting solution to this! Sprays are used, heaters are put on, wipers used endlessly — no easy solution. One who finds a defogging solution will not only gain praise but also prosperity.
The science behind
What is the science behind this? In a mist or a fog, it is the water droplets condensed in the air that restrict visibility, and the size of the drop matters.
Physicists tell us that as the size of the material on which light falls becomes comparable to the wavelength of the light rays, the material scatters light, not allowing it to pass through. Bring the size down, the material becomes transparent.
With water droplets then, we need to bring the size down.
But what makes the water molecules stick together as drops, and not stay single, like gases in the air? The answer is — water molecules like each other more than they are attracted to the molecules in air. And since they are formed by condensing of water vapour, the best shape for such narcissism is a ball or a sphere.
Put the water drops on a surface. Now there is a chance for the water molecules on the outer surface of the drop to interact with the material on the surface.
If the surface is oily, as in a lotus leaf, even if the oil molecules would like to interact with water, the latter does not return the favour. Result — water stays aloof, unaffected by the surroundings. Mystics and philosophers want us to repel the attractions of the world likewise, and be Spartan and ascetic (though not as narcissistic).
Turning to the clouded up windshield, how do we solve the problem — and forever? One way is to cover the glass of the windshield with oil, in the manner of the lotus leaf. Teflon is one such water-hating material. But this could lead to oily dirt collecting on the surface which needs to be cleaned from time to time. And we need to be cost-effective as well.
Nano solution
And it is not just the glass windshield. There are several other situations where anti-mist coating is needed and at as inexpensive a rate as possible.
Enter nanoparticles. Dr. Junhui He and colleagues of the Chinese Academy of Sciences have come out with a way to knock off the water droplets from forming on glass.
They find that nanoparticles shaped like raspberries, hollow spheres put together in a compact manner, with tiny hairlets that reach out to the glass surface, solve the problem.
To do so, they started first with nanoballs of polystyrene. They exposed the balls to oxygen gas under low pressure and hit them with a plasma treatment. This allowed for coating the balls next with silica, decorating them with hydroxy groups all over, thus making them love water (hydro-philic).
These were then placed on glass sheets, and using a polymer mix, they calcinated the material (just as a Chinese potter does in a kiln), which fixed these raspberry nanoparticles on to the glass sheet.
Hydrophilic
The glass surface is now full of hollow spheres which pick up water with great avidity. Indeed, their thirst is so high that the water does not have a chance to form self-centered droplets of any size.
The contact angle, which measures how well the water will get off the surface to form a droplet, plummeted from the usual 40{+o} to 0-6 or even zero.
This drop indicates that the nano-coated surface likes water so much that no droplets are allowed, only a wet layer.
No wonder the surface is now called super hydro-philic. More devout than the priest, just as water on lotus leaf is more denying than an atheist.
The He nano-coat material is thus superior to any other current materials but what about the cost? Given the cost of the starting materials, and the amount needed to be used, it could be dirt-cheap. We now await its entry into the market.
D. BALASUBRAMANIAN
dbala@lvpei.org
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