Hello, and welcome to the Physics World weekly
podcast.
In this episode, you'll hear my colleague, Margaret
Harris,
in conversation
with three
soft matter scientists
with an interest in the squishy,
messy,
and exciting physics of animate droplets.
This episode is sponsored by EPL,
a journal that publishes original
high quality letters
in all areas of physics.
EPL operates under the scientific
policy and control
of the European Physical Society
and is published by EDP
Sciences,
the Italian Physical Society,
and IOP Publishing,
which also publishes
Physics World.
Now over to Margaret.
My guests in today's podcasts are a trio
of soft matter scientists who have a particular
interest in the physics of droplets.
They are Giorgio Volpe, a professor of soft
matter at University College London,
Rob Malinovsky, a research fellow in soft matter
physics who's also at UCL,
and Joe Forth, who's a lecturer at University
of Liverpool, but was at UCL a few
years ago when this collaboration began.
Georgio, Rob, and Joe, hello, and welcome to
the podcast.
Hello. Thank you for having us here. I'm
gonna start with the softest possible question and
maybe the biggest. What is soft matter, and
why is it so fascinating and important?
So soft matter is very generally the the
science of of squishy stuff.
A lot of it is very everyday stuff.
You can think of it as the the
the science of of shampoo, and a lot
of the fundamental findings were made by
large companies trying to figure out how do
we make better soaps, better shampoos.
It very often consists of
materials that are a large fraction of liquid.
You have lots of small particles, lots of
surface area.
Surface tension tends to dominate a lot of
the behavior.
You have lots and lots of weak multivalent
forces. So the system can be moved around
and broken up by thermal motion or or
just by poking it or shaking it and
all of these things.
Physics wise, it's it's interesting because it can
it can change shape. You can incorporate mechanisms
for energy conversion, and that in turn means
that soft matter can can deform itself. It
can move.
You have a whole
diverse range of mechanisms for for elasticity.
If you if you compare it with, say,
typical solids, crystalline solids, they're just they're just
rigid because they consist of repeating patterns. Whereas,
you know, in soft matter, you have all
sorts of mechanisms for different types of elasticity,
viscosity, and viscoelasticity.
With that comes complex deformation and complex flow.
With these
energy conversion mechanisms, you get a whole bunch
of interesting
non equilibrium behavior.
And at that point, the palette of behavior
you can see in in soft matter systems
grows massively. It it still feels like a
field where you can mix a bunch of
stuff together, stick it under a microscope, and
you'll you'll see
something new.
Beyond physics, just sort of scientifically,
it's inherently very interdisciplinary.
You
have the chance to incorporate interesting
chemistry.
The material property sort of underpinning soft matter
are very similar to the material and physical
properties underpinning
biological systems and biological
tissues.
And there's with the sort of study of
non equilibrium systems comes this translational relevance to
active matter, which we'll sort of get to.
I guess it's a matter of debate whether
or not, soft matter,
how what the overlap is between active matter
and and soft matter, but there's certainly a
big big overlap on the the Venn diagram
of those two circles.
As for why I I find it interesting,
my my answer to this has has changed
over the years. When I first learned about
it as an undergrad, the the selling point
for me was that it didn't feel like
the usual physics. It didn't feel like particles
and Maxwell's equations and whatever. It felt every
day,
this idea that of studying the the physics
of gunky liquids or the physics of shampoo.
For me, it was exciting. But, to me,
it was like, wow. You can you can
study this stuff and people will pay you
to do it. That was what excited me.
So I'm getting the impression here that soft
matte is everything that's really messy in physics,
and that's what you love about it.
Yeah. I I so certainly for me, and
it's certainly I think it's something that the
the purists it's sort of on the borderline
of whether one considers it
physics. I I because of this inherent interdisciplinarity,
I I think a lot of it is
certainly not unusual for the the particle physicist
think of it as belonging to something more
like engineering or chemistry, but that interdisciplinarity
is is a big part of the appeal
for me.
Georgio, your work in particular is all on
animate matter. Do you want to touch on
them what the difference is if there are
any differences between soft matter
and animate matter. Because the word animate
to a layperson usually means living, but I
understand that's not necessarily the case with animate
matter.
No. It is not the case. And, actually,
when we talk about animate matter, we're really
thinking about purely synthetic systems, so
artificial. In a nutshell, animate matter is lifelike,
but it doesn't mean that it's alive.
And if you think, for example,
about the droplet, since we're going to talk
about droplets,
you can think that the droplet can move
on its own. It can respond to the
environment.
It can perform task almost
if it was following a purpose. Right? It
looks like it's alive, but it's not.
And,
ultimately, it's just responding to physical and chemical
rules.
So animate,
whether we're talking about soft matter or we're
talking about larger or smaller systems,
it really is about the behavior.
It's a system that is fully synthetic and
it can resemble in terms of properties, in
terms of behavior,
those of living systems.
The way in which we can describe
animal matter is usually through
categorizing
according to three principles. These are called the
three aids of animacies.
And the three aids of animacies are three
principles that tend to describe act this animate
matter based on whether they are active,
adaptive,
or autonomous.
Animate matter, then there is a third field,
which is active matter. And there is definitely
overlap between the freeze of matter, animate matter,
and active matter, but the each one each
one of them has their own characteristics. So
there is overlap, but they're not exactly the
same field.
Okay. I want to dig into these three
a's you say, because you've recently written a
paper for the journal EPL that focuses on
animate droplets. And you've said what animate means.
It comes with these three principles.
Let's start with active. What does active mean
in this context?
So active is the simplest of those three
principles.
When you say a droplet is active, it
simply means that it can take energy from
its environment and convert it from one form
into another.
So a really nice kind of simple example
is there are these oil droplets. If you
take a specific kind of oil and you
put it into into water,
if you impose a chemical gradient
onto this droplet,
various physical forces, they could be surface tension
driven, could be something else, But it will
drive up that chemical gradient or down that
chemical gradient.
And what droplet is doing there is converting
kind of this chemical potential into a kinetic
energy, and, you know, that's the motion.
And these are really nice because you can
think of it as a rudimentary form of
sensing. There's there's no kind of, you know,
thinking or internal processing. It's all physical based,
but it's in its really simplest sense, the
droplet is is sensing this chemical and reacting
to it. But like I said, not like
biological something. There's no thinking. There's no internal
processing. It's just pure physics, and they can
make really neat, neat little swimmers.
What do you mean by a swimmer?
It's just a droplet that can propel itself
pretty much through, through this bulk medium. So
it's not gonna be it's not gonna have
arms. It doesn't have a propeller or anything.
It's usually a quite common example is through
surface tension. So you end up with these
surface tension driven flows. And when you have
a flow, you can
impart a force onto the surrounding medium, and
that causes this this driving, this this causes
this motion. It's swimming.
And then the second a was adaptive.
Do one of you want to tell us
what adaptive is? Sure.
Adaptive material or or or material that's exhibiting
adaptation is the it's the ability of a
of an object, be that a a bacterium,
a bird, or a or in this case,
a droplet, to sense its external environment or
to sense another object and respond to that
change.
As we've sort of seen from from Rob,
a lot of these examples
are are biological.
And going back to what Georgio has said
previously, a lot of the field of animate
matter is really about recapitulating a recapitulating a
lot of phenomena that we see and grasp
intuitively from from biology and and recapitulating them
in a in a synthetic system. And that
and that synthetic system can have biological parts
in it, but our idea here is to
to to produce something synthetic
and maybe from a physicist's perspective, produce something
reductively.
In terms of adaptation,
that change can be both to a change
in its surrounding environment, be that a change
in surfactant concentration or a change of in
pH or whatever, or to the presence of
another,
for instance, bacterium.
So so, you know, a a bacterium might
adjust its its metabolism based on food or
reagent concentrations or might change its swimming direction
in response to the presence of other bacteria.
We need to be a little bit careful
here because bacteria have a a fairly
complex set of stimuli and responses that they
can exhibit. And so these guys, as I
guess, Georgio would tell us, definitely count as
autonomous, which is this third day of animacy.
And so I think arguably what makes something
adaptive is that it can respond to stimuli,
but it can only do so to a
fairly limited
palette
of stimuli and and you respond to only
a fairly limited
respond in a fairly limited,
number of ways.
That adaptation, that that change that your system
can exhibit,
it can be individual or it can be
collective, and that collective response generally emerges from
an individual one.
For instance,
flocking behavior that, again, you'd see in birds,
bacteria, but you might recapitulate in an element
system in a in a in a set
of colloidal particles,
is generally a response to,
local concentration
or some exchange
of information.
In the case of, you know, nature, that
would be bird sensing how far apart they
are from each other. That results in in
flocking. It might be the the bacteria sensing
a local concentration
of a specific compounds, or it might be
colloidal particles being disturbed by the swimming motion
of another colloidal particle.
Most interesting for me in these adaptive systems
is is not the the change between two
equilibrium states, but it's the change between two
dynamical
states. That is states that require the system
to be converting energy to to maintain them.
So rather than simply having a system that
you prepare away from equilibrium and watch it
relax, you have it switching between these two
energy converting states.
There's not necessarily entire agreement in the field
there.
So some, you know, some people would say,
well,
materials
fold themselves in response to illumination from light
count as adaptive.
So we don't wanna rule that out rigidly,
but certainly for me, these this chain it's
this switching between dynamical states that's most interesting.
And what might that look like for a
droplet? Can you give me an example similar
to the Robert's about the chemical,
behavior?
Yeah. So in terms of droplets, a really
simple or a simple ish example is, say,
you have so droplet, you have two liquids.
Right? You have oil and a water. Say
you have some degree of chirality in one
of those
liquids. Could be a liquid crystal. Right?
Under the right circumstances, you can then generate
motion in that system that is inherently somewhat
chiral. It has a handedness. It might spiral
clockwise or anticlockwise.
If you shine a light on the the
chiral phase and it switches handedness,
correspondingly, you can engineer a change in the
handedness of the
the swimming too.
I see. Okay. And I think the third
a, autonomous, that's actually the most difficult to
achieve in a droplet context. Can you wanna
tell us what autonomous means and why it's
so challenging?
Yeah. I'm happy to answer that one. It
it is the one of the most challenges
property to achieve at the microscale.
Let's start with the definition. What is autonomous?
Autonomous
is any behavior that is initiated
due to an internal change of state. It's
not triggered externally.
It is initiated by the object. In this
case, it could be a droplet
or something else, like a bigger a material
or something bigger.
When I think about these, autonomous behaviors, I
like to think about an analogy with something
that people tend to know a bit better,
especially nowadays, which are robots.
Autonomy is not a black and white sort
of property.
It's a spectrum of gray, and you can
go from a system that have some level
of autonomy to system that's extremely autonomous.
Now if we think about, robots,
you can think about some of the robots
that were first introduced in the industrial process.
They were
introduced to help the manufacturing process, but they
were ultimately remotely controlled by humans.
And with the development of technologies, they became
more and more autonomous. The human factor became
less important
to a point that now is being,
put to the side, for example, by development
in artificial intelligence and so on. So if
you look at this, evolution for robots, you
can really see
how you went from systems that were helpful
for, production, but being very not much autonomous,
just helping
with the workforce,
to systems that are becoming more and more
autonomous.
And we can almost start to pinpoint
properties that you will typically associate to,
human beings, like thinking and
capability of extrapolating
information and so on. Now at the microscale,
we want to achieve something similar with droplets
but also with other somatic systems. We want
to develop synthetic materials
that can perform task purposefully
based on a change
of internal state without having,
a remote operator telling them what to do
and when to do it. It will be
intrinsically
triggered by changing conditions.
So in in this sense, if you think
about
increasing autonomy,
this is where really I think that the
lines between
synthetic and living starts to blur.
But
it's also really challenging to achieve because if
you think about
having
this
level of autonomy
comparable to what robots can do nowadays at
the micro scale, It means
including physical and chemical rules in tiny, tiny
volumes
to allow for a certain level of programmability
to develop, and that's not the easy task
to achieve. You know, you really need to
squish down, miniaturize or components to a point
that you can achieve
very
complex life life behaviors, but in tiny, tiny
volumes.
So what's an example of a droplet that's
autonomous? What's some research that's been done that
have has demonstrated
an autonomous droplet?
So in a sense, there is not much
out there that is fully autonomous.
There there is some level of
initial autonomy behavior that kind of connects to
some of the most active and adaptive system
that we have seen, and maybe then Joe
and Rob have also other examples they can
bring up. There is one that I particularly
like or or although we we tend to
have discussion between Joe, Rob, and myself about
whether you can actually classify this as autonomous
or not.
There are some droplets that are
active in as Rob described, so they can
move by consuming chemical fuel.
And if you put these droplets into a
maze structures, they can actually solve it.
I see the insipid of autonomy in there
because it reminds me what
living system, bacteria, slime mold, ants can do.
You know, they can really follow chemical press
to solve
autonomously.
And may I elaborate?
It is borderline whether this can be considered
autonomous or not at this stage. I like
to think that they are, but, you know,
I think Joe and Robert
have slightly different opinions on this. And I
don't know if you guys want to add
any more examples on this.
Yeah. Like, Georgio says the the droplet solving
amaze is a attempting one. I always have
this thing that really if you if you
stick a chemical source at one end of
a maze and you stick a droplet at
the other,
ultimately, the droplet is just gonna move up
the concentration gradient. And so, really, you're just
you're just swimming at the concentration gradient,
under topographical constraint. But I I also I
I see what Georgio says. You have this
you have this system that looks like it's
sort of processing stimuli and and responding. So
I think there's an argument there.
The sort of low hanging fruit in terms
of making an autonomous droplet is to simply
stick a bunch of living material in there.
So if I stick a bunch of swimming
bacteria into a droplet, that droplet can be
can be made to swim, and it incorporates
the same sort of palette of stimulus response
and collective behavior as the bacteria.
Similarly, if I sort of do a cell
free approach, so I take biological
mechanisms for, say, molecular expression and stick them
in a droplet, that also is gonna exhibit
some
degree of autonomy.
I think maybe we me, Georgio, and Rob
would share a frustration there that that system
is not necessarily reducibly complex. We're not we're
showing that we can do a thing. We're
not showing that we can really understand it
in in any meaningful way.
I think the systems that
always appeal to me are these these chemical
oscillators.
And so you have, say,
I don't know if I I'm stealing your
fire here or if you wanna cheat in
this as an example.
But the the classic example is the this
Belisov Jabotinsky reaction where you have, you know,
chemical a reacts to form chemical b, and
then chemical b reacts to form chemical
a. And in systems that incorporate bromine, that
generally results in some change of color. So,
you know, bromine is red, bromide is blue,
and so as the system composition switches from
bromine based to bromide based, you get a
switch from from red to blue, and and
this switch happens at a a characteristic
frequency.
It gets interesting when you start
dimensionally confining these things. So if you which
is a fancy way of saying if you
do it in a petri dish, you have
a, you know, you have an effectively sort
of quasi two d system, and you end
up seeing these traveling
waves of alternating stripe of reds
and blue that look a lot like the
the patterns that Alan Turing studied when he
was trying to
look at morphogenesis
and patterns of animal spots
and describe reaction diffusion mechanisms, I think it
was. Now I might be wrong about that,
though.
Things again get interesting when the catalyst you're
using to make a turn into b or
b turn into a becomes light responsive.
You end up getting
effects that look a lot like analog image
processing.
So you can, you know, you illuminate your
petri dish with a photo mask, and then
you shine a light on the system and
you, you end up getting things like the
skeletonization of the the initial image that you
shone on the the petri dish.
What happens with droplets then is
you
say your say your reagents have a differing
polarity,
so one of them likes the oil more
than the water. And if I have one
of these b zed reactions happening inside a
droplet
and my nonpolar reagent goes travels to another
droplet that also has a a b zed
reaction happening to it, you have exchange of
chemical reagents that are driving
those chemical reactions. And in effect, you have
exchange of chemical information.
And that system is then gonna evolve in
time and space according to rules that are
determined by the transport of reagents and they're
they're partitioning be between the
the two phases, the oil and the water.
And this is a system that then gives
you,
some degree of what I would consider to
be autonomy.
You have complex stimulus response and and processing
of information.
But I don't think we're at the point
where we could really ever call that autonomous
yet. It's just
more this might be an interesting direction to
to go in with this field.
So, Robert, did Joe steal your thunder there?
Do you steal your favorite example of an
autonomous droplet?
No. I actually came up with a new
favorite. There are a few examples of this,
but one system that I really like are
these I wouldn't call these all autonomous.
I think, again, kinda getting there, but for
me, autonomous needs some sort of internal processing.
I I'd I'd really like to see some
sort of, quote, unquote, decision
being made by the droplet. But one one
example of the system I really like are
predator prey interactions.
So,
several examples of kind of droplet pairs of
oil, where if you put them into a
a bath, they like to float on the
surface rather than in in a three d
space, but they will one droplet will chase
the other one.
It's
again, I don't know if I'd call it
autonomous. It's it's a kind of combination of
different sensing and, you know, one droplet likes
to run away from the other one and
one likes to chase the other one. So
it's more of a relationship between between stimuli.
But they're really nice to look
at. And I think they again, it's it's
finding those comparisons with nature. We we like
to call them predator and prey, but it's
it's just, you know, different types of sensing.
There's a really nice example actually where we
can bring in that the soft matter back
into the into the equation where the droplets
can can deform and grow these tendrils,
and
that's part of the chasing. And, you know,
it makes the droplet more terrifying to the
prey. It kind of deforms and grows these
arms, and they follow them along. It's quite
it's quite a complex system. It's not something
you'd see probably
by accident, but, it's a really nice example.
And it kind of shows
the the power the droplets could have. They
especially with this kind of deformation, the shape
change,
and, yeah, there are some really nice examples
of droplets changing shape to in response to
things. And,
that's one of the benefits that that liquids
have over, say, kind of solid micro particles
in that they are soft.
They can change shape. They can flow.
And, yeah, we just we just need to
build that autonomy
more into them. I I guess, Margaret, it
may be something to first take home messages
that,
while animate matter is kind of a framework
to define behaviors,
there is a lot of scope for, reinventing.
And and as you can see,
the definition are still a bit personal. And
one of the things that we're trying to
put forward with this article or with this
perspective is that
there there need to be a conversation
in across communities and the community to
properly define these terms.
And
when do you stop to define autonomy as
autonomy? When do you where do you start?
This is something that we also wanted to
convey with the with the perspective.
So you mentioned in your paper, this perspective,
that it's possible for droplets behave like tissues
or bacterial
colonies.
What does that mean? And can you give
me some examples of how that happens?
So they're two they're two, I guess, very
evocative words that, again, draw more on this
sort of biological inspiration.
When we talk about colonies and the idea
of colonies of of droplets,
I guess the thing it calls to mind
is social insects, things like ants.
You have
information exchange that is, in this case, exchange
of of chemical reagents. It could be in
between individuals in this, you know? And so
so thinking back to the predator prey system
that that Rob was thinking of, you have
a a sort of asymmetric
exchange of chemical reagents between two different
species of droplets, and that can lead to
interesting emergent behavior like transient
and and dynamical clustering.
That exchange of information doesn't need to be
chemical. It can it can be hydrodynamics
as the way a a droplet sort of
swimming through
liquid. That disturbance it makes in the in
the liquid is swimming through will affect the
the swimming of of neighboring droplets. And that,
again, can lead to exchange of information and
and in effect, so clustering behavior.
In terms of tissues, I sort of think
of that in terms of droplets that are
stuck together. Perhaps to give an example to
make it more concrete, there's this wonderful work
on systems called droplet interface bilayers. So you
you take, an aqueous droplet and you print
it with an inkjet printer into, an oily
solution of lipids. The lipid is surfactant, it
will absorb to the the surface of the
droplets. If you print two of these droplets
next to each other, the lipids really like
each other and they'll zip together.
And so these two droplets are then stuck
together and they have a a lipid bilayer
joining them.
What is then very interesting is if you
take pores that enjoy
sitting in lipid bilayers, and there's there's lots
of those in in nature,
because that's just a useful function to have
if you wanna disrupt a cell membrane, and
that's exactly what alpha hemolysin pores do for
instance. They they sit at the the surface
of a lipid bilayer, and they make a
big hole in a cell, and and that's
one of the mechanisms by by which cells
are are killed in nature.
But in this case, it allows information and
transport of material between the two droplets.
What's neat then about sort of mechanization and
printing is you're not limited to just one
or two or three droplets or however many
you can be bothered to place by hand.
You can program your your printer to place
arbitrarily
many
droplets into arbitrarily arbitrarily
complex patterns, and you can select which of
those droplets contain these pores. And so you
can start
building in
complex transportation pathways
for the the the conduction of reagents, for
instance. And so that's sort of what we're
talking about in terms of these tissue like
materials that are that are held together by
by surface tension and exchange information through quite
static
connections as opposed to colony like droplets that
consist of individual things that are sort of
running around and and maybe they exchange,
a chemical reagent with one another.
What I'm hearing here is is just the
such similarities
with life. You talked about things being a
spectrum.
Does this kind of research sort of challenge
the spectrum, what we think of as living
and what we think of as not living?
Or is there for you a very hard
and fast boundary between, okay, this stuff is
living. This is a bacterium, and this stuff
is not living. It's just a collection of
chemicals
in a sort of self assembled bag moving
around. I I don't know if we want
to challenge that because then we will be
bored into with philosophy. But I think anime
and matter is purely synthetic. So in that
sense, it will, by definition, not believing.
But we
the aim is to try to replicate properties
that are usually attributed to living systems, but
still keeping fully synthetic and
controllable in a sense. And
but we want to learn from nature. We
want to learn from what living systems can
do. They're much better at a lot of
things.
So it's really
about
learning from nature and trying to reproduce some
properties, starting from principles that are purely physical
and chemical
to
add some attributes that are beneficial
for materials
like the one we discussed here, but they're
not leaving.
The lines are a bit blurred, but the
distinction is still clear.
That's why it's really tricky, isn't it? I
think, yeah, I don't think we ever go
it's fair to say it's gonna be alive
because even some biological things, I don't I
don't know if I call a virus alive.
And
I don't know. It's it's tricky. Like, what
what level of self sustaining is is alive
is a key question. And I I have
a feeling a lot of these probably won't
be eons
of self sustaining. But
but whether that's a requirement, I'm not sure.
I think that, observation of Robert actually
feeds into
applications or at least applicate aspirational applications of
the field as well. Because if we're being
honest, we don't we don't have applications yet.
But,
you know, what you're what you're
what you're arguably trying to do is
figure out what are the functionally useful properties
of biological systems. You know? The the idea
to change shape in response to
stimulus or change behavior in response to a
stimulus
without
having to create a system that has to
worry about sustaining itself and and has all
the
inherent,
both chemical and philosophical complexity that comes with
that.
On the subject of applications since Joe's brought
it up, your EPL piece is what's called
the perspectives article explaining the current state of
the field and sort of suggesting how it
might develop in the future.
So So focusing on the future, what are
some things that soft matter scientists such as
yourself would like to be able to do
with animate droplets in the future?
I think one of the things that it
would be nice to be able to do
is to have programmable soft robots that are
made of droplets.
They will allow to make
robotic systems that are closer to the microscale
potentially
that are much more deformable than some of
the
robots that we are typically thinking of.
And they can have
can be useful for for applications that
can range from making materials that are
adapting to changes in stimuli for you know,
can go from construction purposes to drug delivery
purposes.
That that will be one of the things
where
further develop development in the level of nemesis
or droplets can actually lead to
creating some impactful applications
down the line.
Robert and Joe?
One thing that I think I'd really like
to see happen is,
a lot of this, like, soft matter and
droplet physics it arises with with quite simple
chemicals,
and you can get these complex behaviors with
these quite simple chemicals. And
that's quite important, say, if we wanted to
apply these things in the environment.
So, you know, if you think about a
lot of, environmental cleanup, sometimes you dump chemicals
in the environment to remove chemicals,
and you obviously have to make sure those
chemicals you're adding are at least not as
bad as the ones you're taking away.
But
what this field can kind of do is
is use this this physics that can arise
from just physical prop and you're using physics
rather than chemistry almost to to do this.
I think
the scope of finding function in the physics
of some more
innocuous chemicals
than using more advanced chemistry, if that makes
sense. So you might have something like you
you'd pour it on an oil spill, and
it would sort of semi autonomously
and in an adaptive and inactive way seek
out the bits of oil
so you wouldn't have to manually apply this
this chemical, whatever it was, to clean up
the the oil spill.
Yeah. One thing I'd really love is if
you add this, like, little bit of liquid
into your oil spill and just sends out
little swimmers,
drags everything back, and then, you know, brings
it back to you. That'd be, you know,
incredible. But I don't know how close we
are to that yet.
Alright, Joe. Take out take out your crystal
ball and show us what, what you think
animate droplets will do in the future. Yeah.
I guess I have I sort of have
two what I think are reasonably promising directions.
One that doesn't have an immediate use, but
I just think is a promising direction in
terms of making an interesting material is is
so I was I was, waxing lyrical about
these droplet interface by layers that we print
into tissues. Right?
And my a lot of my own research
interests
focus on printing soft materials.
And so a very simple question you could
ask is, what happens if we take these
printed droplet tissues that are now technologically
quite well established even if the techniques to
make them are reasonably niche and and only
a few labs have the capability to do
it? And we start incorporating
the sort of well established
energy conversion mechanisms that that we've been talking
about today. And this feeds into what Rob
was saying. Can we even take those energy
conversion mechanisms to move away from needing to
even print a system in the first place?
Can we can we make a system that
chooses
seven sets of quotation marks there, but chooses,
how to build itself, how to how to
build itself into a complex structure?
More
immediately
relevant,
I think,
is actually a lot of the rules being
developed to describe animate droplets in particular, you
know, exchange of materials in solution, evaporation, volatility.
These are extremely relevant to a huge range
of consumer goods.
If you think of what a deodorant is,
right, that is an evaporating, partially volatile phase
separating system, lots of liquid components evaporated on
a heated surface of complex topography.
The the the sort of effects we're seeing
are almost certainly seen on a daily basis
in their systems. And if you wanna improve
the
the the performance of those products, you you
probably want to be digging into the the
fundamental
mechanisms that describe animal droplets.
So going from cleaning up an oil spill
to having a deodorant that seeks out the
exact areas that are smelly and and prevents
them from from driving people away? Can you
make a deodorant that wants wants to live
in a sweat plant?
Excellent. So what are the barriers to achieving
some of these, you know, exciting soft matter
developments of the future? You know, what are
the challenge? You've talked a little bit about
the equipment you use to to make them.
Are there challenges there? Are they more theoretical?
What are the what are the issues?
There there are different
issues. Some are more
fundamental, some are practical.
I would say fundamental,
we start to
enter a realm of non equilibrium, far from
equilibrium physics.
This is a new frontier.
People have been studying non equilibrium physics for
a long time, but
I think that we need to further our
level of understanding. So often when you play
with the systems in the lab, you find
behaviors that are completely
alien to you, and then it's actually quite
fun to try to reverse engineer and understand
what's happening.
But
sometimes starting from first principles with this non
equilibrium physics is not possible.
Another challenge that I see is we already
mentioned it earlier, it's miniaturization.
We want to achieve behaviors that typically we
associate to
living organism.
We have been successful
to a degree
at the macroscopic scale,
and biology is extremely good at achieving very
complex behaviors,
packing a lot of information,
a lot of complex machinery in tiny cells,
but we're not there yet. We're really, really
far from achieving anything that looks like that.
Biologists, the inspiration
for that, we are still quite far.
Even just working with liquid materials is a
challenge because
if you go to macroscopic scale where surface
tension starts to dominate, you can say that
liquid starts to have a shape,
but, technically, they're still liquid. So even achieving
all this complexity
in terms of adaptation,
responding to the environment,
sensing the
environment, having complex reactions happening in these tiny
liquid compartments.
All of this has to happen to achieve
activity,
adaptiveness,
and autonomy while keeping the droplet stable, and
it might not be
compatible
and,
you you know, you want the droplet to
survive the process rather than be consumed with
it.
There is interdisciplinarity.
As you've seen, all of these
topics we've been discussing, it's somehow the crossroad
between physics, chemistry, and biology.
We're getting better at discussing things among disciplines,
but there is still a lot of, technical
language, technical barriers, language barriers that need to
be overcome
to be able to work together.
And that's already complex when we think about
different soft matter systems. Imagine something that can
span across different case like this animate matter
behaviors.
So there there are a lot of challenges
that, need to be tackled,
and I'm sure that,
both Joe and Rob will have their own
challenges that can add up to this. But
as you can see, it's a very
rounded and complex problem, and that's why it's
also fun to to study it.
Robert and Joe?
I would like to add to what Georgi
was saying is, I think we do focus
a lot on equilibrium physics, but
there's really exciting stuff happening in non equilibrium
physics. And some of us actually were hiding
in plain sight. There's a really nice paper
from Stanford
where they looked at food coloring.
And if you put it down on a
glass slide, they just chase each other and
sense each other and, like, dance around on
the slide.
It's it's stuff that's everywhere. It's just we
it tends to be overlooked.
So finding the thing that's in plain sight
and understanding it for what it is?
Yeah. In plain sight or maybe just hiding
a little bit under the surface with some
really complicated physics behind it for for some
reason.
And, Joe, you want to add to that?
No. I I think from my perspective,
both Giorgio and Rob have mostly nailed it.
I think the the challenge
to design animate materials, animate droplets is is
a challenge of interdisciplinary
collaboration.
Fundamentally,
the chemist is scared of physics. The physicists
are scared of chemists. Everyone's scared of biology.
This is
a huge overgeneralization.
There's obviously tons of people doing great work
across those disciplines, but I I think maybe
we do.
We sit too much in our comfort zone
and maybe we sit too much in what
we perceive to be prestigious. Like Rob says,
I think there's a huge value
in studying and understanding
the everyday. And I think there's
a huge
a huge wealth of really interesting stuff
just sort of sat there in, to quote
his example, a bottle of food coloring.
Hopefully, that inspires some more listeners to go
out and look for the interesting soft matter
phenomena in their in their daily lives. Georgiou
Volpe, Rob Malinovsky, and Joe Fort, thank you
so much for coming on the podcast to
talk about this work, which you can read
in full in the journal EPL. Thank you
very much, Margaret.
Thank you very much. Thank you.
That was Margaret Harris in conversation with Giorgio
Volpe,
Rob Malinowski,
and Joe Forth
about the mini review article
or perspective
that they wrote for EPL
about animate droplets.
Their article is entitled Towards
Animate Droplets,
Active, Adaptive,
and Autonomous.
And if you'd like to read it, it's
available for free on the IOP science website.
Just search for the journal EPL.
Our thanks to Giorgio,
Rob, and Joe,
and to the journal EPL,
which sponsored this episode of the Physics World
weekly podcast.
I'm Hamish Johnston, and our producer is Fred
Iles.
The theme music for this podcast is called
one three seven, and it was composed
and performed
by the physicist
Philip Moriarty.
Join us next week for more fascinating stories
from the world of physics.