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Pratik Barve, Author at PhysicStuff

Author: Pratik Barve

What is Leidenfrost Effect?

What is Leidenfrost Effect?

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.

Circular Grooves Hold the droplet in the center.
The concentric edges hold the droplet in the center of the pan. Image credits: Martin Ristic.

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 anomalous expansion of water?

What is anomalous expansion of water?

First of all let us see why is it called anomalous ( meaning unusual ) expansion of water. Raising the temperature of liquid or gas increases the average separation between the molecules because they start vibrating more vigorously, leading to decrease in the density. The same should happen with water , heating it to 373 K (100°Celsius) increases the average separation up to a point where it becomes a gas and lowering it to 273 K (0° Celsius) decreases the average separation and becomes ice. So according to that logic ice should have the highest density, right?

But it is observed that as we cool water, density goes on increasing till 4°C and then starts decreasing again. Weird, right? In-fact this is the reason why ice floats on water. At about 4°C water reaches its maximum density.

A water molecule consists of 2 Hydrogen and 1 Oxygen atom. As we cool the water, the inter-molecular distance decreases and molecules come closer thus increasing the density. At about 4° C the water molecules come close enough that Hydrogen bonding takes place between Hydrogen atom of one molecule and Oxygen atom of other molecule. This type of bonding is not as strong as covalent bonds. Now the H2O molecule has a specific structure. Due to Hydrogen bonding the atoms begin to arrange themselves in a lattice like structure and at 0°C the state changes to solid when water crystallizes. The water expands after 4°C because the oxygen atoms repel other oxygen atoms of water molecules and hydrogen bonds maintain the lattice structure.

Hydrogen forms a bond with Oxygen atoms of neighbouring molecules.

The positive and negative signs in the above figure denote the effective charge on atoms in water molecules. Water molecule is a polar molecule. It means that there in uneven distribution of electron density in the atoms. In one water molecule, the probability of finding the electron of hydrogen atom is more near the oxygen atom due to the bond between them. So effectively hydrogen atom has effective positive polarity and oxygen atom has negative polarity. An electrostatic attraction between the partial positive charge near the hydrogen atoms and the partial negative charge near the oxygen results in the formation of a hydrogen bond between 2 water molecules.

Effects of anomalous expansion

This very strange but interesting property of water was actually very significant in the survival of aquatic life.

Aquatic Life flourishes even if the top layer of entire lake freezes.

As the water has highest density at 4°C, in very frigid and cool atmospheres the lakes used to have liquid water at bottom. Only the top layer of lake would turn into ice, further insulating the water below ice sheet. This helped aquatic life survive in the depths. Same with ocean caps and icebergs. The density of ice is 9% less than water at 4°C so it floats. So this anomalous expansion of water in-fact helps the aquatic animals to live. Even under icebergs or we can think back to the ice-age where ocean layers froze but deep layers were water due to this property of water.

Other such effect is bursting of water pipelines in really cold regions. Due to extreme cold weather, the water freezes inside pipes, expands and due to this immense pressure the pipes burst. You can observe this really easily by filling a plastic water bottle upto the brim and keeping it in the freezer. The bottle expands or in some cases bursts. (If you are really adventurous try freezing a glass bottle full of water. Cleaning is not fun.)

What is Doppler Effect?

What is Doppler Effect?

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 :

  • 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.


Star Wobbles due to exoplanet.

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