Experiment

The experiment was carried out inside a single-door cold room of dimensions 1.52m, 1.52m, and 1.83m as length, breadth, and height, respectively (Fig. 1). The body of the room is made of galvanized iron sheet and is insulated by PUF. The temperature of the room can be reduced up to — 35° C. An air-conditioner is operated outside the room so that the temperature difference between outside and inside the cold room can be reduced. One thermistor is used to note the temperature inside the cold room and another is used to note that of the outside. Both temperatures have digital display. Inside the room, a spherical glass vessel of diameter 35.6 x 10~3 m and 20 x 10~3 m3 capacity is placed, and it has several outlets on its body.

A cloud of supercooled droplets was produced by cooling the vessel and passing steam through a hole near the floor of the vessel. A closed container with water was used to produce steam and it entered the vessel through a pipeline. The amount of water vapor inside the vessel was controlled by adjusting the steam flow. The seeding material was injected inside the vessel through a port. Two thermistors were used to measure the temperature inside the vessel. Of the two, one was kept near the floor of the vessel and the other, kept just above the seeding port. The seeding temperature being mentioned in the literature was that noted in the second thermistor, as nucleation mostly occurs in its neighborhood. Two thermistors were used to find the temperature gradient inside the vessel. All the thermistors were

Fig. 1. Schematic arrangement of experimental apparatus inside the cold room.

Schematic Cold Rooms

Fig. 1. Schematic arrangement of experimental apparatus inside the cold room.

calibrated at ice and steam points, and the calibration error was within 0.1°C. The temperature difference between the two sensors within the vessel remained within 0.1°C, when a steady condition was reached.

One circulator was fixed at the roof of the cold room to keep the temperature uniform inside the room. A light was also kept inside the room for working purposes.

At first, the temperature of the room was brought to a pre-decided value of low temperature, and then steam was passed into the vessel until a steady temperature was reached. This steady condition can be controlled in two ways, i.e. by fixing the temperature of the vessel when steam was passed and controlling the flow of steam. The steady temperature can be retained for about 5min. The seeding was done at that steady temperature. The entire experiment was performed within this steady condition. One should note that, as steam was passed, the temperature of the room remained almost constant, but that of the vessel underwent quick change. In all the experiments the total water content of the vessel was kept fixed within a narrow range of 130—150 x 10~3 kg.

Commercially available DMSO being used here is 99.5% chemically pure. As the freezing point of DMSO is 17-18°C, it remains in liquid state at the normal temperature of the laboratory. A volume of 20 x 10~6 m3 of the liquid, brought quickly from room temperature, was sprayed inside the vessel in one stroke. The syringe being used here had a nozzle and admitted only droplets having diameter with a peak of 0.7¡xm and standard deviation of 0.21 yU,m. It is expected that DMSO will form solid particulate inside the chilled vessel.

To study the optical property, the laser beam was passed through a port located in the middle of the vessel. Five other ports are located in the same horizontal section of the vessel so that the directly scattered beam as well as those scattered at angles 30°, 36°, 144°, and 150° with respect to the forward direction can be received. Five photodiodes are placed behind the five ports to receive the scattered light. A current-regulated circuit then amplifies the signal, and the amplified voltage output then goes to a data logger. Finally, all the data registered in the data logger are transferred to a PC for archiving. Immediately after the seeding the scattered intensities are found to change with time. The scattered intensity is automatically noted and archived at an interval of 1s. When the voltage output returns to the base value, one can assume that the medium has become clear of ice cloud.

Crystals formed within the ice cloud finally settle down at the base of the vessel. To collect the falling-crystals formvar-coated glass slides were used. Formvar is a brand name of polyvinyl formal resin. A solution was prepared with 0.4 x 10-3 kg of formvar in 10 x 10-6 m3 of chloroform. A thin layer of this solution was coated in two slides at a time, and these were kept in a tray near the floor of the vessel to collect the signature of the crystals falling there. The lid over the tray can be opened or closed using external control. Crystals were collected for 2 min only after seeding was done. When the slides were brought outside after the experiment, the chloroform quickly evaporated leaving behind a plastic film of formvar. On the other hand, the ice part of crystals melted to water. Details about these formvar-coated slides are already available in the literature.25'26

The slides were then kept inside a desiccator with sufficient silica gel, so that water would evaporate and be quickly absorbed by the gel. A replica of the arrested crystals is then retained in the slide, and in the case of heterogeneous nucleation the nucleating agent is left behind at the core. One can observe the replicas through a microscope.

The concentration, size, and basic habits of the crystals in a slide were noted at different temperatures. The microscope being used here has a magnification of 400. On each slide, 10 randomly chosen fields of view are taken for counting.

To study the nucleation ability of DMSO-coated Ammonium Sulfate dust, 5g of Ammonium Sulfate dust was coated with 0.7 g of DMSO in liquid state. Ammonium Sulfate dust had dimension in the range of 0.10.7 yU,m. In fact, the above-mentioned amount of DMSO was just sufficient to coat fully the dust particulates of Ammonium sulfate. An amount of 20 x 10-6 kg of coated Ammonium Sulfate was sprayed inside the vessel in one stroke, keeping consistency with the first experiment. All the conditions were kept identical like the experiment with DMSO only. Like the first experiment, the concentration, size, and basic habits of the crystals formed in a slide were noted at different temperatures.

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