Thermodynamics Archives

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 anomalous expansion of water?

By Pratik Barve | June 1, 2019 | Comments 0 Comment

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 Tripple point?

By Pratik Barve | March 22, 2018 | Comments 0 Comment

Tripple point is a term coined in 1873 by James Thomson, brother of Lord Kelvin. Tripple point of a substance means the temperature and pressure at which a substance exists in all the 3 states (gas, liquid, solid) simultaneously in thermodynamic equilibrium. Now that sounds pretty weird. For instance let us consider the substance to be water. Water boils at 373 K (100 degrees celcius), it is liquid above 273K (above 0 degrees Celcius) and it’s solid i.e ice below 273K (below 0 degrees Celcius). So how is it even possible that all three states are able to coexist.

First let’s go through some basic thermodynamics.
PV=nRT
This is the famous Ideal Gas equation.
P : Pressure
V : Volume
n : No. of mole i.e amount of substance
R : Gas constant
T : Temperature

Now when we try to achieve tripple point of any substance, the system or the container is isolated from the surrounding. So the volume remains constant, amount of substance is constant, the gas constant will always be a constant. So to keep the equation valid Pressure has to be directly proportional to Temperature.

Here things become interesting. So if we reduce the pressure in the system we actually reduce the boiling point of the substance. That’s why in vacuum chamber water boils instantly. So now all we have to do it carefully maintain pressure and temperature of the system in such a way that it freezes the substance into solid but low pressure starts boiling it and in between there will be liquid state. That’s how you obtain a tripple point of a substance.

And believe me it just looks too weird that a substance, for instance water, is boiling but freezing at the same time in the same container!
The tripple point for water is achieved at 273.16K (0.01 degrees celcius) and at the Pressure of 0.0060 atm i.e 0.611… kPa.

Take a look at this interesting video : https://youtu.be/r3zP9Rj7lnc

Just like water has tripple point at specific parameters of pressure and temperature other substances have thier own tripple points where they coexits in all 3 states.

What is the significance of tripple point?

What exactly is this parameter used for? Well triple points make ideal reference points for the calibration of thermometers. They can be realised by using a sealed, evacuated, cylindrical glass cell filled with the pure substance, with an axial re-entrant well for the insertion of the thermometer. This device is called tripple point cell used for calibration of thermometers.
The triple point of water has a unique place in metrology since it is the basis of the definition of the units of temperature, the kelvin and the degree Celsius. Its temperature is 273.16 K and 0.01 °C by definition. Additionally, the triple points of mercury and several gases – argon, oxygen, neon and hydrogen – are used as low temperature reference points on the ITS-90. Triple point cells containing organic substances can also be made. Ethylene carbonate has a triple point temperature of 36.315 °C which, being close to body temperature, makes it a highly useful reference point for the calibration of clinical thermometers, while benzoic acid has a triple point temperature of 122.33 °C, close to the sterilising temperature of medical drip solutions.

Triple point cells are so effective at achieving highly precise, reproducible temperatures, an international calibration standard for thermometers called ITS–90 relies upon triple point cells of hydrogen, neon, oxygen, argon, mercury, and water for delineating six of its defined temperature points.