 Capillary tea, ladies and gentlemen welcome to today's video we are going to discuss a phenomenon called capillary tea and capillary tea by definition is the rise or depression of a fluid inside very close space now we create something called artificial capillary tubes which are very small diameter tubes but it can also be seen in real life the plants always have me intrigued you know because all the minerals in the water is located at the bottom and some of the leaves also get the water in the minerals how? because water has to travel against gravity the answer is capillary tea OMG I spilled some water I need to clean this using a tissue paper I'm gonna put it like this but what the hell is this? the water is rising against gravity and it's absorbing the water through the pores in the tissue paper what is this? capillary tea in the next bit we are going to do an experiment on the capillary tea for this experiment I'm gonna need three things right here the first one is obviously a beaker of 100 ml it can contain 100 ml of water then I'm gonna need something called the capillary tea now these tubes have really really small volumes the one that I'm holding has just 1 ml diameter and the third thing I'm gonna need is gonna be water because usually capillary tea happens for any kind of fluid but the easiest fluid we can find is water right here so I'm gonna fill this completely with water and then let's do the experiment so what I'm gonna do is I'm gonna put this capillary tea tube on the surface of the water and you can see the level of water is going to be higher than the surface of the water it's going to be higher than the surface of water rise and this is exactly the phenomenon called capillarity to understand why capillary rise happens there are two physics concepts that we need to know about first is surface tension and the other one is meniscus formation let's first understand what surface tension is consider the case of water which is the most common fluid of all the molecular structure of water has three atoms hydrogen, oxygen and hydrogen now the problem with this is that oxygen is much more electronegative compared to hydrogen molecules now what exactly happens is the electron cloud is slightly pulled towards the oxygen and the oxygen gains a slight negative charge and the hydrogen gets a slight positive charge now in this entire setup you will notice all of those molecules which contain hydrogen as well as oxygen will have slight positive charges and slight negative charges over them now what happens is this hydrogen which is part of this molecule number one will start attracting the electronegative oxygen from a different molecule so this molecule as a whole will start experiencing a net force in this direction similarly it will also be pulled in this direction it will also be pulled in this direction it will also be pulled in this direction in this direction and this direction and in this direction because of all these molecules around it and I am talking about a molecule which is inside the fluid thing we have to understand that there is no net force on a molecule of water which is existing inside the liquid itself. So that is the first concept. But if you think about an imaginary surface on top of this liquid surface, so let me make an imaginary surface right here. This is the top surface right here, top surface of the liquid. Now you do understand Keith. Yes there will be forces on these molecules pulling downwards okay as well as sideways okay but there will be no forces acting on them on the upward direction because I'm considering it is air molecules up here okay and we'll talk about them in a bit but there are no strong water forces or forces hydrogen bonding forces on the upper upward direction. Now what happens is because of this the molecules the water molecules on the surface of the liquids are able to are able to essentially you know pack themselves in a better manner compared to compared to the molecules which are inside the fluid okay and that creates a tension on the surface okay and an ability of the surface of the water to hold little bit tension from the upward compared to the molecules inside and this is what we call surface tension. To visualize surface tension what we're going to do is we're going to take a beaker full of water okay and it's not moving the surface is not moving and what we're going to do is we're going to put small metal pins on top of this surface okay and if I put them very slightly you will see that they will start floating okay now you have to understand that these metal pins are much more dense than water okay let me try one more time let's try to slowly put them over the surface of the water okay as you can see these are floating okay and the reason behind their floating even though they are more dense than water is because of surface tension I can prove that by taking a small pencil right here and breaking the surface tension by just pushing this pin below the surface and you will see that this pin will immediately drop down okay so as you can see the the pins have dropped down to the bottom okay so this might give you this should give you a little bit feel about what exactly surface tension is. Next let us understand why exactly meniscus are formed over a liquid. So as we know everything is made up of atoms and molecules but it's not like everything is spread out like a fine dust of atoms things have rigidity because the atoms are pulled together by forces of attraction. Now the same kind of atoms and molecules attract each other but different kinds of atoms and molecules also attract each other so if I take water in a beaker then the molecules of the water are being pulled together due to a force of attraction between the molecules of the same kind that is water this is called the force of cohesion. Meanwhile the glass sides walls of the beaker are also pulling the molecules of the water and this attraction between molecules of different type glass on one hand and water on another is called the force of adhesion that is why we have the name adhesive for the heating that sticks to different materials together. We know that the glass molecules are made of silicon and oxygen atoms now oxygen is highly electronegative that we have already discussed before water on the other hand are H2O molecules okay the oxygen and hydrogen we have already discussed about something called hydrogen bonding in which the hydrogen or the partial charges on the hydrogen and the oxygen start attracting each other into a phenomena which we call hydrogen bonding okay now these kind of forces between same molecules is known as cohesive forces and of when the water molecules start interacting with the glass there is also a bond or there's a there's a attraction between the hydrogen the slightly positive hydrogen of water to the slightly negative negative to the slightly negatively charged oxygen of the glass atom okay these kind of forces are known as adhesive forces because these are forces between different molecules now here what happens is the adhesive forces are actually greater than cohesive forces okay and because of that instead of a straight profile the surface of the water looks something like this because molecules of water jump up and start getting attached to the glass surface on top here is a picture here is a picture of water in a small burette as you can see the surface of the water looks something like this if you look from top this looks like a concave structure so the name of this thing is a concave meniscus okay now so how does the knowledge of surface tension and meniscus formation help us to understand the capillary rise the surface of the water has surface tension in it so if I if I make a structure of a small tube the surface because of meniscus formation is like this and we know that on the surface there is also the surface tension force so you will see in the tension on there is an additional force acting on the surface of the water in in in not so horizontal manner this force will have two components one along the tube which we call T cos theta and theta is the angle between the fluid and the surface we we have a special name for this angle we call this angle of contact okay and there will also be a horizontal component T sin theta T sin theta which is of no use for us right now but T cos theta that is acting all around the surface okay all around the surface will pull up the water and that is the cause of the capillary rise okay and this force is enough to balance out whatever weight of water is this whole column of water has T cos theta balances that out and that is one way of explaining how capillary rise works and there is another approach to understanding how capillary rise happens and that is by the form of something called surface energy so typically everything in this universe wants to have as less energy as possible so the natural tendency of the molecules of the of a liquid wants to have wants to be get attracted towards lesser energy scenario now what you have seen in the capillary rise what happens is molecules of the water it is getting attracted towards the glass now why is that because the interface between the glass and the water the molecules trapped between that is having lesser energy compared to the molecules which are inside the water as well as the molecules which are on the water surface so the natural tendency to decrease the energy will lead to have more surface area between the glass and the water and hence the water inside the capillary get pulled up and because of that the surface area between the water and the glass increases