258 HALLGREN AND HOSLER 
Environment saturated with respect 
to ice. 
Curve A. 
Environment dry 
Curve B. 
FORCE REQUIRED TO SEPARATE SPHERES(IN DYNES) 
-80 -25 -20 1h} -10 -5 ie} 
TEMPERATURE , °C. 
Fra. 1—Relationship of mean force required to 
separate the ice spheres to temperature 
Experimental apparatus—An overall view of 
the freezer, apparatus, and control panel is 
shown in Figure 2. The apparatus is mounted 
in a commercial freezer approximately 2 X 2 ft 
and 6 feet high. The temperature can be varied 
from room temperature to —26°C, with the 
temperature difference from the top to bottom 
of the freezer kept to approximately 1°C or less 
by a mixing fan located at the bottom of the 
freezer. 
A schematic of the basic components of the 
apparatus is shown in Figure 3. It is, basically, 
a variation of the techniques used by Kinzer and 
Cobb [1956, 1958] in their studies on small wa- 
ter drops. Region A represents an inner chamber 
with a 4-ft® volume in which an ice-crystal cloud 
is formed by means of a regulated flow of satu- 
rated vapor and subsequent seeding by dry ice. 
The concentration and form of ice crystals vary 
with temperature in the usual manner. The aver- 
age diameter of the ice crystals is between 8 and 
20 microns being in general larger at higher 
temperatures and smaller at lower temperatures. 
The cloud is slowly transported up to the mouth 
of the vertical tube (2-inch inside diameter) con- 
taining the sampling device by a negative pres- 
sure created by a pump. The crystals are then 
transported through the smaller tube (34-inch 
inside diameter plastic tube) in which the veloc- 
ity can be varied from 0 to 100 cm/sec by vary- 
ing the voltage applied to the motor of a blower 
which in turn varies the negative pressure cre- 
ated at the top of the tube by the Venturi ef- 
fect. A rack gear at B permits the insertion of an 
ice sphere into the center of the tube. The ice 
sphere is mounted on the end of a fine rabbit 
hair or on a spider silk and is observed through 
a microscope. Immediately below the observa- 
tional area is a sampler with which we measure 
the concentration of ice crystals passing through 
the test section. It permits a section of the tube 
to be shifted out of the flow, and subsequently 
the crystals contained in the volume precipitate 
onto a cover glass covered with a one-percent 
solution of Formvar and placed at the bottom 
of the shifted section of the tube. The height of 
the sampling section is two inches, and the cover 
glass is placed on a holder and then inserted into 
the center of the tube. Since the cover glass is 
located in the center of the tube, losses to the 
Fic. 2—Overall view of freezer, control panel, 
and apparatus 
