USE OF LOW TEMPERATURE SCANNING ELECTRON MICROSCOPY TO OBSERVE 
ICICLES, ICE FABRIC, RIME AND FROST

William P. Wergin1, Albert Rango2 and Eric F. Erbe1

1Nematology Laboratory and 2Hydrology Laboratory
Agricultural Research Service, USDA; Beltsville, MD 20705

Previous studies showed that low temperature scanning electron microscopy (SEM) can be used to 
record images of precipitated snow crystals, which collectively form structures that are commonly 
known as snowflakes1,2.  Information about the structure of snow crystals can be used to improve 
models that estimate the water equivalent of the winter snowpack3.  These models, which are based on 
satellite microwave data, have practical use in approximating the quantity of water that will be available 
for crop irrigation and hydroelectric power.  Our previous success of using low temperature SEM to 
image snow crystals has encouraged us to utilize this technique for other types of frozen aqueous 
specimens.

Icicles were collected at Beltsville, MD; snow crystals, ice fabric and frost were collected from sites at 
Bearden Mountain, WV.  Samples, which were obtained when air temperatures ranged from -12 C to -
2 C, were collected on flat copper plates (15mm x 27mm) that contained a thin layer of methyl 
cellulose solution that was precooled to the ambient outdoor temperature and then rapidly plunged into 
LN2.  After a few minutes, the plates were inserted diagonally into 20 cm segments of square brass 
channelling and lowered into a LN2 storage dewar where they remained until they were sputter coated 
for observation with low temperature SEM.

Small forming icicles, which were fractured and observed in cross section, revealed concentric rings of 
air bubbles that may be released and entrapped as successive layers of water freeze (Fig. 1).  The 
weight of a ski was used to compress newly fallen snow.  This process, which flattened the surface of 
the snowpack, caused fragmentation and compaction of crystals that could be readily characterized (Fig. 
2).  This type of surface is generally referred to as ice fabric.  Rime, which could be imaged on the 
surfaces of precipitating crystals, probably formed as the crystals descended through clouds containing 
supercooled water droplets that condensed on their surfaces (Fig. 3).  The frost consisted of needles that 
presumably formed from the condensation of water vapor (Fig. 4).  These results suggest that low 
temperature SEM is useful for imaging frozen aqueous specimens that could also include ice core 
samples and particulate pollutants that become incorporated into snow and ice.  In addition, this 
technique could be used to image such dynamic phenomenon as icing on the surface of aircraft wings.

1.  W.P. Wergin, E.F. Erbe, Proc. Royal Microsc. Soc 29(1994)138.
2.  W.P. Wergin et al., Scanning, 17(1995)41.
3.  A. Rango et al., IEEE Trans Geoscience and Remote Sensing, 27(1989)740.

Figure 1. Cross fracture of a portion of an icicle illustrating concentric rings of small air bubbles.
Figure 2. Surface of ice fabric resulting from pressure, fracture and compaction of snow crystals by a 
snow ski.
Figure 3. Rime resulting from the atmospheric collision of a snow crystal (hexagonal plate) with super 
cooled water droplets.
Figure 4. Needles of frost formed by condensation or sublimation of water vapor.