I so wanted to talk today
about my experiments with

nickel discs in these acting the magic
systems and I like to start there by

thanking the people who have contributed
to this work including Robert Henry and

Professor Daniel Rice and
of course my advisor Robert Laney.

So as many of you know these active
systems are out of equilibrium liquid

crystalline systems which are driven by
an internal energy source for the produces

approachable changes in the magic director
field as you can see in this video.

This system was of course developed by
the Dr group at Brandeis University and

they kindly provided us with
samples to use that we could create

insights system ourselves.

It's composed of micro to build switch
as Orban oil water interface and

forms are qualified to dimensional layer.

The elements form bundles to
attractive depletion forces and

as efficiently high density.

Flowing in turbulent
pneumatic layer forms.

The motion is driven by their motors which
cause sliding between the bundles and

this produces an extensional stress
which produces the motion as well as

the spontaneous creation and
I Alesha of one half the facts of

the plus one half the facts self propagate
in the direction of their head and

the minus one have two facts skip pushed
around by the flow in the film so

we were interested in using nickel discs
to probe the system for really two reasons

one is to use them to investigate some of
the physical properties of the system and

another is to try to affect the flow
within the film by by spinning them.

We have observed these disks and

the film has an inverted microscope
on which are mounted so.

Illinois which allow us to apply
magnetic torques to the probe.

So we put these disks
in the active gel and

allow them sediment to the interface now
we observe that they don't actually embed

within the film but actually they
rest some distance above what we do

is we spin the disks with
rotating memetic fields and

then when the disk spins it creates
flow within the ball fluid and this

flow actually produces a viscous stress
on the in the magic film underneath.

Sharon in the plot on the right is
simulated result of the viscous stress

on a static surface produced by
he's spinning disk above and

so this this stress actually
influences the motion within

the film and so one thing we have observed
actually is that the motion of defects

in the vicinity of the disk
is in fact are altered

to quantify this we measured the average
cross product train the position unit

vector of a defect and
its velocity unit vector.

Since this is in two dimensions
this cross product is

basically just a number to consider vector
and as you can see from the plot there is

this net positive value which persists
out to about two hundred my computers and

this basically indicates that
the defects tend to move

in a direction that coincides with
the direction of rotation of the disk.

So you can see that the viscous
stress that is imposed by the disk

actually directs
the defect motion locally.

Another thing we've observed is that as
the fission in the high frequency when

the viscous stresses large enough there's
a more dramatic change to the film

namely that the vortex structure actually
forms and I'm Since a vortex structure.

Actually has a top logical charger
plus one one can ask how the film

accommodates the structure and has
the snapshots on the right to demonstrate

it does this basically by absorbing
two plus one half defects and

you can see just from
comparing the first and

last images there's a net decrease of top
logical charge of one in the film and

this is compensated by the increase
in charge that you get from

creating the vortex so that the overall
charge in the system is conserved

we also observe nucleation occasionally
of defects at the edge of the vortex

as well as another interesting thing which
is that so we've observed is sometimes

the disk is unable to maintain the vortex
and it collapses and when it does this it

actually decomposes into two plus one
half the facts which then propagate

away from one another again this conserve
the overall charge of the system.

So too with the goal of sort of
learning more about the physical

properties of the system we measure
the velocity within the four tacks.

So with how we did this base to
retract the microtubule bundles and

shown here on a log log plot is
the average velocity from the center of

the disk in a vortex for
many trials and so

the data has actually been normalized so
that you can see that there's an overall

consistent shape to the to
the data although the overall

magnitudes of Flossie's do change by
a factor of four so from trial the trial.

Shown Also are these one over
on one of our squared lines and

they basically are shown to just
represent some simple cases that one

will get ns in certain situations for
example a sphere rotating in a ball fluid

in three dimensions will produce a flow
such that the velocity will fall off as.

One of the R.

squared now a descriptive
in two dimensions.

Will produce a flow the follow up is one
over our now in our case we have a disk

rotating in a fluid which is imposing
a stress on the film so it's a little

bit different so to understand exactly
what one would get we simulated.

This system and how we did this is
basically model the act in a magic film

as a thin fluid layer sandwiched
between two ball fluids and

we vary the viscosity of the film and

we also impose a boundary condition in
this case at one hundred Michael meters

which corresponds to a typical
cortex size and as you can see on

the plot on the left there's these curves
which are the simulation results for

the velocity within this vortex
as a function of distance for

various discuss the days of the of
the film so as rate increase the viscosity

of the film you can see that the overall
magnitude of the loss of these go down.

So in fact one can use this as a sort
of gain an estimate of the sky city

of the film by using our philosophy data
that we found so as an example of this.

Shown here is a comparison of the data for
one trial shown in the red in the red

dots as well as a simulation result in
which an average was taken over a range

of four tech sizes which corresponded to
the range of orthotics sizes observed in

the video so in this case we found
an estimate of about two point four Pascal

second like computers which we got
from basically taking the three D.

viscosity that we use
in the simulations and

multiplying it by
the thickness of the film.

By comparing the velocity magnitudes
also in many trials we get

sort of a range of values of discuss
the of the film between about point.

Fifteen Pascale second Michael meters.

So Susan clued we've shown that
an extremely impose of a stress created

by a rotating dist I was able to locally
direct the motion defects of the center of

the of the disk and we used the disk as
sort of an affective microbiological probe

to estimate the viscosity of the film so
on thanks.