Water Freezes Differently on Positively and Negatively Charged Surfaces of Pyroelectric Materials
Entry by Sandeep Koshy, AP 225, Fall 2010
Title: Water Freezes Differently on Positively and Negatively Charged Surfaces of Pyroelectric Materials
Authors: David Ehre, Etay Lavert, Meir Lahav and Igor Lubomirsky
The authors use pyroelectric materials, which have induced surface charges when subjected to a temperature change, to isolate the effect of electric field on the freezing of water. They find that charged surfaces increase the freezing temperature of water relative to uncharged surfaces. More closely, they observe that positively charged surfaces allow for freezing at higher temperatures than negatively charged surfaces. They show using X-ray diffraction, that freezing begins at the surface on positively charged surfaces and at the air liquid interface on negatively charged surfaces. This paper is important for understanding the behavior of water, a substance with importance that does not need to be explained.
Soft Matter Keywords: phase change, charged surface, crystal structure, nucleation, electric fields
The ability to control water freezing temperature is important in many processes including from animal survival in cold climates to cloud seeding. Electric fields have been thought to enhance freezing of super cooled water (SCW) for over a century. Many techniques have suggested clustering of water at charged surfaces differs from that in the bulk.
The use of charged metallic surfaces confounds the effect of the electric field since SCW will freeze at non-charged metallic surfaces as well due to mirror charge effect. This problem can be overcome by using pyroelectric materials, which are insulators that can have their surface charge changed with cooling and heating. Specifically, pyroelectric plates cut perpendicular to their polar axis transiently develop opposite charges on its two ends. This effect can be sustained by varying the temperature of the plate. The current study aimed to study the effect of electric field on SCW freezing using such a material.
A pyroelectric plate was used to study the effect of the electric field on water freezing temperature. Some general concepts are explained in Fig. 1.
When a surface charge is first induced, the electric field is confined to the interior of the crystal (Fig 1. A). Over time, equilibrium is reached as external depolarization occurs, with the electric field still confined to the crystal (Fig 1. B). When placed in a conducting cylinder, the charge at the bottom of the crystal redistributes quickly while that at the top takes a much longer time to dissipate, creating an electric field (Fig. 1 C). This electric field dissipates over a very short distance (~0.8 um), but is enough to cause supramolecular clustering of water at the surface. Fig. 1 D defines the directions within the crystal with Z+ being the direction of the positive induced charge and Z- being the direction of the negative induced charge.
The experimenter used surfaces oriented with either Z+ or Z- on the top along with , two different surface coatings at the bottom: chromium (allows a field) or aluminum oxide (does not allow a field). Chromium allowed good conduction with the outer cylinder allowing an electric field to form. Aluminum oxide served as an insulator and served to retard field formation. Thus the four conditions tested were: Z+field, Z+nofield, Z-field, Z-nofield.
The temperature was lowered by 2 °C/min from 24 °C in a humidified chamber until droplets formed and subsequently froze in order to determine the freezing temperature on all surfaces.
Freezing temperatures on various surfaces
When the “no field” conditions were used (Z+nofield and Z-nofield), a freezing temperature of –12.5° ± 3°C was seen. When a positive field was used (Z+field) the freezing temperature was –7° ± 1°C. A negative field (Z-field) gave a freezing temperature of –18° ± 1°C with freezing occurring at the air-water interface where the electric field effects are low.
On Z-field conditions, water could be held at -11 °C without freezing even after allowing the field to dissipate. Subsequent application of a positive field (requiring heating to -8 °C) caused heating induced freezing. Overall a negatively charged surface was found to retard freezing while a positively charged surface promoted it.
Powder diffraction studies of ice crystals
X-ray diffraction (XRD) was used to look at the structure of the ice crystals formed (Fig 2). The “no field” conditions showed nearly identical crystal structures (Fig 2). The conditions with positive and negative surface charge had very different XRD patterns. The effect of inclination angle on the “no field” condition was observed using rocking curves of the (0 0 2) peak. A scan of the Z+field conition at 7°showed no difference compared to at 0°. A rocking curve at (1 0 0) showed the presence of a small population of crystals oriented perfectly perpendicular to the surface on Z+field surfaces. This indicates nucleation at the surface, potentially by rearrangement of water at the surface to form cubic ice. The Z-field condition showed a peak at (0 0 2) which showed two broad maxima on a rocking curve, indicating formation on a curved surface, thought to be the air water interface.
These results suggest that on a positive surface, the oxygen end of the water molecule orients towards the surface whereas on a negatively charged surfaces, the hydrogen atoms will face the surface. The differences in the electron behavior of these two ends of the molecule will result in unique molecular assembly under these two conditions which results in different freezing behavior on substrates of varying charge.