Physics
-
Thomas Young's Double Slit 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
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.
⇐ Use this menu to view and help create subtitles for this video in many different languages.
You'll probably want to hide YouTube's captions if using these subtitles.
- What causes water droplets to bounce 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 unison mutants commuter
- which is for screen at length
- ordeal for me to squared disparaging per unit area
- service in japan acts like a stretch elastic membrane kind of like a belated
- because surface tension is is contractile force each section of the
- blue is pulling on each other resisting change in cinci
- what causes service and jane
- it's caused by the attractive oracle he supports a speech in water molecules
- if we look at a poll of likely the molecules that are inside the poor are
- experiencing cohesive forces with neighboring molecules
- they're completely surrounded by other molecules
- and are enjoying their interactions
- the interactive slower the energy seed of these molecules
- these are happy molecules
- molecules on the surface of the four are only surrounded by half the number of
- other molecules so they only experience half the amount of cohesive interactions
- these are unhappy models
- they read a high-ranking d_c_-ten molecules inside the poor
- to minimize the number of unhappy molecules
- but with a justice shape to expose the smallest possible surface area
- that's why water droplets hysterical and why in space
- lots of water all city departments peers
- but what about water droplets resting on the surface
- what determines whether they will be dot the role of or spread out completely
- when a water drop of complexes service it takes a cheap of hysterical cap
- before have we learned that all liquids have service energy
- actually every single surface-to-surface energy
- a sentence can be thought of as the interface between two phases
- before when we're talking about the surface tension of the liquid
- you were talking about the service and she between a liquid and air
- there's also the service and he retrieved solid in their
- and a certain sense into liquid and solid
- if the congress and the equilibrium content angle
- we can do a force balance on the line of contact with the surface
- we want to balance of forces in the extraction
- we have the surface and he became solid and a prayer
- the service angie we can sell it and they put in the opposite direction andy
- excluded at the same percentage in tune with the n_b_a_ prayer
- rearranging gives this which is the answer is
- once relationships at the continental that a droplet links of the service is
- related to all of the service and cheese
- if the equilibrium continental is greater than ninety degrees the surfaces
- hydrophobic
- on the other hand at the continental is less than ninety degrees the services
- hydrophilic
- intercontinental is bigger than a hundred fifty degrees
- the surface is defined as being super hydrophobic
- water drum set of super hydrophobic services will follow up
- if the continental is less than five degrees
- the surface is defined as being super hydrophilic
- large on the set of super hydrophilic services will spread out completely
- this is useful for anti file coatings
- advanced services super hydrophilic any water that contact the surface will form
- a thin film instead of performing droplets on the surface
- so what makes a surface super hydrophobic person for hydrophilic
- there to mean factors and press the surface chemistry
- the service can see determines whether the service has slowed or high surface
- seventy
- which then determines whether the services hydrophobic or hydrophilic
- generally speaking surfaces with the both surface energies are hydrophobic
- and services with high energies are hydrophilic
- thinks i just have fun and other classic several energy out things such as metals
- have high energies
- the second factors service rep
- in general surface roughness will make a hydrophobic surface even more
- hydrophobic anna hydrophilic surface even more hydrophilic
- scientists have been trying to determine what kind of trainee structures mexicans
- is super hydrophobic person who had to fill it
- they've been looking and exams need chairs such as the lunacy to obtain the
- special properties
- they're two different states have water drop it can be and when it comes in some
- rough surface
- to go over these two different miles we'll cult favorite evite equilibrium
- continental
- which is the continental for and ideal facts are fixed
- mccarthy to start the apparent contact angle which is the content and go on a
- rough surface
- these two miles were developed by wenzel and by calcium baxter and they shall
- have service records can affect the water droplets contact angle
- the first state that a water droplet npm when a context of surfaces the wenzel
- state
- in this state there are no air bubbles underneath the droplet and the droplet
- is incomplete contact with the surface
- the drop to actually sticks very well to the surface and it's called attend
- droplet
- in the wenzel model
- the surface roughness is quantified by our which is the real surface area
- divided by the projected service area
- since every surface has some sort of roughness
- because no surface it's completely smooth at the molecular level
- we can assume that are is greater than one
- the wenzel mock states that could save data star is eager to our tenth
- co-signed a_t_t_
- inspire is greater than one the cosine of data star is greater than the
- co-signer dat
- this is a very important statement
- let's look at what happens when fear he is less than ninety degrees
- if they take he is forty five degrees
- n_r_a_'s nonpoint too
- we can calculate the value of the estar
- it is starting to have to be thirty two degrees
- so twenty-eight years that's in ninety degrees we can see that data star is
- less than petey
- now with the devil happens twenty-eight years greater than ninety degrees
- if in fact the value of the akita hundred thirty five degrees
- an article to one point two
- we can calculate that the start
- is equal to
- under forty eight degrees
- now we can see that ada star is greater than the egg
- so when the services hydrophilic
- data star is smaller than dating e
- month surfaces hydrophobic
- a two-star is bigger than payday
- this equation shows that roughness will make a hydrophobic surface even more
- hydrophobic
- and hydrophilic surfaces even more hydrophilic
- in the chocolate is in the kasi back sixty
- the water drama actually sits on top of tiny air bubbles
- in this state
- water droplets will bounce a 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
- grow up
- picking up a meter along the way
- generally speaking that kasi baxter's the occurs for a very very rough
- services
- a special form of the kasi baxter model
- just update a star is dependent on the percent of solid anything contact the
- droplet
- as is valued approaches zero or in other words if the tropics sitting mostly on
- the air pockets
- because an f_a_a_ test our approach is negative one and data star approaches a
- hundred eighty degrees
- so to summarize
- how water drop it behaves in a complex a solid is dependent on service energies a
- conflict angle describes whether the surfaces hydrophobic or hydrophilic
- surface roughness can also cause surfaces to become super hydrophobic or
- super hydrophilic as shown by the wenzel or kasi faster model
- apologetic love how hydrophobicity can be useful is how the namic decir de
- tocqueville its water to drink
- this field listened and then a desert in africa
- the beetle has a very special back where they are a little hydrophilic islands
- that are surrounded by hydrophobic areas
- tiny farm droplets can collect on the hydrophilic islands and grow too large a
- droplet
- once the troubles are large enough
- the droplets rolled down the beatles back and collected to drink
- a
Be specific, and indicate a time in the video:
At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
|
Have something that's not a question about this content? |
This discussion area is not meant for answering homework questions.
Discuss the site
For general discussions about Khan Academy, visit our Reddit discussion page.
Flag inappropriate posts
Here are posts to avoid making. If you do encounter them, flag them for attention from our Guardians.
abuse
- disrespectful or offensive
- an advertisement
not helpful
- low quality
- not about the video topic
- soliciting votes or seeking badges
- a homework question
- a duplicate answer
- repeatedly making the same post
wrong category
- a tip or feedback in Questions
- a question in Tips & Feedback
- an answer that should be its own question
about the site
Share a tip
Suggest a fix
Have something that's not a tip or feedback about this content?
This discussion area is not meant for answering homework questions.