There is a wide public perception that a cell phone that has been accidentally immersed in water can be dried out by placing it in a sealed container of rice for a day or two. This report describes an experiment to evaluate the hygroscopic properties of rice, specifically its ability to remove water from a simulated cell phone.
The evaluation demonstrates that rice does not work as an effective desiccant. In the experimental measurements, slightly more water was lost to evaporation simply by leaving the waterlogged device in an open room than by enclosing it in a container of rice.
The phone simulator was used in two measurements. First, a known weight of water (around 5ml = 5 grams) was inserted into the simulator and the device was accurately weighed. Then the simulator was placed inside a container, which held approximately one pound of rice, and the airtight top of the container was fitted. After 24 hours, the container was opened, the simulator removed and reweighed, this was repeated after approximately 50 hours.
The second experiment set up the simulator in the same way, but the simulator was then left in the open air in a room at approximately 21.5C, and periodically reweighed over a 48-hour period.
The weight measurements were carried out using an electronic scale with a resolution of 10 milligrams. This is about 3% of the smallest weight differences encountered, sufficient to give a high confidence in the resolution of the results.
When immersed, a real phone draws in water through the various openings and seams. The water is held in the interstices around the internal components and printed circuit boards, and is difficult to remove with passive methods. The only way that water is drawn from a wet phone at room temperature and pressure is by evaporation through convection, and this is limited by the small surface area of water exposed by the case seams and openings.
Other techniques for removing water include heat, forced air (with or without disassembly) and reduction of atmospheric pressure. Pressure reduction lowers the boiling point of water so it turns to vapor at room temperature.
Using a real cell phone for this experiment brings up a significant practical issue – a waterlogged phone tends to drip because of openings and seams in the case, which makes establishing the initial and subsequent weights of a wet phone problematic. For this reason, a phone simulator was devised for this experiment. The simulator is a plastic box with openings in the top. The openings total approximately 42 mm2, representative of the openings in several current phone designs that were examined.
Inside is a small folded piece of paper towel, which at the beginning of the experiment is wetted with an accurately known weight of water. The weight of the water was chosen to be about 10% of the volume of a smart phone, as it is estimated that the internal spaces of a real phone total around 10% of its total volume.
