Bouncing Droplets: Superhydrophobic and Superhydrophilic Surfaces

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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
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