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Course: MIT+K12 > Unit 1
Lesson 3: Physics- The physics of skydiving
- The physics of invisibility cloaks
- The science of bouncing
- How do ships float?
- Thomas Young's double slit experiment
- Newton's prism experiment
- Bridge design and destruction! (part 1)
- Bridge design and destruction! (part 2)
- Shifts in equilibrium
- The Marangoni effect: How to make a soap propelled boat!
- The invention of the battery
- The forces on an airplane
- Bouncing droplets: Superhydrophobic and superhydrophilic surfaces
- A crash course on indoor flying robots
- Heat transfer
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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?
- 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)
- 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)
- At3:17you 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)
- 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)
- will there be more?(6 votes)
- 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)
- 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)
- Why is Low Energy=Hydrophobic and High Energy=Hydrophilic?(2 votes)
- 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)
- Does the calming effect of rain on seawater have something to do with surface tension?(2 votes)
- not really,the droplets just fall into the water(1 vote)
- 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)
- it depends on the smartness of the student and the standard of the school
in my opinion they should teach it in 6th grade but in my school they teach it in 9th grade(2 votes)
- Is there an experiment to test this?(1 vote)
- yes, first you get soap, and then you cut out a little boat like in the video and then follow the other steps in the video(2 votes)
- what are two different ways to change drag on an airplane?(1 vote)
- At3:21, 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)
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]