Crack formation in desiccating Laponite® films under AC field: Effect of varying frequency

An experimental study of the role of voltage and frequency on desiccation crack pattern of Laponite® films in a non-uniform Alternating Current (AC) field gradient is reported. The cracks always emerged from the stronger end of the field and proceeded parallel to the field direction. Depending on the value of the applied field voltage and frequency, the cracks then curved in a direction perpendicular to the applied field direction, before reaching the weaker end of the field. This is reminiscent of fatigue cracking. The curvature was a maximum at a particular frequency for every voltage. The time of appearance of the first crack ta, was an exponential function of the applied AC voltage and frequency. A system characteristic relaxation time, independent of applied voltage was found when ta was scaled appropriately. Scanning electron microscope images of the cracked Laponite® film exhibited different textures for different frequency and field strengths. The images indicated that there exists a frequency window for any applied voltage for which the Laponite® particles gather sufficient energy from the electric field to reorient themselves from a configuration in which the particle long axis is aligned along the electric field to a configuration with the particle axis perpendicular to the field direction. Cracks developed along particle boundaries following their changing orientation.