Wednesday, November 8, 2017

Crystalline gels

What happened this summer, part 2/n

We have a paper out in Nature Materials on how crystallisation can help make a gel.
Tsurusawa, H., Russo, J., Leocmach, M., & Tanaka, H. (2017). Formation of porous crystals via viscoelastic phase separation. Nature Materials, 16(10), 1022–1028.

This is the second result of my collaboration with the lab of Hajime Tanaka, where I did my PhD. You haven't heard about the first result? We are still writing it down. In academia chronological order is not always granted.

It all started with a crazy idea from the boss and an even crazier “I'll try that” from a fresh PhD student around April 2008. It took Hideyo almost three years, including a depression, to hammer this idea into a working experimental setup and one more year to perform the experiments before graduating. Then he joined a private company and left me with a mountain of data to analyse. I had followed the story from the beginning as we started our PhD at the same time. I had completed my PhD on colloidal glass a year before he finished his about colloidal gels. Different subjects but many common methods. Hideyo was a skilled experimentalist, I was more keen on developing analysis methods.

We were both working with large colloidal particles (plastic balls of 3 microns in diameter) dispersed in an oily solvent. This system is perfect for observing in 3D at the particle level whatever physical phenomenon. The particles are charged and repel each other at long range. In water, you usually have plenty of dissolved ions. These ions tend to screen electrostatic repulsion, which means that same charges do not repel each other if they are further than 10 nm, at most 100 nm in extremely pure and neutral water. However in our oily solvent there are very few dissolved ions and electrostatic charges still can feel each other 10 microns apart.

Engineering interactions between colloidal particles. Refractive index matching and hairs ensures hard repulsion at contact. Charges provide long range repulsion but can be screened by ions. Non adsorbing polymers induces short range depletion attraction.

For me it was a bane since I wanted colloidal glasses, that is concentrated suspensions with particles close to contact. So I added some strange salts able to dissolve into the oil, providing ions that would shorten the range of repulsion. At that time, it was known that if you added polymers to the mix, they induced a short ranged attraction between particles and even at low particle content you could obtain a network of particles.

A colloidal gel observed by confocal microscopy

Such a solid network intertwined with a liquid solvent is called a gel. Yogurt is a gel, as most of our foods, organs, and many cosmetics. How gels form is still quite mysterious. We know that particles that attract each other would prefer to be grouped all together, leaving a large swath of empty solvent aside. A bit like when salad dressing unmix with oil on one side and vinegar on the other. Except that a yoghurt does not expel all the whey otherwise nobody wants to eat it. Some phenomenon stops the separation and allows the stability of the network. Except that your skin cream that was nice and stable yesterday just collapsed in a separated mess this morning. Nobody knows why and when a gel will suddenly lose its stability.

To understand that, we have to know why the phase separation arrests in the first place. We have to know how a gel forms. And for this we want to observe gel formation with our large colloids to be able to follow what is going on in every detail. But nobody knew how to do that.

Sketch of the semi-permeable cell

That when Hajime Tanaka had his crazy idea. Let's mix particles and polymers together but without salt. We have a short range attraction that is hidden by the long range repulsion. The suspension is stable with particles that stay far away from each other. We put this suspension in a chamber suitable for microscopy where the bottom is a membrane filter. Pore size is chosen so that neither colloidal particles nor polymers can go through, but salt can. Now we put everything under the microscope, add a salty solution on the other side of the membrane, and see the suspension forming a gel.

What is especially great with this idea is that with large particles and thin enough sample chamber ions diffuse so fast across the chamber that switching off the repulsion is practically instantaneous from the point of view of the slowly diffusing particles.

It seems simple but it took a couple of years or engineering to get the method right. Thanks Hideyo for toiling on that. Thanks also to John who analyzed a subset of the data and found that there was something strange going on; something that was at odd with existing models. Thanks also for calling me back to participate in that exciting story, even if I originally had to analyse a different subset of the data.

The most common model for gel formation relies on glass transition. If you cool a liquid without crystallising it it will slow down so much that it hardly flows anymore. One could wait the age of the universe before observing any flow. So practically what was a liquid is now a solid, disordered like a liquid. This is the definition of a glass. You can also obtain a glass by compression rather than cooling. The phase separation between a particle poor phase and a particle rich phase is analog to a gas-liquid phase separation. The colloidal gas contains almost only solvent and the colloidal liquid is very concentrated in particles. What the model says is that the liquid is son concentrated, so dense, that it become a glass. To sum up, a special type of phase separation (a “spinodal decomposition”) creates a liquid network, but this liquid becomes a glass and so the network arrests and becomes solid.

Reconstruction of gel structure. Gas particles in orange, liquid in grey, crystal in purple. Left: strong attration, thin network. Right: weaker attraction, stress-driven rearrangement is possible and thus crystallisation.

In our experiments we observed a very different scenario. The network formed, but strands were so thin (one or two particles thick) that we had a hard time calling it a liquid or a glass phase. In some conditions (weaker attraction) some strands snapped and the network could coarsen a bit more, maybe 4-5 particles thick. Here occurred the unexpected: crystallisation.

Because there was enough space available in the liquid network, the particles in the network were able to rearrange locally to form crystal nuclei. Instead of a disordered glass we obtained a network full of ordered crystals.

Growth of the crystals beyond the original liquid network

At some point, a crystal reached the edge of the liquid network. And it did not stop there. Particles in the gas adsorbed on the crystal, allowing it to grow further than the envelope of the original network. But there are so few particles in the gas, they should be quickly spent? Not if other parts of the system sublimate or vaporize. What we found is that particles in the liquid network vaporized and then adsorb on crystals far away, making them grow.

Ice crystal grow at the expense of surrounding supercooled water droplets.

Interestingly, this sequence of events is very probably what is responsible for rain. Atmospheric clouds are made of water droplets. It is cold up there and the water is supercooled, liquid despite a temperature below freezing. Thermodynamics tells us that water vapor in contact with supercooled liquid water has to be at higher pressure than water vapor in contact with ice at the same temperature. Actually when one droplet eventually freezes we obtain an ice crystal in contact with vapor which pressure is too high because set by the surrounding supercooled water droplets. So the gas condenses on the crystal, and the crystal grows at the expense of surrounding droplets that need to evaporate to maintain the vapor pressure. The crystal sucks the water out of the droplets without touching them. At some point the crystal becomes large enough to fall out of the cloud. Tens of meters below the temperature is hot enough to melt the crystal into a rain drop. This is called the Bergeron process. Next time you open your umbrella, think about this complicated sequence of events involving three phases - crystal, liquid and vapor.

Of course, if the temperature is below freezing down to the floor we get snow. The delicate geometric shapes of snowflakes are made possible by their formation process. Vapor adsorption on crystals allows a much slower and more harmonious growth than direct liquid condensation. Crystalline planes are more ordered, cleanly arranged. That is what we observed in our crystal-gels.

So, to sum up, we have observed an other way to arrest a phase separation into a gel that involves crystallisation rather than glass transition. This process creates a crystalline network with a large area of neat, vapor-deposited crystalline planes. It screams “catalysis” to my ear, or any kind of application that needs a large area of crystalline materials. And we demonstrated that such porous materials could be made in one pot, with the system tumbling down all the different steps by itself: phase separation, stress-driven rearrangements, crystal nucleation, vapor deposition. I hope people designing “real materials” (not made with fancy colloids) will be able to make such crystal-gels with nice applications down the line.

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