Pattern Directed Dewetting – I

Pattern Directed Dewetting – I


welcome back now to the thirty six lecture
of the course so we are near in the completion i am not very sure how you have enjoyed why
are you really hope that you have enjoyed the course and you have been following the
lectures and have submitted the assignments and i would really like all of you to sit
for the examination get a certificate and i mean its very simple if you have just hard
the courses the course the lectures and gone through the whatever material that i we have
uploaded i am sure we will do the very very well in the examination i never ask the questions
which i have not talk and therefore i just check how your able to grab whatever i have
taught so i i really ah hope to see all of your names
appearing in the answer scripts that we are going to evaluate in in few weeks time form
now any way you of course you are the other thing is that you can always send in the discussion
forum or through the form of email any specific suggestion that you may want to have a specific
improvement you would like to have of course please do not forget that the frame work does
not allow us to interact inside a class which is of course the most sort after and well
desired ah way of teaching at least high feel it that way where at least you get a glimpse
of what subject is and what do and stuff like that
so considering the limitation of this ah framework i am sure you have enjoyed the course anyway
coming back to topic we have been talking about spontaneous instability and dewetting
of ultra thin films and we have looked into the issues from a from somewhat theoretical
details and now you understand that it is essentially one can look into the magnitude
and the sin of lest say a parameter like effective hamarker effective ah inter phase potential
all thats say the ah disjoining pressure and one can sort of have a qualitative idea whether
the about the stability of the film of course ah the entire ah theoretical framework
that we have discuss so for is considering vander waals sources because hamraker constant
is the constant that is defined only where vander waals interaction so you can always
ah ask what is going to happen in real situations because we have even the neglected the polar
interactions well ah turns out that most of the polymers until and analysis actually take
a semi question line polymers something like that or or ah ah conjugated polymer to be
more precise ah they lack the polar interactions and therefore ah their behavior it has been
surprisingly found in fact that is rather well captured based on a theory or formulation
that only considers vander wall vander waals interaction
of course the other important aspect ah since we are using a polymer so what we gain is
that polar interaction is mostly absent in the system that is thorough but what is really
ah lacking in the formulation that we have talk so for and thats where actually deviations
in experiments do come in i i must admit that is we are also neglecting the satiric interactions
and now you should all understand that the ah for a long chain pol for long chain polymer
molecules the entanglement is indeed there static interactions is there ah ah ah are
there which are neglected in whatever we have talk so for in fact hones honestly speaking
considering static interaction and issues like that is starrer the research level i
mean there is lot of where going on but it will provide be take some more time before
these ah ah top its ah make their way into text books
ah the other issue is that ah i did not talk each particularly because of the positive
of the time as well as ah due to i did not want to make the course really very heavy
and complicated i did not one did not teach the hydrodynamics of thin film which is even
ah in fact rather simple approach in one can just look into the naive stokes equation and
we are linear stability analysis and in fact one of the resources that i have referred
to that is my n p t e l ah lectures and the wed material we can find it their its not
included in this course but any of view who is interested who want to studied further
you can always do that ah you will find there that viscosity the
polymer viscosity or the liquid viscosity has been consider ah similar to that of nutronian
fluid and thats i also not correct because because of the entanglement effects and others
long represents of the long chain molecules ah polymer mails in fact exuviates significant
amount of non linear behavior or non nutronian behavior and those things needs to be incorporated
in the development of more precise and accurate theory but one thing is very convincing that
whatever be the precise rheological model that in fact does not affect the stability
that sort of all those the dynamics evolution sequence and some recent experimental studies
of course suggest that they might also altered the morphology during evolutions so for example
ah i briefly mentioned that we talked about this rim formation around a growing whole
and depending on the level of visco elasticity so if issue is that we are looking at a dewetting
of a rather viscous film ah the profile of the out after the periphery of the rim or
the rim profile is rather symmetric so what it means is for a viscous film ah the rim
cross section is symmetric that is its something like this however if you ah looking at dewetting
of a strongly visco elasticity film we know you can choose the molecular weight and such
a way that the film execute significant level of visco elasticity ah the rim profile of
an terms out to be something like this i am not going into the detail the why it happens
but this has been experimentally ah observed and so this is how the visco elasticity ah
effects the evolution morphology in addition to the dynamics
but what we will focus in this lecture is something different see we have we are now
all familiar with this particular lecture ah which is essentially the key evolution
sequences of a polymer films so you have flat film and what i had hinted that one fascinating
aspect is you only spin coat a flat film and do nothing all you do is this film behave
like solid a room temperature because there glass transition temperature is about room
temperature if if it is a polyester rim or a p m m a film in fact there t g is a close
to hundred degree centigrade so a room temperature irrespective of the weight ability of the
substrate sin of the effective inter phase potential we are not going to see any dynamics
but all you have done in order to trigger this evolution sequence easy is to heat of
the film or ah to expose it to its solvent way for ah there are certain differences i
must admit between solvent paper exposure and heating up of the film ah i dont want
to going to all of them but one of the major things that you may one to consider is that
though you consider a spite you considering the fact that there is coefficient of thermal
expansion ah the expansion in a polymeric material is not ah very larger therefore after
heating when the film is about to dewett its thickness is roughly of the same order as
that of the initial thickness of the film but when you ah do the second approach that
is you do a solvent viper exposure i have already mentioned that what happens is the
solvent molecules entered in to the ah film matrix and this well so one immediate difference
that you can observed is that in in the later case at the point when the film is about to
dewett its actually thicker than its originally thickness anyway ah for this course again
i yesterday ah when i one of the earlier lectures i mentioned that if i i would like to talk
about many more detail aspects of the dewetting if i take a dedicated course on dewetting
that means see ah depending on the feedback and response of this particular particular
course that i will decide so if i ever take that i will provide be discuss
all this entreat detail ah however what is important is irrespective of the presides
route is this this form of spontaneous instability is undesirable as a coating and i already
mentioned that work research work is already going on to suppress this instability and
one of the preferred route is in fact to add nano particles in very small proportion
so what happens is in fact very exciting instead of a pure homo polymer film now you actually
have a nano composite thin film you have this nano particles and these nano particles exhibit
ah dynamics with respect to the polymer chain in fact based on the entropic interactions
is has been shown by several workers that these nano particles huge to migrate to or
the liquid substrate inter phase because what happens is the polymer chains which are again
the long chain molecules their ah rotational entropy ah sort of gets truncated ah because
of the fact that they are sticking the once there are sticking to the substrate so if
a if a polymer molecule is release from the substrate and is replaced by a nano particle
in fat that polymer chain gives and conformational entropy and that thas how it the system is
thermodynamically favored right ah but again that is a topic i will not touch upon but
what is moved interesting is that what i will trying to highlight and got dewetted is that
you started off with a flat film and you work creating some nano and meso scale structures
so in fact instability spontaneous instability in in a way is a route for nano fabrication
or novel meso scale ah fabrication with soft materials limitation is ah new have seen how
beautiful the structures you can make while we are staring sof lithography and photolithography
and these structures are random and isotropic so such structures are often of no use so
significant research is in fact aimed at aligning the structures ok you cannot really take a
flat film on a flat substrate an align it in fact you need to do something and that
something turns out to be there are very popular approaches and some not so popular research
level approaches which people have tried out in fact that there approaches are to use a
patterned substrate and that patterned substrates in fact includes you can either use a chemically
patterned substrate or you can use a topographically patterned substrate to and so what you do
is in flat smooth film is what we have talk so for you either heat up you either heat up or expose to solvent viper
and you get some random structures in contrast what you do is in order to align
them you would like to take either a chemically patterned substrate
or a topographically patterned substrate i am drawing the film on this topographically
patterned substrate a big later ah lets consider a chemically patterned substrate so now ah
you see you seamlessly know in fact how to perform this experiments how do you make this
chemically patterns substrates you simply make them my macro conduct painting and therefore
what are the chemical patterns you typically have chemical patterns with alternating domains
of hydrophobic or hydrophobic areas and now you also know what is hydrophobic
and hydrophilic in fact ah with respect to what are typically hydrophobic patches will
have low surface energy hydrophilic patches will have high surface energy
so that in fact translates most likely to the fact that the spreading coefficient on
one of the patches is in fact going to be negative in fact probably over the hydrophobic
patches and the local spreading coefficient over the hydrophilic patch hydrophilic patches
are going to be ah positive so variation to be different and do i really tell you more
answer is no so suppose if this are the areas where the spreading coefficients are negative
lets say these are the hydrophobic patches ah thermodynamically the film is unstable
over here whereas here the if the spreading coefficients are positive or they have much
lower value ah in in principle the film is stable over these patches
so what are you essentially doing you have a film which is periodically unstable over
certain locations and essentially you would like the film now what you are doing is who
are specifying a hydrogenate in the form of the substrate in fact its a chemically hydrogenous
surface you want the film to rupture over the hydrophobic areas and thats exactly what
people have done ah people have take an micro contact painting surfaces so these are in
facts some simulation results excellence simulation results and you can create or you can achieve
what is known as tinplating that is ah whatever is the geometry of the chemical patterns that
gets reflected in the topography of the dewetted film
so under perfect tinplating this is what the original papers talk about so if this is ah
please understand these are chemically patterned patches so topographically they will be roughly
smooth so you take this is the substrate lets say rate corresponds to low surface area domains
and you take the film on top of that so let us consider we are taking a cross sectional
view we are talking about so this is the film what you would like to expect under appropriate condition and what are the
conditions lets not get into that you can look into turns of wonderful word by k kajari
kargupta and ashito sharma around two thousand to two thousand three this is the area they
have lots of talk plus publications on this particular area and you would expect in fact
if these are low surface area domains so you would like expect that after dewetting the
film profile could be something like this why because i hope the picture is clear to
you because over these rape zones so low surface area patches and therefore it is lightly there
the film will rupture over these areas so you essentially ah probably the picture got
a bit clumsy you would expect the film to however ah simulation and subsequent experiments
also shows that of course this ah first let me discuss this that there is a compete i
mean there are whole lot of parameters that effect the ordering please do not understand
that the movement you take a chemically patterned surface you are going to get order thats not
go look it possible because film thickness is a critical parameter film thickness also
ah gives you are the natural length scale of instability ah the periodicity of the patches
the duty ratio of the patches the duty ratio essentially is the term which is like you
have lets say this alternating patches that a periodicity of lets say lambda p
but you see you can also have structures like this same periodicity but here the ratio o
or here the width of the lets say hydrophobic patches is more as compared to that of the
hydrophilic patch so duty ratio is something like a ratio of lets say l hydrophobic minus
l hydrophilic and for this type of a surface of course you can immediately ah figure out
that summation of this phobic plus phallic is going to give lambda p and most of the
structure the gratings we have we consider because as a easiest to fabricate often we
have this ratio is equal to one for a surface like this
so this ratio is often refer to as the duty ratio duty ratio you can just check out the
net what is the classical definition so ah film thickness periodicity of the structures
so let me just make a quick list film thickness periodicity of the structures duty ratio deference
in the wet ability of the two patches thats an important parameter so if you have lambda
one and lambda two over here the magnitude of lambda one and lambda two in fact matter
is there is a beautiful paper ah simply say something around two thousand seven or eight
which in fact shows that there is a critical value of this lambda one minus lambda two
below which the effect of ordering its completely destroyed
anyway so this is the beautiful area ah i must also mentioned a wonderful story and
which ah highlights the strength of stimulation related or theoretical work this entire work
has i mentioned is are the two pioneers in this field and both are for india i i t kanpur
ah the entire development was first stimulated and theoretical predicted and experiment subsequently
followed so this is what good theory can in fact do this is how good theory can in which
science ok moving on what is the other approach so the other approach so one is patterned
direct a dewetting but to use a chemically patterned substrate and in fact this has receive
to more attention the second option is to use a topographically patterned substrate
now and you see we already have some a f m images ah which actually shows that this dewetted
droplets which where random can be beautifully ordered on a topographically pattered substrate
so for example if you take the this particular figure on the right side each one of the stingy
droplets or dewetted droplets and the cross patterned is a lithographically soft lithographically
fact fabricated template but we talk about this ah topographically
patterned substrate in fact i am go to spend some time on this but does it rim any question
on your mind as to why i havent drawn the film on top of the topographically patterned
substrate its in fact a very good question to ask and the answer is it is quest the answer
to this is not trivial because so for we have only considered or always considered
on a smooth flat substitute which and therefore in the context of dewetting even on on in
the context of chemically patterned or chemical patterned directed dewetting ah the initial
thickness of the film is uniform
but if you look at this condition is it possible for you to get an uniform film thickness if
you spin coated and this is not a hypothetical problem because one thing i would look to
remained you that in photolithography the spin coding is not popular because you would
like to do soft lithography or dewetting spin coding is popular because photolithography
extensively requires spin coding and in photolithography when we talked about this concepts of mescaline
and things like that you have seen that you might have to code several layers or you might
have to perform photolithography several times and over a pre existing patterned
so essentially the question of spin coding on a patterned substitute is non trivial and
i am sorry to say fundamental understanding was rather port which sort of got corrected
very recently however initial patterned directed dewetting experiments on topographic pattern
surfaces people where little scare to ah on honestly speaking nobody knew how exactly
this variation takes place ah on a topographic lee patterned substrate the film thickness
variation so what was done was very pain staking lease the samples were prepared ah by coating
it on a flagged uniform silicon way for a micro sheet and then floating it on water
and then transferring it on to the topographically patterning substrate so this way even show that your film thickness
at least remains on constant of uniform it was also shown that the way you transform
that is the way you recapture so the ah this is sort of a cartoon that gives you an idea
what was done this is a layer of water you take the film ah you you need to have really
really very good hance so this this was coated on a flat surface then you float it on water
is every possibility that there will lot of rankles those are going to samples ah and
you capture it with a patterned surface i will repeat it so you coated spi spin coating
on a smooth surface then you float it on water and then you recapture it with another ah
topographically patterned substrate so this was how initial samples for these where these
experiments where created and it was also shown the depending on the
angle at which you pick up your sample you can have two different initial configurations
so if you picked up your substrate vertically you actually get a completely conformal addition
of the film which respect to the substrate in contrast if you pick it up horizontally
you sort of get a film morphology which is hanging which is in contact with only the
protrusions and hanging over the other areas so depending on how you prepare your film
you could eitherly this lead to this and lead to this ah running out of time in this particular
lecture so i will stop my discussion for the subsequent lecture which will also continue
on patterned divert dewetting from here thank you

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