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Circular Grooves Hold the droplet in the center.

What is Leidenfrost Effect?

By Pratik Barve | July 26, 2019 | Comments 0 Comment

Now this is something very cool we observe when water is put on a very hot surface. We can see little droplets bouncing around which should actually be evaporated instantly because of such high temperatures. This is due to a phenomenon called Leidenfrost Effect (named after Johann Gottlob Leidenfrost).

Leidenfrost Effect Droplet Schematic — Drop levitated due to layer of vapour. Image credits: By Vystrix Nexoth at the English Wikipedia, CC BY-SA 3.0, Link

When liquid droplets come into contact with a hot surface (around boiling point temperatures) it evaporates almost immediately. If the temperature is high enough such that the layer of liquid drop which comes into contact with hot surface vaporises immediately then this layer of vapour acts like a cushion. This layer of vapour cushion prevents heat transfer to rest of the liquid droplet as steam has poorer thermal conductivity than the pan/hot surface. The steam cushion levitates the droplet and it skitters around without any friction and thus surviving for much longer times. This “high enough” temperature is called the Leidenfrost Temperature.

For instance water has boiling temperature of 100° C. If we put water drops on a pan with temperature of 100°C the droplets will just hiss and spread out vaporising rapidly. But when the pan temperature reaches around 193°C, which is the Leidenfrost point for water, it levitates and skitters around.

So the Leidenfrost temperature is different for different liquids. There’s also something called as inverse Leidenfrost effect where hot liquid droplets are levitated on cold liquid surfaces as the liquid vaporises levitating the hot droplet. For example Anaïs Gauthier’s team at University of Twente have studied this effect by depositing a room temperature droplet of alcohol on top of pool of liquid nitrogen at – 196 degrees celcius.

There’s one application for which we can use this property of liquids. Water droplets just skitter around on a flat hot surface, but if we change the texture of the hot surface we can give a direction to the motion of droplet undergoing Leidefrost effect. If the texture if made of sharp steps at an angle / ratchet like texture , some vapour from below the droplet exits and propels the droplet.

This also allows the droplet to move uphill against gravity. This can be used to make Thermostats with no moving parts. Thermostat is a device which senses the temperature of a system and then can be used to control or regulate the temperature of the system. The Leidenfrost thermostat works by using the cooling power of water droplets. It moves the water droplets in one direction to cool the system when the temperature is too high, but discards the drops by moving them in the opposite direction when the temperature is too low, allowing the system to heat up to the correct temperature. This would be better understood by watching this video.

Published in the Journal of Heat Transfer, the thermostat is demonstrated in a short film made by undergraduate students:

Leidenfrost effect is also the explanation for some bizzare stunts some people perform without harming themselves like hitting a stream of molten metal or dipping wet finger in molten lead or blow out a mouthful of liquid nitrogen. The drastic temperature differences creates an heat insulating layer between the skin and materials for a very short duration.

What is Doppler Effect?

By Pratik Barve | March 11, 2019 | Comments 0 Comment

Everyone has observed the phenomenon of Doppler Effect at some point. An ambulance coming towards you sounds a bit high pitched and then when it goes away from you it sounds a bit low pitched. So basically Doppler Effect is the change in frequency or wavelength due to the relative motion between source and observer. This phenomenon was named after the Austrian physicist, Christian Doppler, who proposed it in 1842 during his time at Prague Polytechnic University.

Doppler effect can be observed in sound waves as well as electromagnetic waves i.e light. The apparent change in wavelength/frequency due to the motion of source object is called as Doppler Effect. Consider a scenario where an observer is observing a moving object. If the object is moving towards the observer the wavelength is shorter due to the motion of source, and hence the frequency increases (higher pitched sound). Whereas if the object is moving away from observer the wavelength is longer as source is moving away from the observer and hence frequency decreases (lower pitched sound).

In case of light, if the object is moving towards you its called as blue shift because the wavelength reduces i.e it shifts towards blue side of the Electromagnetic Spectrum and if the object is moving away its called as red shift because the wavelength increases i.e it shifts towards red side of the Electromagnetic spectrum.

Note that blue shift and red shift doesn’t actually mean the object appears blue or red, it just means that frequency increases or decreases. A stellar object’s spectrum may be in ultraviolet region which is already beyond blue, in that case blue shift means the spectrum shifts towards the higher frequency range.

Red Shift and Blue Shift of Electromagnetic Spectrum of a Star.

Some applications of the Doppler Effect

Police radars make use of Doppler effect. The device is pointed at the target (vehicle), radio waves are emitted which hit the target and are reflected back. Depending on whether the vehicle is moving towards or away the change in wavelength is measured and instantly speed of the vehicle is calculated by the electronic circuits in the device. Such device is a good for non-intrusive way of traffic rule enforcement.
Handheld Police Radar.

Image Credits : stealthveil.com
Doppler Radars are used by Meteorologists to study the weather. Similar to Police radar it uses radio waves, they have large enough wavelength to interact with clouds and precipitation. This can be used to determine the speed of cloud and using other parameters like wind speed, temperature, air currents,etc the prediction of weather becomes more accurate.
Doppler Radar at the National Weather Service in Dodge City, Kansas.
Doppler Echo-cardiogram is a device used to take images of heart. It uses sound waves which makes it relatively safe medical imaging technique. The sound waves bounce off the walls of heart and the red blood cells hence we obtain an image which helps determine the rate of blood flow and direction.
Doppler Mitral Valve.
Image : Wikipedia.
In Astronomy and Cosmology Doppler effect is used to determine if a stellar object is moving towards or away from us. It is also used to determine the distances of stellar objects. Click here to read more about determining stellar distances.
When a planet orbits a star, the star wobbles around the center of mass of the star planet system also called as barycenter (common center of mass for star and their planets). So the wobble means that the star moves away from us and towards us. That’s it! We can use Doppler Shift to detect exoplanets!!

Infact our sun also wobble mostly due to Jupiter.

To read more in detail about Doppler effect and also it’s mathematical formulation refer to this pdf.

What is a mirage?

By Pratik Barve | May 7, 2018 | Comments 1 comment

Mirage is an optical phenomenon which creates an illusion of water. The most common occurrences are during hot sunny days and most of us are familiar with mirage we see on highways. We have heard popular stories of weary traveller who sees a lake at a distance, that’s just the reflection of the sky above which creates the illusion of blue lake.

Reflection can be seen on the road which looks like there’s water on the road. This is the usual highway mirage which is formed as the surface of the road heats up the air just above it.

So how is this illusion created in the first place? When light travels through a medium of equal temperature it follows a straight line path. But when there’s a temperature gradient i.e different layers of medium (air) have different temperature, light doesn’t follow a straight line path. This has to do with the refractive index of cold air and hot air. Refractive index is the ratio of velocity of light in vacuum to velocity of light in medium. So if a medium has higher refractive index the speed of light decreases in that medium. Hot air is less dense as compared to cold air so light travels faster in hot air than in cold air. During hot summer days the road or surface of earth gets heated a lot. This heats up the layers of air just above the surface and temperature gradient is created (regions of hot air above the surface). So the light coming from objects far away instead of following a straight line path towards us bend towards hot region as it travels faster through it creating a reflection like illusion.

Vertical Temperature gradient is how the temperature varies as we move from surface towards vertical direction. The surface heats up the air just above it. So the light rays from tree which should travel in a straight line towards us bends towards hot region as it travels faster in hot region creating an inverted image of the object.

Another explanation according to quantum electrodynamics is that the photons take the path of minimum time when travelling from one point to another. Even if the path is curve it will bend to reach other point in minimum time. So when a vertical temperature gradient is present during hot days mirages are formed.

Types of Mirage

Inferior Mirage

Inferior mirage is when the image is formed under the real object. Usually in desert or highway mirage the real object is sky and the mirage is formed below the object which looks like reflection of sky from water. The light rays from object bent in hot region by same amount. Therefore inverted image is formed. This is the most common type of mirage. It is not much stable as the hot air rises above cold air which creates distortions in the image. As you walk towards mirages they seem to be moving away from you.

Superior Mirage

A superior mirage occurs when the temperature of air below the line of sight is colder than the air above it. This is unusual since warm air above cold air is unusual gradient and hence it is called temperature inversion. So now the light rays are bent downwards from the hot region, this creates the image above the object. This looks kinda weird and is not usually observed. They tend to be more stable than inferior mirages as there is no turbulent flow between cold and warm air. Superior mirages are common in polar regions especially over large sheets of ice that have a uniform low temperature.

These mirages can be pretty weird, some light from objects beyond horizon can bent and form an image above but the object cannot be seen as it is beyond horizon. This may explain some stories about flying ships or coastal cities in the sky, as described by some polar explorers. These are examples of so-called Arctic mirages, or hillingar in Icelandic.

Fata Morgana

Now this is something cool. Fata Morgana is an unusual type of superior mirage. A Fata Morgana may be described as a very complex superior mirage with more than three distorted erect and inverted images. Because of the constantly changing conditions of the atmosphere, a Fata Morgana may change in various ways within just a few seconds of time, including changing to become a straightforward superior mirage. The rays will bend and create arcs. An observer needs to be within an atmospheric duct to be able to see a Fata Morgana. Fata Morgana mirages may be observed from any altitude within the Earth’s atmosphere, including from mountaintops or airplanes.