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Crystals Via Lasers

Crystallization is voodoo. I have not changed my opinion one bit since this 2015 post – if anything, I’m more convinced than ever. We chemists tend to talk about such fields with some pride, because we have to deal with them by our hard work and our wits, but when you get right down to it, it’s kind of an embarrassing situation. Not being able to predict melting points is a related embarrassment, and what this tells you is that there are things about the transition to an ordered solid phase (or out of it) that we do not understand very well at all. A lot of very tricky, counterbalancing, inter-related thermodynamics are in play as you move in or out of a liquid state, and we’re not up to handling all of them yet.

So what we rely on, until that far-off day when we can just calculate our way out of such difficulties, are empirical tricks and techniques. The number of these in the crystallization/solubility field are beyond counting (a sure voodootropic indicator if ever there was). And I just read about a new one, which seems to have now been put on a bit firmer theoretical framework.

It was discovered in 1996 that shining a small, bright laser beam into a concentrated solution can induce crystallization, and no one has been quite sure how that works. Or maybe it’s better to say that several attempts have been made to be sure about that, without general agreement. There have been many reports since then of this effect, even to the point of producing different polymorphs in a controlled fashion. That area has had some reproducibility problems, but the general laser-induced crystallization phenomenon does not seem to be in doubt.

Several proposals have been made for a mechanism, but this new paper has an interesting one: it appears that the intense laser flux promotes movement of those molecules with a higher refractive index into the beam. This “laser tweezer” effect concentrates a given species in one zone, then, and depletes it in the surrounding one. This works for liquid-liquid separations (as shown in this paper, using nitrobenzene and decane), which is an easier test system. But it should apply to all sorts of phase transitions, including crystallization:

Because the laser-tweezing potential depth scales with the refractive index of the new phase and because nearly all solids have a higher refractive index than their corresponding liquid or solution phase, this effect can in principle explain all known laser-induced crystal-nucleation results, with the exception of nucleation induced by pulsed-laser-induced vapour or plasma bubbles.

. . .This is a generic effect that does not only apply to poorly mixing liquids but to any mixture or solution. However, the ease with which the laser-tweezing potential can initiate phase separation is enhanced near a liquid–liquid demixing critical point or binodal line. Phase manipulation and nucleation can be induced with a straightforward low-power laser diode. This suggests that this effect can be used to control matter in a range of practical applications.

There will need to be some careful work done to control for things like local heating and viscosity (the present authors have thought about this as well), but overall, this looks like a potentially very interesting way to induce crystallization, to separate liquid phases, and more. Worth keeping an eye on!

22 comments on “Crystals Via Lasers”

  1. TWS says:

    Not at all familiar with this field but if this sort of thing works well with cheap CW diode lasers you could easily imagine such a system being bolted on to a flask to crystallize out your product.

    1. Derek Lowe says:

      One also wonders about nucleating protein crystallization. . .

      1. TWS says:

        Yes, I’d imagine it could be even more valuable in protein crystallization. If it is possible to take advantage of the intrinsic polarization of the laser radiation, you can easily imagine some very interesting applications, but I’m just not sure if that’s a dream that could ever really crystallize (sorry). Perhaps tricky crystallizations will require breaking out your laser pen in future?

      2. b says:

        My first thought as well. Perhaps an interesting observation – I’ve done a decent amount of Microscale Thermophoresis, which involves shining an infrared laser on a capillary of protein solution that on occasion is highly concentrated. You can experimentally monitor the way the protein solution moves towards or away from the heating effect induced by the infrared laser. Sometimes, you see the movement happening then just instantly stop and the signal dramatically changes shape. Never had an explanation for this, but maybe it is inducing some form of crystallization in the capillary.

  2. sgc says:

    Can it be a “simple” thermophoresis ?
    One particles are moving into the beam due to local heating, others out and you get separation and nucleation.

    1. Finlay Walton says:

      Heating has been ruled out as a mechanism – there is a small degree of heating in the focus (0.4°C at full power), but any heating would push the system further above the critical point into the mixed regime, negating the effect.

      1. sgcox says:

        I think that is even more than in Nanotemper Microscale Thermophoresis…
        Effect depends on the gradient, not absolute value and it is huge in a focused laser beam.
        I, like “b”, did see weird protein motions on that machine.

  3. John Wayne says:

    Lasers are good for everything

  4. Hap says:

    Is there any decomposition? Laser-induced crystallization would be good so long as you don’t generate and crystallize decomposition products.

  5. scotty says:

    Exposing a concentrated solution to a warp field works too

    1. Pennpenn says:

      Don’t you need ionising radiation to re-crystalise dilithium?

  6. Anon says:

    “The 34-year-old wept at a hearing as a federal court judge in Brooklyn, New York, handed down the prison term.”

    Guess who? 🙂

    1. chiz says:

      The world’s most successful prisoner?

    2. Pennpenn says:

      If it is who I think you’re talking about, it’s just a pity he isn’t in jail for for the reason most people want to see him pilloried. The court transcripts when they were trying to find an impartial jury is worth a chuckle, at least.

  7. ScientistSailor says:

    It works even better if the lasers are attached to sharks…

    1. Micha Elyi says:

      Only use land sharks. Else too much danger of forming an ice-9 precipitate.

      1. Lawrence says:

        If you do you will likely neve unfreeze IIRC

  8. exGlaxoid says:

    If you use a polarized filter can you get only the “R” or “S” crystals from the racemate? That would be really cool.

  9. Marcin says:

    So would it make sense to hypothesize that polarized light being chiral could initiate enantiospecific crystal formation from the primordial soup of early life?

    1. Matthew Feinstein says:

      A related thought– a laser beam could apply a torque to a molecule. This is already done with optical OAM (Orbital Angular Momentum) states and could help answer the embarassing question of where the entropy goes.

  10. Insecure dumb PI says:

    Check out my hardcore treatment of grad students, and they love me… i am one….bad….ass. no one can touch my 50 nature papers.

  11. appa says:

    Might be related to laser cooling. Every molecule approaching the laser beam with the right speed and angle gets a recoil from the photons and is therefore slowed down. All the other molecules don’t interact with the photons because of doppler shift effects.

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