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    Wednesday, 19 February 2014 10:23

    Tech Tip: Handling Small Crystals (Analysis and Solutions)

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    Handling and mounting tiny crystals is challenging

    Small crystals dehydrate quickly and are difficult to mount

    Small crystals (<100 μm) seem to swim away from a mounting tool. If you do manage to mount them, they end up on the neck instead of in the aperture, and by the time you’re ready to flash cool, the crystal and/or your drop have dried out.



    Analysing the problem

    Stoke’s law and laminar flow

    Mounts seem to push small crystals away, as if the mount were too hydrophobic

    What you see is a simple demonstration of laminar flow and Stoke’s law. When you move the mount through a crystal-containing drop, the liquid flows laminarly around it. If the crystal’s density matched that of the liquid, it, too, would just flow around the mount, and it would be nearly impossible to snag it.

    Gravitational pull and viscous drag

    Fd = 6π μ R v

    If the crystal’s density is larger than that of the liquid, the crystal will sediment under the influence of gravity toward the mount below it. The crystal’s sedimentation speed is determined by the balance between the gravitational force pulling down and the viscous drag force given by Stoke’s law that opposes motion.
    It’s an easy matter to show that this sedimentation speed varies as the square of the crystal diameter. Thus, the time you have to wait for a crystal to sediment onto the mount increases rapidly as the crystal gets smaller. A 10 μm crystal sediments at less than 10 μm/s, 100 times more slowly than a 100 μm crystal.
    In the videos below, the white teflon balls falling through glycerol differ in diameter by only a factor of two. How do their terminal speeds compare?

    The video to the left, in which we try to pick up a small white teflon ball in glycerol using a plastic spoon, illustrates the problem.

    Small teflon ball

    Medium teflon ball

     

    Dehydration

    Dehydration is a serious risk when working with very small crystals. Crystals smaller than 50 μm can dry out in seconds, degrading crystal diffraction. If you see diffraction spots that look like they come from salt, you know you’ve got a problem.


     

    Solutions

    Choose the right tool for the job

    With enough time and a steady hand, any mount style will do when dealing with small crystals. But to simplify and expidite your research process, Gentaur provides the best solution for handling small crystals with the Small Crystal Harvesting Kit. Reduce the problems associated with small crystals. Spend less time chasing them and eliminate dehydration issues. Included in this kit are:

    MicroMounts™ with a special wicking aperture that reduces laminar flow problems
    Low Viscosity CryoOil™ to store crystals and prevent dehydration prior to mounting
    MicroMesh™ to easily mount very tiny (<30 μm) crystals
    Reusable goniometer bases designed to decrease costs and increase throughput
    • Goniometer base holders for easy storage and cleaning
    Heavy–tweezers; ideal for handling mounts with reusable bases

    Small Crystal Harvesting Kit 

    Move slowly and carefully

    The presence of the mount’s aperture makes things a bit easier. As you move the mount through the liquid, liquid flows through the aperture. The crystal can flow along with this liquid and be sieved out. However, as the aperture gets smaller, the flow speed through it gets much smaller. If you move the mount too quickly, the liquid won’t have time to flow through the aperture. Your crystal may just flow around the aperture and end up on the neck of the mount.

    The key to mounting very tiny crystals, then, is to move the mount very slowly toward the crystal, allowing enough time for the crystal to sediment through the liquid toward the mount, and for liquid to flow through the aperture.
    This is illustrated in the video to the left, in which we retrieve a teflon ball in glycerol using fork.

    Dehydration

    To minimize dehydration, we recommend the following:
    1. Using Gentaur’s MicroMounts™, MicroLoops™, or MicroMeshes™ to quickly and easily mount your samples, minimizing the time for dehydration to set in
    2. Transferring your crystal to a drop of Low Viscosity CryoOil. Oils block evaporation during and after mounting
    3. Working in a humidified environment or in a cold room.
    o Evaporation rates are proportional to the quantity Δr.h. = 100% – r.h. so that increasing humidity from a typical laboratory value of 50% to 90% can reduce the evaporation rate by a factor of 5.
    o Evaporation rates plummet with decreasing temperature. The saturated vapor pressure of water at 4°C is 1/4 that at 25°c.



    Good luck!

    We hope this Tech Tip has given you further understanding about the complicated processes of crystallography. Please don’t hesitate to provide us with comments or suggestions at This email address is being protected from spambots. You need JavaScript enabled to view it.

     

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