last 4th of July we had a barbecue at my
sister’s house
however we end up spending most of the
time inside because of the heat we end
up congregating in her living room and I
noticed that she had two different types
of windows she explained to me that she
recently replaced most of her windows
with these expensive energy-efficient
models but she did keep a few of her old
ones for aesthetic purposes even though
they are letting in all the summer sun’s
heat and this is fairly common because
windows installed before the 1990s have
the efficiency close to that of a car
window and we know that car windows let
in a ton of heat if you’re like me in
the summer months you end up spending
your trip to the grocery store just
trying to cool down the car and get
comfortable you buy a few items and by
the time you’re out the car is already
hot to the touch
but we don’t really think about how much
heat comes through our windows of our
buildings because we’re constantly
regulating the temperature inside but if
we scale up this example of a car to a
building it shouldn’t be too surprising
that we waste 4 percent of all US energy
due to these and efficiencies this ends
up costing the u.s. 40 billion dollars a
year it results in 24 billion kilograms
of co2 released into our atmosphere
that’s the same amount of emissions that
would be caused by 120 million cars
annually a hundred and twenty million so
this is obviously a problem that we need
to address somehow and because of the
environmental and the economic problem
presented there are companies that are
trying to address this but the solutions
have problems in them themselves first
these energy-efficient windows usually
have a tint or discoloration to them
which prevents adoption from residential
homeowners second it requires you to
hire professional to come out to measure
your windows to cut these windows to
size move your furniture away from the
windows
and then actually install them and this
can cost upwards of $10,000 so the
windows don’t pay for themselves for at
least 10 years and the savings you get
in your energy bill so there’s not
really an option for someone like my
sister who wants to keep her old windows
or for lower-income families
specifically those that might be in
hotter urban areas that don’t have ten
years to wait for their investment to
return so how do we make a more
energy-efficient window at least one
that can block heat that doesn’t have
this tin or discoloration and would pay
for itself relatively quickly
well to address any problem we have to
understand the fundamental science
behind the problem and when we think
about the Sun coming through our windows
we typically think about ultraviolet
light the stuff that can give us
sunburns ruin our furniture in our
floors we think about visible light
everything that you’re able to see and
we think about infrared irradiation
which for the purposes of today’s talk
is just a way that he can get through
our windows and is relatively unwanted
about 43 percent of the solar spectrum
is this visible light which we obviously
want but a majority of it a staggering
52 percent is this infrared radiation
which is just bringing heat through our
windows so how do we specifically target
the infrared radiation without getting
these discolored or these tinted windows
well to address this we need to
understand how color works and usually
when we think about color we think about
pigments and dyes your shirt can be dyed
you can add a pigment to a paint and
apply it to your wall both pigments and
dyes work in the same way and that works
through what the pigment or the dye
absorbs so in this first example we have
a red surface and the dye that’s in this
red surface is absorbing every color
except for red so you only see what’s
remaining which is the red light and in
a more complicated way the dye can
absorb Reds complimentary color on the
color wheel which is green so what’s
happening here is all the colors that
are left and are not absorbed appear red
to your eye
now there’s a second more complicated
type of color and that’s color from
structure I didn’t even know what color
from structure was until I started my
PhD but it’s relatively common and used
by nature a lot such as by these opals
or these butterflies to get the blue the
green and the yellow color as you see
here and the way that structural color
works is it’s a special arrangement of
atoms on the small nano scale and how
light interacts with these structures or
these geometries so let’s take a little
bit of a dive into how structural color
can help us with our window problem in
science we call these color giving
structures photonic crystals a photonic
crystal is a really fancy way of saying
that we have a selective mirror and what
I mean by a selective mirror is that we
can selectively reflect one wavelength
away from the photonic crystal and allow
everything else to go through and using
a simple equation called the Bragg
equation we know that we can change the
domain sizes or the spacing of these
alternating layers in the photonic
crystal shown here and we can target
different wavelengths of reflection that
we’re mirroring thus we can also figure
out what’s going to go through so let’s
see how this works in our first example
shown here we have a relatively small
photonic crystal when we shine the solar
spectrum through it we’re going to
reflect short wavelengths because the
domain sizes are small so short
wavelengths correspond to something like
ultraviolet light violet light in blue
light like our butterfly example shown
here but everything that’s not going to
be reflected is going to go through our
photonic crystal so the longer
wavelengths like green yellow or red but
also our infrared light which causes
heat so if we increase the domain size
over photonic crystal just a little bit
now we’re going to be reflecting longer
wavelengths of light which could
correspond to something like our green
our yellow and our reds now our short
wavelengths are going to pass right
through the photonic crystal the stuff
we were reflecting earlier but our
really long wavelengths are still going
to go through so our heat is going
through our photonic crystal
and it’s not until we increase the
domain size over photonic crystals
sufficiently enough that we’re now
reflecting these really long wavelengths
of light which correspond to the
infrared light which heats up our house
but the color is able to go right
through so you could imagine if there’s
some way that we could put these
photonic crystals on our window we could
reflect the heat back out but the
visible light would come right through
our windows so how do we get these
photonic crystals on our window without
expensive manufacturing processes well
to do that we leverage something called
self-assembly and let nature do the work
for us
self-assembly works by trying to lower
the energy of a system now when I think
about this I think about mixing oil and
water when you mix oil and water the oil
beads up it doesn’t want to mix with the
water but it rapidly self-assembles to
the lowest energy it could possibly be
you would have to input energy to mix
this back up either by stirring by
yourself or some other means and in
chemistry we have this very cool
technology called block copolymers a
block copolymer is a way that we can
link two things together kind of like
gummy worms so if we imagine these gummy
worms are like the green parts like oil
and the yellow part is like water
they’re chemically linked together in
our block copolymer they want to
separate but they can because again
they’re linked together so our gummy
worms have to assemble in such a way
that it has that low energy it’s self
assembles and the yellow wants to be
with the yellow and the green wants to
be with the dream so hopefully this
alternating structure is starting to
look like the photonic crystals we
talked about earlier and by changing the
size of our gummy worms we’re changing
the domain spacing which means we can
change the wavelength of reflection so
using this block copolymer technology we
can tune our photonic beam to reflect
ultraviolet visible light and all the
way through infrared so
what if we could put this photonic paint
in a spray can that you could buy at
your local home department store well
that’s exactly what we’re working on at
Colorado State University in
collaboration with California Institute
of Technology and Lawrence Berkeley
National Laboratory we want to put this
photonic paint in a spray can so that
you can retrofit your old and
inefficient windows to be able to be
energy-efficient again you would apply
the spray paint yourself in a thin film
that would self assemble on your window
– this thin transparent film that would
have no tint or discoloration and this
would cost ten times less than anything
else on the market and pay for itself
within a year this would be ideal for
something like skyscrapers which are
constantly getting beat down on by the
Sun but it’d also be great in
residential areas where the homes are
constantly under cooling loads and want
to be comfortable reflecting the heat
you wouldn’t want this in an application
where you’d want that natural sunlight
to come in because we’re reflecting the
heat away from your windows not
reflecting it back in so while we’re not
painting your house quite yet we are
able to paint windows in the lab scale
and there is no tint there is no
discoloration the solution is clear
using our photonic paint you’ll be able
to reflect a majority of the sun’s heat
in the form of infrared radiation so
hopefully the next time I return to my
sister’s house on a hot summer month we
can apply these to our windows and stay
cool and comfortable thank you
[Applause] [Music]