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  • How were the helium pinch-off pictures taken?
    • A Kodak DCS315 Professional digital still camera was
      attached to a long range microscope outside the cryostat
      and focused on the pinch-off location.
      From the camera mount, a 630nm laser beam is redirected
      out of the microscope aperture, where it passes under the
      pinch-off location, and enters a photodetector on the
      opposite side of the cryostat. A blue glass notch filter
      is used to attenuate the partial transmission of red
      laser light into the camera. The image of the drop is
      allowed to pass through this attenuating filter. 630nm
      red light is removed, giving the image a blue tint.
      The disruption of the laser beam by the falling drop
      triggers the xenon flashlamp.



      By installing a delayed pulse generator between the
      photodiode and the flashlamp, several photographs of the
      drop in various stages of pinch-off can be easily
      obtained, allowing the frames to be pieced together into
      a video clip.
  • How do QCMs (Quartz Crystal Microbalances) work?
    • Quartz is a material that exhibits piezoelectric behavior;
      that is, an applied voltage to the crystal causes a physical
      distortion of the material. Likewise, a physical distortion
      of the crystal produces a voltage.



      When an AC voltage is applied to a QCM such that its frequency
      matches the resonant frequency of the crystal, a standing wave
      is established in the crystal. When a mass is applied to the
      crystal, as would occur in film deposition, or allowing droplets
      of liquids to exist on its surface, the resonant frequency of
      the crystal drops. The drop in frequency is linearly
      proportional to the mass coupled to the crystal's oscillation.
      The employment of some clever mathematics will convert this
      drop in frequency into a known mass.
  • Why not put the camera inside the cryostat?
    • Outwardly, it seems like a good idea. Why fuss with all the
      expense of mounting windows and just put the electronics inside the
      cryostat itself?



      The reason we can't do this is because at the temperatures our
      experiments take place, which is about -456║ F, all electronics
      that involve semiconductors, such as CCD cameras, turn into
      electric insulators, rendering them useless.
  • Liquid helium boils at 4.2K. How do you achieve temperatures lower than this?
    • Helium-4 evaporative cooling: - Liquid helium can be cooled from its boiling
      point by pumping on the bath of liquid helium. Using this method alone, temperatures
      as low as 1K can be achieved. For anything less, other methods must be employed.
    • Helium-3 evaporative cooling- 3He is expensive (about $200 per STP liter of gas),
      and efforts to recycle the evaporated gas must be employed. Temperatures as low as
      about 400mK can be obtained with this method.
    • He3-He4 dilution refrigeration- about 100mK
    • Adiabatic spin demagnetization: paramagnetic salt- <10mK
    • Adiabatic nuclear demagnetization- <1ÁK
  • How did you tilt the cryostat in those sliding droplets videos?
    • We held the dewar aloft with a chain hoist while ratcheting a rope tied to the base of
      the cryostat and proceeded to hold our breath!