If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Bouncing droplets: Superhydrophobic and superhydrophilic surfaces

This video introduces the concept of surface tension, and shows how roughness can make a surface superhydrophobic or superhydrophilic. The Wenzel and Cassie-Baxter models are explained. Special thanks to the MIT BioInstrumentation Lab. Created by MIT+K12.

Want to join the conversation?

  • mr pants purple style avatar for user Me
    Doesn't the postfix "phobic" derive from the word phobia? and phobia means a fear of, so wouldn't superhydrophobic mean something like "scared of superwater?"
    (3 votes)
    Default Khan Academy avatar avatar for user
    • piceratops ultimate style avatar for user Mehrab Jamee
      Almost. So water droplets on a superhydrophobic surface will ball up as if scared of the surface. The prefix super- will imply that the water droplets will "try harder" to not touch the surface. If you were hung from a string over a pool of lava, wouldn't you ball up and try not to touch the lava? Water droplets will behave similarly to a superhydrophobic surface. Hope this helps. :D
      (8 votes)
  • leaf green style avatar for user Garfield Maitland
    At you spoke of fog repellents being superhydrophilic as a practical purpose. I was wondering are there any other uses of superhydrophilic surfaces as well as superhydrophobic?
    (4 votes)
    Default Khan Academy avatar avatar for user
    • female robot grace style avatar for user Deena
      Yes, there are lots!
      Superhydrophilics are used a lot in commercial equipment to remove oil stains (since oil and water will not mix).
      Superhydrophobics can be used for anything that you wouldn't want to get wet. For example, let's say I accidentally spill a drink on my laptop. If the laptop was coated with a superhydrophobic compound, then the drink could be easily removed from the surface and wouldn't cause any damage by getting in between the keys.
      These are just a few examples, but the really fun part is imagining new ways the principles could be put to use. Check out this really cool new discovery (also by MIT students) at http://www.liqui-glide.com/ What do YOU think - is this an example of hydrophilic or hydrophobic behavior?
      (6 votes)
  • old spice man green style avatar for user Jonathan
    will there be more?
    (6 votes)
    Default Khan Academy avatar avatar for user
  • leafers seedling style avatar for user samhatchettcleary
    I think Surface Tension was very interesting, and I want to do a science experiment for a project. Can anyone think of any variables I could manipulate with the floating-paper-clip scenario?
    (2 votes)
    Default Khan Academy avatar avatar for user
    • leaf green style avatar for user Bogar
      Here are a few potential variables to play with - water temp, kind of fluid, shape of paperclip, length of paper clip (linear dimension), size of paperclip (small, large), temperature of paperclip (how would a red hot one respond? or a frozen one?), water depth, fluid container volume, fluid container surface area (tray vs cup), Surface condition of paper clip (scuffed, polished, powdered, greased)
      (2 votes)
  • purple pi teal style avatar for user rajarshi
    Why is Low Energy=Hydrophobic and High Energy=Hydrophilic?
    (2 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user wolmot
      All interactions universally tend towards a thermodynamic equilibrium. The energy states of the molecules are determined by the amount of cohesive interactions between them. "Happy" molecules with a low energy state would be found deeper and deeper in the fluid tank. Whereas "sad" molecules with a high energy state would be found higher up. If you imagine that the molecules with the higher energy are in a vapor state and molecules with lower energy are in a solid state, then what're you've said makes sense. Solids would have a higher surface tension and roughness thereby a greater impact on an outside body in terms of surface contact as opposed to a high energy vapor state. Further imagine a droplet of water being thrown onto the low energy solid vs high energy vapor. Intuitively, the solid surface would be much more hydrophobic as compared to the hydrophilic vapor. Thus proving your statement, that low energy "solid, happy" states are more hydrophobic than high energy "vapor, sad" states which would be hydrophilic .
      (1 vote)
  • piceratops seedling style avatar for user Neodymia
    Does the calming effect of rain on seawater have something to do with surface tension?
    (2 votes)
    Default Khan Academy avatar avatar for user
  • hopper cool style avatar for user ☣Ƹ̵̡Ӝ̵̨̄Ʒ☢ Ŧeaçheя  Simρsoɳ ☢Ƹ̵̡Ӝ̵̨̄Ʒ☣
    What grade level do you start to pick up Theata?, and Theata star, and Theata E? I teach middle grades math and we don't ever mention Theata, Where does that come in? Trig? Thanks T.S.
    (1 vote)
    Default Khan Academy avatar avatar for user
  • old spice man blue style avatar for user Mikeify
    Is there an experiment to test this?
    (1 vote)
    Default Khan Academy avatar avatar for user
  • mr pants teal style avatar for user anoushka.goyal
    what are two different ways to change drag on an airplane?
    (1 vote)
    Default Khan Academy avatar avatar for user
  • purple pi teal style avatar for user dsgo
    At , wouldn't this be bad because of refraction? If this was on a windshield, The driver probably wouldn't be able to see objects correctly.
    (1 vote)
    Default Khan Academy avatar avatar for user

Video transcript

[MUSIC PLAYING] What causes water droplets to balance when they hit a surface? What determines whether a surface is hydrophobic or hydrophilic? And what does it mean when a surface is superhydrophobic? It all comes down to surface tension. Surface tension, or surface energy, is a tensile or contractile force. It's given in units of newtons per meter, which is force per unit length; or joules per meter squared, which is energy per unit area. Surface tension kind of acts like a stretched elastic membrane, kind of like a balloon. Because surface tension is a contractile force, each section of the balloon is pulling on each other, resisting changes in shape. What causes surface tension? It's caused by the attractive or cohesive forces between water molecules. If we look at a pool of liquid, the molecules that are inside the pool are experiencing cohesive forces with neighboring molecules. They are completely surrounded by other molecules and are enjoying their interactions. The interactions lower the energy state of these molecules. These are happy molecules. But molecules on the surface of the pool are only surrounded by half the number of other molecules, so they only experience half the amount of cohesive interactions. These are unhappy molecules. They are at a higher energy state than molecules inside the pool. To minimize the number of unhappy molecules, liquids adjust their shape to expose the smallest possible surface area. That's why water droplets are spherical and, while in space, blobs of water also take the form of spheres. But what about water droplets resting on the surface? What determines whether they will bead up and roll off or spread out completely? When a water droplet contacts a surface, it takes the shape of a spherical cap. Before, we learned that all liquids have surface energy. Actually, every single surface has surface energy. A service can be thought of as the interface between two phases. Before, when we were talking about the surface tension of the liquid, we were talking about the service energy between a liquid and air. There's also the surface energy between solid and air, and the service energy between liquid and solid. If we call this angle the equilibrium contact angle, we can do a force balance on the line of contact with the surface. We want to balance the forces in the x direction. We have the surface energy between solid and vapor, the service energy between solid and liquid in the opposite direction, and the x component of the surface energy between liquid and vapor. Rearranging gives this, which is Young's relation. Young's relation shows at the contact angle that a droplet mixed with a service is related to all of these surface energies. If the equilibrium contact angle is greater than 90 degrees, the surface is hydrophobic. On the other hand, if the contact angle is less than 90 degrees, the surface is hydrophilic. If the contact angle is greater than 150 degrees, the surface is defined as being superhydrophobic. Water droplets that touch superhydrophobic surfaces will ball up. If the contact angle is less than five degrees, the surface is defined as being superhydrophilic. Water droplets that touch superhydrophilic surfaces will spread out completely. This is useful for anti-fog coatings. If the surface is superhydrophilic, then any water that contact the surface will form a thin film instead of forming droplets on the surface. So what makes a surface superhydrophobic or superhydrophilic? There are two main factors, and the first is surface chemistry. The service chemistry determines whether the service has low or high surface energy, which then determines whether the service is hydrophobic or hydrophilic. Generally speaking, surfaces with both surface energies are hydrophobic and services with high energies are hydrophilic. Things such as Teflon and other plastics have low energy while things such as metals have high energies. The second factor is surface roughness. In general, service roughness will make a hydrophobic surface even more hydrophobic and a hydrophilic surface even more hydrophilic. Scientists have been trying to determine what kind of tiny structures make surfaces superhydrophobic or superhydrophilic. They've been looking at examples in nature, such as the lotus leaf, to obtain these special properties. There are two different states a water droplet can be in when it contacts a rough surface. To go over these two different models, we will call theta e the equilibrium contact angle, which is the contact angle for an ideal flat surface. We'll call theta star the apparent contact angle, which is the contact angle on a rough surface. These two models were developed by one Wenzel and by Cassie and Baxter, and they show how service roughness can affect a water droplet's contact angle. The first state that a water droplet can be in when it contacts a surface is a Wenzel state. In this state, there are no air bubbles underneath the droplet and the droplet is in complete contact with the surface. The droplet actually sticks very well to the surface and it's called a pinned droplet. In the Wenzel model, the surface roughness quantify by r, which is the real surface area divided by the projected surface area. Since every surface has some sort of roughness-- because no surface is completely smooth at the molecular level-- we can assume that r is greater than 1. The Wenzel model states that cosine theta star is equal to r times cosine theta e. Since r is greater than 1, the cosine of theta star is greater than the cosine of theta e. This is a very important statement. Let's look at what happens when theta e is less than 90 degrees. If theta e is 45 degrees and r is 1.2, we can calculate the value of theta star. Theta star turns out to be 32 degrees. So when theta e is less than 90 degrees, we can see that theta star is less than theta e. Now let's look at what happens when theta e is greater than 90 degrees. If we set the value of theta e to 135 degrees and r equal to 1.2, we can calculate that theta star is equal to 148 degrees. Now we can see that theta star is greater than theta e. So when the surface is hydrophilic, theta star is smaller than theta e. When the surface is hydrophobic, theta star is bigger than theta a. This equation shows that roughness will make a hydrophobic surface even more hydrophobic and hydrophilic surfaces even more hydrophilic. If a droplet is in the Cassie-Baxter state, the water droplet actually sits on top of tiny air bubbles. In this state, water droplets will bounce or roll off. This is useful for water repellent and self-cleaning surfaces. A service can be self-cleaning because any water droplets that contact it will roll off, picking up any dirt along the way. Generally speaking, the Cassie-Baxter state occurs for very, very rough surfaces. A special form of the Cassie-Baxter model shows that theta star is dependent on the percent of solid that is in contact with the droplet. As this value approaches zero-- or in other words, if the droplet is sitting mostly on air pockets-- the cosine of theta star approaches negative 1 and theta star approaches 180 degrees. So to summarize, how a water droplet behaves when it contacts a solid is dependent on surface energies. The contact angle describes whether the surface is hydrophobic or hydrophilic. Surface roughness can also cause surfaces to become superhydrophobic or superhydrophilic, as shown by the Wenzel or Cassie-Baxter model. A cool example of how hydrophobicity can be useful is how the Namib desert beetle collects water to drink. This beetle lives in the Namib desert in Africa. The beetle has a very special back where there are little hydrophilic islands that are surrounded by hydrophobic areas. Tiny fog droplets can collect on the hydrophilic islands and grow to larger droplets. Once the droplets are large enough, the droplets roll down the beetle's back and is collected to drink. [MUSIC PLAYING]