snotitle.jpg (19016 bytes)

grow.gif (10037 bytes) Home
Natural Snowflakes
  --Photo Gallery I
  --Photo Gallery II
  --Photo Gallery III
  --Guide to Snowflakes
  --Snowflake Books
  --Historic Snowflakes
  --Ice Crystal Halos
  --Snowflake Store
Designer Snowflakes
  --I: First Attempts
  --II: Better Snowflakes
  --III: Precision Snow
  --Snowflake Movies
  --Free-falling Snow
  --Designer's Page
Frost Crystals
  --Guide to Frost
  --Frost Photos
Snowflake Physics
  --Snowflake Primer
  --Snow Crystal FAQs
  --No Two Alike?
  --Crystal Faceting
  --Snowflake Branching
  --Electric Growth
  --Ice Properties
  --Myths and Nonsense
Snow Activities
  --Snowflake Watching
  --Photographing Snow
  --Make Your Own
  --Snowflake Fossils
  --Ice Spikes
  --Activities for Kids
Snowflake Touring
  --Snowflake Hot Spots
  --Northern Ontario
  --Hokkaido, Japan (2) (3)
  --Michigan U. P.
  --California Mountains
Copyright Issues
Electric Snow Crystal Growth
   ... Using electric fields to alter the growth of snow crystals ...

x1-s.jpg (898 bytes)   Electric fields can affect the growth of ice crystals in several interesting ways, and can be used to grow long thin needles of crystalline ice (see the Designer's Page).  The electric effects come about because electric fields influence the way water molecules diffuse through the air in the vicinity of the ice surface. If we charge up a growing crystal, then electrostatics dictates that strong electric fields will be set up around the crystal. The fields and field gradients will be particularly strong near any sharp points on the crystal. Since water molecules have an intrinsic electric polarizability, the electric fields tend to polarize the water molecules. If the field also has a strong gradient, then the polarized molecules are attracted in the direction of stronger fields.

    We analyzed dendrite growth in the presence of an applied voltage [1], for the special case where the growing crystal has no facets. This is simpler than the growth of real ice, but it seems to describe the phenomenon quite well. For a small applied voltage, the dendrite looks about the same as with no voltage, but the tip becomes sharper, and the tip growth velocity increases. This is because electrically enhanced diffusion brings more molecules to the high-voltage tip. The crystal surface tension stabilizes the growth in this case, just as it does for growth without an applied voltage. (For information on normal dendrites see Snowflake Branching.)

fig4a-s.jpg (3007 bytes)    Once the applied voltage reaches a certain critical value, however, the crystal growth becomes unstable, and the tip velocity increases dramatically.  The image at right shows an electric needle growing out of a normal ice dendrite.  A normal dendrite first was grown on a metal wire, and allowed to reach a steady state, where it grew with a constant tip velocity of about 3 microns/sec.  Then a voltage was applied to the wire, and voltage slowly increased with time.  As the voltage increased the dendrite tip velocity increased until at 1300 volts it was growing at about 4 microns/sec.  The dendrite continued to produce sidebranches as the voltage increased.  At 1400 volts an electric needle shot out from the dendrite tip at about 30 microns/sec and kept growing at that velocity.
   We believe the electric needle forms because surface tension can no longer stabilize the needle growth, which must be stabilized by some other mechanism.  We're not sure what this is, but the leading candidate is tip heating: molecules condense on the tip and heat it up, which prevents the tip from growing too rapidly.

side-s.jpg (1594 bytes)fig9-x.jpg (4901 bytes)   Electric fields also have other odd effects on ice crystal growth. For example, the dendrite at right was grown essentially like the one above.  However, when the applied voltage reached a threshold value, no electric needle formed.   Instead the tip split; in fact it split, and then the two front branches split again.  What happened is that the crystal axis rotated 30 degrees; at the first split, vertical in the image went from a [2 -1 -1 0] axis to a [1 0 -1 0] axis (see the Snowflake Primer).  We don't know why this rotation took place, but it happened at just the threshold voltage where normal growth became unstable.  This behavior remains a mystery.  The second image shows a side view taken just after the first image.

compx.jpg (8197 bytes)   The image at left is another composite view (enhanced to show detail) of a tip-splitting dendrite. This time as the voltage approached the threshold value, the sidebranches became evenly spaced (compared to the last image). Another mystery!

elec2x.jpg (2342 bytes)elec1x.jpg (2312 bytes)   The images at right show what happens when a high voltage, above the threshold value, is applied to an ice covered wire.   Sharp points on the ice crystals quickly develop ice needles, which grow out, occasionally splitting and changing direction as they grow.

caxisx.jpg (5816 bytes)aaxisx.jpg (5636 bytes)interx.jpg (4848 bytes)

   Removing the applied voltage allows one to see the crystalline axis of the electric needles, as can be seen in the above three images [2].   The first set was grown at a high supersaturation, which produced a lot of electric needles.  These needles were grown at -5 C, and most grew along the c-axis.   After the voltage was turned off, and the whole collection of needles was moved to a place in the growth chamber at -15 C.  At that point snow stars grew at the ends of all the needles.
   For the second image the wire was placed at -15 C before the high voltage was applied.  A collection of electric needles grew from the wire, each along an a-axis.   When they got long, the voltage was removed, and normal growth resumed at the needle tips, revealing the crystallographic orientation.
   The electric needles in the third image were grown at an intermediate temperature.  When the field was removed, and the needles moved to -15 C, their subsequent growth revealed that the needles each grew along a [1 0 -1 0] direction.
highv1x.jpg (4067 bytes)    Finally, we find that at very high voltages the electric needle growth itself becomes unstable, resulting in the erratic growth shown at right.
   We are currently mapping out the electric needle morphology as a function of temperature, supersaturation, and voltage, in order to understand this phenomenon better.

[1] "Electrically Induced Morphological Instabilities in Free Dendrite Growth," K. G. Libbrecht and V. M. Tanusheva, Phys. Rev. Lett. 81, 176 (1998).

[2] J. T. Bartlett, A. P. van den Heuval, B. J. Mason, Z. angue. Math. Phys. 14, 509 (1963).

Return to was created by Kenneth G. Libbrecht, Caltech
Comments?  Send an e-mail....
page views since February 1, 1999