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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.
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- 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