Title: Water and solutions in nanoporous media: capillarity, osmosis and phase change
Speaker: Dr. Olivier Vincent
Affiliation: Permanent Research Scientist at CNRS, Institut Lumière Matière, Lyon, France
Abstract:
I will discuss a series of experiments that illustrate the rich physics of phase transitions and transport in nanopores, using water and salt solutions. I will first present results about the dynamics of water uptake and evaporation in nanoporous media, triggered by humidity changes. These results show different behaviors and different physics (including capillary imbibition, cavitation etc.) depending on the applied conditions and their history. I will also show how these behaviors change when adding solutes to the water; from these measurements we can also obtain useful information about how nanoscale confinement modify the conditions for crystallization and deliquescence of salts. Most of these observations can be rationalized using a combination of capillarity, osmosis and nucleation in confinement. They also connect to various areas of science and technology, including plant physiology, geophysics and heritage preservation, among others.
Hello I think you can see me from here I’m paa and for today’s prab lecture we are pleased to introduce Dr oliv a physicist at The Institute of light and matter in Leon before holding his current position Olivia his PhD in physics in the University of then worked for five years
As a research associate in the University of forel his research is very relevant to our group revolving around transport and pH changing for media citation and bubo Dynamics the physics of plant among other things uh today’s lectur is entitled water and Solutions in Anor media capity as Moses and face
Change and Olivia if you’re ready the floor is yours thanks and thanks paa for the introduction for and for the invitation I think it’s a it’s a great Community to talk to and it’d be interesting to have feedback on what I will present so today what I wanted to present is a
Combination of different experiments that I’ve done over the past five to 10 years uh that show the uh some interesting aspects of water and Solutions in nanoporous media both in terms of the Dynamics and the thermodynamics actually as you will see most of this talk can be kind of
Summarized by playing around with Kelvin equation and so of course for this community there’s not much uh need to introduce why studying porous media is important in my um in my experience with porous media I started to be interested actually from Plants um and plants in general have a
Lot of um of issues where they need to interact with water um um um and one of the driving forces for how water interacts with porest media is humidity and a lot of one I’m going to show today is how porous media interact with humidity either with fixed humidity or with humidity
Cycles and a very important uh phenomenon that arises in plants is um or the transport of water in trees uh this is driven by a deficit of humidity in the atmosphere that will generate a driving force from the leaf that will pull up the water colum from
The Roots this is the phenomenon of evapo transpiration and here humidity in the atmosphere plays a huge role where it will drive flow during the day and uh stop driving flow at night so that’s humidity uh driving flow through a poest medium but humidity changes can also Drive the formation and generate
Stresses and a very famous example is pine cones that open and close depending on the humidity and also these examples where spores from ferns can actually move and deform under humidity cycles and this place uh due to this change of shape due to humidity cycles and there
Are many other contexts where the intera of humidity and and surfaces or porme is important I can site the atmosphere with aerosols for example or the problems of Soul solutions that get damag in pors media in civil engine ing geop physics or artwor so a common theme to all these
Context is that uh the interaction of the water in the POR medium with external humidity generates stresses and one of the reason that uh confined liquids that dry or condense Drive stresses is through capillary forces so of course if we have a medium that is filled with water uh with many
Sky that are flat at the the surface there is not much stress but if you start drying the system then you generate curve interfaces and through lapas law this generates a stress in the liquid that can be transmitted to the structure as well and can also Drive
Flow of course the maximum stress you can get depends on the size of the pores and the smaller the pore size the larger the stress and if you get to nanometer size pores the stresses can be massive of order a thousand atmospheres an interesting aspect as well of these
Curved confined minis sky is that they have a vapor pressure that is different from bulk liquid water and this is driven I mean this is explained by the Kelvin equation that relates the vapor pressure or the relative humidity to the capillary pressure in the liquid and of
Course if you have microscopic miny this effect doesn’t matter much but if you go to nanom nanometric size Min Sky then this effect can play a large role and the paper pressure deficit that you can get is important another way of looking at Kelvin equation is that if you impose
Humidity over a nanoporous medium if you fix the humidity and then you will fix the stress in the medium and you will um achieve some stresses or negative pressure in the liquid or capillary stress in the liquid just by playing on the relative humidity and the way this
This gets achieved is because the curvature of these interfaces will reach some equilibrium with the outside humidity and the low the humidity the the higher the curvature of this Minis and this is all described by Kelvin equation and the stresses that are implied are again very large um typical
Examples here is if you if you are at 100% rative humidity like here you have no stress in the liquid if you go to 90% you already have um minus 145 atmospheres of stress of negative pressure and if you go to 50% rative humity then you reach a th000
Atmospheres typically of course these stresses can be achieved only if the porest medium has po small enough to sustain these stresses and that’s why they’re important for um nanoporous Medium as I said before these traces are important and they can generate mechanical deformations either in pine cones but also in solid por structures
Like por silicon here and you can generate these reversible um deformations of the medium just by changing the humidity and so the humidity is kind of a dial of the stress and strain in this medium of course the stresses can be so large that they can uh um Drive Boiling
Spontaneous boiling on the liquid which is called cavitation in this case because the pressure gets much lower and in fact much lower than the vapor pressure of the liquid but in fact absolutely negative and this is a highly metastable State and this can lead to spontaneous bubble nucleation and and
This is a movie actually filmed during my PhD thesis some time ago of one of these cavitation events in a liquid that is at minus 200 100 bars of negative pressure and of course as I said it’s important these these capillary forces interacting with humidity for driving
Flows in trees and these capillary flows because the capillary forces are so large they can large they can drive large flows and they are tunable with the humidity so we will see a bit more later in the talk about these two aspects cavitation and capillary flows and I’m
Going to also look today into what happens when we don’t have a pure liquid in the system but a solution and typically a Sal solution and this uh relates to other areas of U of Science and Technology but I would say one of the most prevalent thing where uh the problem of salty
Water in por media is important is uh Heritage conservation and geophysics where if you have humidity Cycles on a porous medium that contains a salt solution you can drive crystallization or delance of the salt and this can generate damage and also different problems like salination of soils and
Etc and so the solute itself and the liquid has additional additional effects in compared to Kelvin equation in capillary forces first the solute will change the surface tension of the liquid it will also introduce collative effects like an effect of osmotic pressure a change of vapor pressure Etc and also of
Course in can crystallize or uh introduce the liance in the system so you can get all these kind of new phenomena that appear so here today I will talk about these different aspects focusing first on the effect of capillary forces we will look at evaporation or inhibition so reverse
Processes and the phenomenon of cavitation in some of the system and in the second part I will look at the effect of solutes um looking at the Dynamics that induce the solutes like osmotic flows interacting with the humidity aborption isotherms that look at what happens um in the thermodynamics
Of the system when you introduce salt and also the phenomena of crystallization the liquis so first about capillary forces um I’m going to show some experiments that we did mostly with silicon and these experiments were done at Cornell where were still there what we use is
Poral silicon with that is kind of it’s a por medium that is nanopores the pores are typically 3 to four nanometers in diameter but it’s also highly isotropic and interconnected and we have a thin layer of this system where we sandwich uh this layer between glass and silicon uh
Meaning it’s a wafer basically it’s a quasi 2D system and it’s open just on one side here and that’s the side where it can the liquid can evaporate or condense in the system this can be seen kind of an artificial tree where you have a root system with a reservoir here where the
Water can be sucked from you have a porous medium that is kind of similar to the trunk or Leaf of a of a tree system and here uh the evaporating surface is kind similar to the Lea and so what we did in terms of experiments with this system is very
Simple we fill the system with water we have the reservoir here and then we let it dry or let it sit in an environment with control humidity and again the only way the water from the reservoir here can evaporate or can leave the system is by evaporation from this surface and we
Measure the speed of the emptying of the reservoir uh to measure the flow rate in the system basically and the reservoir is not like sketched here of course it’s a serpentine Channel and we will see some of uh its um some of its length here and each line that I show here is
An experiment with different humidity so when we put the system in a given relative humidity you see the system starting to empty so the bright part here is where the gas and here uh the um the liquid and it’s just the reservoir that we see here again and The
Ting of the reservoir informs us that the system is driving flow at different speeds depending on the humidity I we play again so you can see of course that the lower the humidity the higher the flow rate uh but at low humidity it starts to saturate it
Seem and so if you look at the data of the speed of the Flow versus the humidity you have this first very linear relationship when you lower the humidity the flow increases and then it saturates and I will look later at the saturation but the explanation to why why the flow
Is linear is because I didn’t plot here the velocity as a function of humidity but in terms of the driving force that is implied by Kelvin equation which is the theoretical capillary pressure you get in the poe by equilibrium between the liquid at the edge here and the humidity
Outside and so uh it seems to work very well so that means that the linear relationship informs that the Kelvin equation is really driving the flow in the system here and what it implies as well is that the curvature here we can tune the curvature of this many sky that
Are at the edge here just by changing your humidity and the flow responds to that and the saturation of course is I would say kind of obvious because you cannot curve the miniscus infinitely and at some point you will reach the maximum curvature associated with the size of
The poe and then when you reach that point the flow doesn’t depend on relative humidity anymore because it’s just driven by the intrinsic capillary pressure of the pore so that’s the first um type of um system that we can look at this drying induced uh flow in these systems
And of course what we did is Al look at the reverse process of instead of drying the system we drive imbibition through it and the way we do this is we take kind of the exact same system but without a reservoir and we uh just take the system completely
Dry and we put it in some relative humidity that is relatively high I would say so you see here for example 98% 80% 50% and once we do this so starts from the dry system and increase the relative humidity you will see that there’s a front that develops from the edge in the
System so we’re looking at the layer from the top here and the front Dynamics depends on the relative humidity again and first it can be surprising that just by raising the humidity we see a front that invades the system and the reason that this happens here is because of
Capillary condensation and because these nanopores are very small and hydrophilic there are some critical reative Unity above which the the liquid Vapor sorry the the water vapor in the atmosphere will spontaneous condense in the poe and this condensate that you get by capillary forces will start to invade the
System uh and with this image analysis to look at the Dynamics of this front and for those who know inhibition it looks very much like a square root of time behavior that is typical of Lucas for bur dynamics of course compared to the standard Lucas for bur Dynamics where we
Have in an idealized single pore we have a meniscus that has some negative capillary pressure that drives flow within the system from a reservoir that’s on the outside here we have a slightly different system where we have have a miniscus from the condensate that’s trying to invade the system but
Also there’s some another miniscus at the edge between the liquid and the outside Vapor phase that kind of resist the invasion and of course the the speed at which the miniscus will in I mean the liquid will invade the system depends a lot on the competition between these two
Minis this meniscus is just uh driven by the the intrinsic cap pressure of the poe but again this menisc is curvature is driven by Kelvin equation and it’s fixed by the humidity outside from the equations we saw before um and so the more the lower the
Humidity you have the the the more curve this meniscus is and the the bigger the competition is is with meniscus and that’s why the the the lower the humidity is the lower the speed of inhibition because these two minis Sky compete more and more and we could verify that
The modified Lucas Lucas W equation that takes into account this Kelvin effect works very well with the data we have for the Dynamics of inhibition by capillary condensation at different humidities and of course we can even reach a point where the two Min Sky have the same curvature and there’s no invasion
In h skip this so that’s that’s capillary Invasion with speed that depends on humidity due to Kelvin equation and I’m going to look at another phenomenon looking again at a situation where we dry the system and here we have a slightly more complex systems where we have the same
Uh porous medium nanoporous medium that is sandwiched between silicon and glass but we also introduce micro cavities or micro channels in the system that are in contact with the um the nanoporous layer and the one we fill the system with water uh the Water embid both the nanop layer and
The the capillary sorry the the water micro cavities and once it’s full we start drying the system here we dry the system at 84 per rative Unity which is relatively high but you will see that still things happen this is an actual photo of the sample from above and that’s the kind of
System we will look at so here I’m going to show a movie of uh what happens when we drrive this system and here probably from your screen you won’t see much but there’s an array of these micro cavities and they’re full at the moment and they become very bright when they empty and
You will see them kind of appearing from the the the medium and so when we put the system full at 84% relative Unity you see that the system starts dry drying but it dries by emptying these micro cavities that we have in the system and the the interesting aspect
Here is that if you look at what happens in the nanoporous layer that’s below the the micro cavities it remains full at all times because at these humidities it will not empty but these micro cavities empty uh without having a drying phone from The Edge and the only way that can be done
Is by Bubble nucleation or cavitation and so what happens here is that when you dry the system by either Kelvin effect you start developing negative pressures at the edge here and these negative pressures will transmit to the liquid in the pores and then to the microscopic cavi is here and the
Pressure can be so negative that it drives spontaneous bubble nucleation and that’s how the systems dry um and we could measure that uh the drying of these cavities of the cavitation events happen between minus 200 to minus 300 Bars of liquid pressure we also saw that there’s some
Interesting Dynamics if you look closely at how these systems empty over time in in the sense that they will have a bunch of cavitation events and then nothing and then a bunch and then nothing and there seems to be a periodic emptying of these cavities separated with period where
Nothing happens uh much and we could explain this by I won’t go into details here but this is all due to the nonlinear coupling between the nucleation kinetics where the rate of nucleation depends on the local pressure and the parastic evolution of pressure in the system and this is a simulation
Of these um the Dynamics that we simulate in these systems where each of these um red dots is a cavitation events and then once locally the the liquid in the micro cavities has emptied the pressure can go back down again and you generate these Cycles uh of self-organized burst of gravitation
Events uh that are uh due to these uh Complex Evolution of the pressure field over time over space and dimension so that was a brief uh overview without too much details of what we did on these systems of por silicon systems about capillary forces the flow driven by capillary forces and
The phase change like cavitation induced by capillary forces and this is um several references you can look at if you want to hear more about I mean read more about these systems and right now I will move to some more recent uh studies where we look at the effect of adding Source uh
In this um in similar systems so that’s what I’m going to talk about now and um so the a question of the prev yeah could I ask you in the previous one are don’t you see any cavitation in the forest silica Su layer so we don’t see it it doesn’t mean
That it doesn’t happen um because the scales are so that there may be Tiny Bubbles in some locations but I don’t really expect it because of um of confinement effects and I expect that the nucleation of the bubbles Within These nanop pores would take much more much larger driving forces to occur and
We can also measure that so if you look at sorption isotherms of water in the systems you can see that um the layer uh the nanopulse layer itself stays full as long as the I mean completely full without any little dim due to citation as long as the humidity is above 50 to
60% and here we dry the system at 84% so it’s way higher than any drying or cavitation in these NE layers so essentially that means that the critical radius for a for a nucleated cavity is too large to yes develop in the the substrate exactly and so the critical radius uh depends on
The negative pressure you have in the liquid so it depends on the humidity and I think um the the moment at which the critical radius reaches the size of the pores which is a few nanometers uh it it should happen around 40 to around 40% relative humid so way lower than but
It’s it’s true that it’s an effect of comparing the critical nucleus to the PO size basically okay thank you sure very nice thanks so should I move to Sals y let’s go so I this cavitation things is um an interesting aspect because it’s actually something that happens in trees and that dries that
Drives uh the drying and mortality of trees in the summer because you have these large D forces by evaporation in the trees and you get cavitation events and one of the questions that was open at that time and it’s still kind of unresolved in the plant science Community is whether once you have
Cavitation events you can refill the the the channels that have emptied by cavitation and as I said it’s still not resolved whether this happens in plants or not but we could show that by adding solutes to the the the water in our systems we could actually uh resolve
Cavitation and refill the the micr channels so here what I showing is is very similar to the systems I’ve shown before except that instead of being these circular cavities is these long channels but the system is exactly the same apart from this uh the the black
Spots you see here you can forget about them is just some um artifacts that are on the external surface of glass and they don’t matter at all in the in the Dynamics here and so again when we dry the systems we have negative pressures developing and we have cavitation and
You will see that each white line that is appearing is a cavitation event that propagates through the channel and then completely empties the channels and this is the same Dynamics as I’ve shown just before with the circular channels except with these long channels and what we did
Is um this the the liquid that embies the systems here is not pure water as before but it’s it contains a solute I think in this case it was Ura but it doesn’t really matter if it’s Ura or salt and we did it with salt as well and
The interesting thing where when you introduce solute in the system is that if you so this is driven at let’s say low relative humidity on the left side but once you have a solute you can actually induce condensation within the system by the osmotic effect of the
Solute and you can refill these these systems and I will show that when we raise the humidity again after cavitation we can completely refill these cavities and they complet Ely collaps and the system is full again in the end and at first it’s not obvious what
Drives the flow to go uh within the system to collapse these cavities at first but if you look there’s some hint at if you look at the outside surface of the samples here so these systems maybe I forgot to say it but they’re we see it from above it’s closed on the top
Surface and the only places where they’re exposed to the humidity is the lateral size here so here it’s open on all four siid I think but if you look closely at this side you see that I will play the video again you see that there are little droplets forming on the edge here
And these droplets are actually very important in uh refilling these systems and I will show uh exactly the mechanism just just now and so we could figure out that what happens here is um if you look at the SK a sketch of the system here is
Very simplified so the the big pore on the left here is actually one of these micr channels that I’ve shown before the little pore here schematizes the the POR the nanoporous layer and this is the external surface here and the thing is when you have a solute in the system you
Can impose a vapor pressure uh that is above the vapor pressure of the solution if you have pure water the equilibrium is at 100% relative humidity and you would have to impose a super saturated Vapor to induce condensation but here with a solute the the solute lowers the
Vapor pressure it’s one of these cative effect I’ve shown I’ve uh talked about in the introduction and due to this if you have a solute and sufficiently high reative humidity you can induce vapor condensation uh in the solution due to this osmotic driving force so this
Cative driving force and if you do that and you have a hydrophilic system the condensation will will tend to make the solution spread out of the system and actually flow out and once the solution flows out in forms either films or droplets on the external surface and due
To the equilibrium between the the this film of solution or the droplet solution and the humidity you will tend to dilute the solution because once again we are imposing a humidity above the vapor pressure of the solution and so it will drive condensation into the solution as
Long as the solution is not dilute enough and this dilution of the solution on the outside surface makes it less concentrated than the solution that’s still in the inside of the system and this drives a solute concentration gradient that drive an osmotic effect and if the pores are small enough this
Tic effect can be quite large and this is actually what drives the the feeling of this uh um of these micr channels after gravitation in this case and of course an interesting aspect when you see this is that you can think that what what happens if you have a um
The reverse effect so here we we impose a higher relative humidity on the outside but you can say oh let’s impose a low erative humidity and of course if you start with A system that has a miniscus here and a low rative Unity you will drive dying and cavitation but if
You already have a film of solution formed on the outside of the system what you will drive first is not citation but a concentration of that uh solute fil solution film and by concentrating the solution film here more than the inside you will drive an osmotic effect that is opposite
To before and the osmotic driving force here is from the inside to the outside so we tend to empty the system in in in instead of uh refilling it and we check that this happens by we just played around with a sample here with a sample
Where we just have one of these cavities that is still empty and by similar to before when we impose High rity it will shrink the bubble but when we put when we switch back and forth between high and low relative humidity we can um trigger reversible fing and emptying of
These cavities Jo by anosmic effect and I won’t go into details here but this is um what we measured and you see that we have a very uh steady uh very quickly we have a steady state driving flow uh osmotically driven flow in these systems that respond very
Quickly to the humidity changes we impose uh okay and so uh that’s interesting because we can show that we have first a very high control of the flow rate in the system of filling and emptying just by the humidity and also that we can refill systems completely by
Osmosis and the systems I shown before up to now where we added souls are systems where we don’t we we and re I mean we empty and refill cavities in the system that are microscopic but we don’t look at the drying of the nanopulse layer itself and what happens when this
Happens and of course to drive and to drrive these nanop systems we need to impose very low rative unities again because of the very small pore size and what we did to just evaluate what happens in these systems uh with the complete range of relative humidities but within the nanopores themselves is
Water aborption isotherms which is just a measure of the water content in the system uh at different creative humidities and we cycle the humidity between very low let’s say um uh 20 20 to 30% and to very high humidi close to 100% And this is well known that if you
Have a nanoporous medium and you cycle humidity above it when it’s at low rative humidity the medium is basically dry with potentially some absorbed molecules and the poor walls but not bulk liquid in it if you increase the relative humidity by going to the right here at some point you will get
Capillary condensation that I talked about before and that drives capillary inition in some cases and so here is because the medium is hydrophilic with small pores uh the condensation of liquid in the system is at um at relative humid is lower than 100% here the pores are about 10 nanometers and
The condensation is around 90% reative humidity and once in condensed you have a res versible Plateau here where it stays full no matter what the relative humidity is and if you go to the left and start dry the system by lowering the reative humidity there’s some hysterisis
But it it evaporates also at a humidity that is lower than 100% And here which is around 85% and so this is with when the systems contain no salt so it’s just a nanop medium that interacts with humidity and we looked at what happens when we introduce salt and things change a lot
First so the the curve with salt is this one and you see that first it it’s shifted to much lower rative humidities which is in itself not surprising because I said before when you introduce salt you lower the vapor pressure of the liquid itself so all the humidities are
Kind of shifted to the left but also you see new shapes that appear in the system you have condensation here again but then condensation is not complete and it’s it’s continues here until the sample is full there are some things here that I won’t talk about much but
Then when you dry the system instead of drawing very quickly like here in terms of the humidity so this system there’s a very sharp transition between wet and dry here there’s a very smooth transition so the systems empty very uh smoothly I would say until there’s a
Very sharp kink in the curve where the systems empties abruptly up to its dry State and and we can cycle and this is reversible and um the reason for all of these effects um is because of the solute of course and what happens here is that we
Start from A system that contains a solution uh and the solution fills the complete volume of the pores and here we reach a point where the curvature is too large and the system starts drying uh but of course when drying the system we concentrate the solute and
This reaches a new equilibrium where the system is happy in equilibrium with the outside humidity but just we concentrate when we lower the humidity we concentrate the system just more and more but the concentration cannot increase indefinitely and at some point there will be crystallization because the concentration is too high so the
Little Kink we see here is the abrupt crystallization of the salt within the system and the interesting thing here is that salt is known that when you take a bulk salt crystal and increase the relative humidity there’s a critical humidity at which it will start absorbing the water around and
Spontaneous ly dissolving from the water vapor in the in the atmosphere and for NAC that we used here is about 75% reative humidity and here we see that the uptake of water in the system is much lower than 75% relative humidity which will be around here and meaning that there
There’s something that is Shifting the delance point in the system and of course it’s confinement and so the message here is uh that by introducing solutes you change a lot the the absorption properties of water in these nanoporous layers and you also see the phase transitions of the solute that are
Shifted compared to the bulk and we see clearly crystallization here and the liquisense here and the interesting aspect is so I won’t go into details in this curse here but we did Cycles with a lot of different concentrations of different sorry concentrations of the solution and what’s obvious is that no matter what’s
The amount of salt in the system the total amount of salt or the initial concentration you will use to fill the system the crystallization point in terms of humidity is always at the same spot and the delence point is also always at the same spot which means that
No matter the concentration we use we can define a confined crystallization point and a confined delence point and we did a systematic study of where these points sits depending on the characteristics of the poest medium and especially the poor size and so that’s what we did here and we looked at how
The delance point in terms of humidity and the crystallization point in terms of humidity depend on the pore size and we use different types of por silicon here um that’s actually the same por silicon as I’ve shown for the previous experiments for the osmotic Dynamics and cavitation Dynamics we have some other
Types of por silicon here and we have some um porous uh oxide alumina oxide here AO and so what we see here is that when we so that’s bulk delance here 75% as I mentioned before uh and you see that when you reach four sizes that are let’s
Say above a few tens of nanometers you get very close to bur delance but once you start going below 20 to 10 nanometers the delance points is is very very Shifty compared to to the B uh and there’s the same behavior for crystallization uh and again the reason
For this is solely Kelvin equation and we could fit this curve here through the data points with a modified version of Kelvin equation but without any adjustable parameter uh the only modifications we have to put in kelvin equation is an activity term that takes into account the osmotic effect of the
Solute or the cative chemical potential effect that the solute has on the liquid and also the fact that the surface tension of the liquid changes because of the solute and just with this equation we can fit this data without any adjustable parameter and the same happens for crystallization except that here there’s an
Additional fitting par there’s one fitting parameter which is the super saturation at which uh the the solute crystallizes in the pore and the super saturation um is I me for for for NAC Solutions the maximum super saturations that have been measured by levitating water droplets very um carefully and drying them slowly
Is around 2.2 and we reach bul super saturations I mean Super saturations in our system that are very uh um similar to the max maximum super saturations that we can measure uh very carefully for NAC in in in control systems so the thing is uh the interesting thing I would say
Is Again by just applying Kelvin equations to these systems we can very successfully describe what what’s happening in terms of the phase change of salt in these systems and this can have interesting consequences for uh the the delance and crystallization damages that we can observe for real materials for example
And I’m going to finish this presentation by just a few examples of what we are doing uh now with similar systems and actually we notice that if we just look at our systems with a camera when the system is around here in terms of the Cycles just before
Crystallization we can actually see the crystallization happening in the in the layer and this is a video that we we took of crystallization appearing in nanopore silicon just by looking with a camera and for and with some contrast enhancement and so you see that you start to get these nucleation spots in
Several parts in the system and these nucleation spots grow then in a circular fashion and then merge so don’t don’t really pay attention to this dark Center dark um Circle in the center it’s an ARA due to some other experiments that I’m going to show you a bit later but so
Right now we’re looking at these nucleation and growth Dynamics and trying to extract the nucleation rate and the growth rate Dynamics and look at how confinement so all these Crystal that we see are basically growing in this nanoporous sponge and we are trying to see whether there there’s any
Interesting Dynamics in the growth Dynamics and in the nucleation pattern that we see here so that’s one of the directions we have right now another one which is actually the reason why there’s this dark spot in the center is that we also look at what happens when we uh we
Have drops impacting or just deposited on these nanop layers drop of Salt Solutions and how they embibe and dry depending on the humidity and and stuff like that and so here is the movie where you see a a droplet of NAC solution deposited in one of these nonporous
Layers and you see a lot of things appearing But first you have a circular inhibition around the droplet and due to the fact that it’s also drying at the same time due to evaporation you have crystallization and we are trying to look at the patterns we get Of crystallization both outside and inside
Of the system depending on a lot of parameters and the last thing I wanted to show you is uh once we have this depos this salt deposit basically on top of the system on top of the monop layers we also look at what the effect of humidity Cycles have on these systems
And for example here is we start from these deposit and we increase the humidity and then we lower it again and you see that there’s lots of things appearing there’s spontaneous delence of the salt on the outside also delance inside and then they form these little droplets and the droplets grow
And merge and there’s lots of of funny Dynamics here and and when we dry the system you see all the droplet disappearing and another deposit that is formed that is very different from the initial deposit so cycling humidity in these systems that have salt and to or short droplets um raises some
Interesting questions of how the the deposit will evolve as a function of the Cycles okay so that’s uh I’m reaching the end of my presentation here so what I’ve shown is different uh experiments but the common theme is uh the behavior and dynamics of water and Solutions in
Nanopores and what I’ve shown is a lot of his Dynamics and thermodynamics has to do with the Kelvin equation and the Kelvin lapas equation entails capillary flows like inition or evaporation like have shown it can also Drive phase transitions I’ve shown examples of cavitation crystallization and the liqu
And also uh it can couple to other effects like osmosis or poity to generate Dynamics and patterns uh so finally I would like to acknowledge the different uh people have worked with me uh on these tees both at Cornell University and here in Leon uh
And I know that very soon there will be a postdoc position available and open uh for working on these them so if you are interested please contact me and this is the funding we have right now for developing these themes so thank you for your attention and be happy to take any questions