Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay Research paper Crack formation in desiccating Laponite® films under AC field: Effect of varying frequency Sudeshna Sircar a , Sujata Tarafdar a , Tapati Dutta b,a,* a Condensed Matter Physics Research Centre, Physics Department, Jadavpur University, Kolkata 700032, India b Physics Department, St. Xavier's College, Kolkata 700016, India ARTICLE INFO Keywords: Desiccation cracks AC field Fatigue cracking ABSTRACT 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 t a , was an exponential function of the applied AC voltage and frequency. A system characteristic relaxation time, independent of applied voltage was found when t a 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. 1. Introduction Colloidal particles in aqueous solution usually develop a surface charge due to the dissociation of cations from particles into the solvent. The charged state of the particles is dependant on the pH of the solution (Tawari et al., 2001; Thompson and Butterworth, 1992) and can change with ageing (Bandyopadhyay et al., 2004; Mourchid and Levitz, 1998). In solution, an electric double layer of counterions develops around the particles. When subject to direct (DC) or alternating (AC) fields, the colloidal particles exhibit a wide range of phenomena which arise from particle polarization, motion of the ions in the electric double layer coupled with fluid flow, and forces resulting from gradients in the electric field. The recent experimental advances in the area of micro- fluids, nanoscience and micro-electromechanical systems have kindled an intense interest in understanding the motion and manipulation of colloidal particles by electric fields (Abe et al., 2004; Blaaderen et al., 2013; Mittal et al., 2008). The applications of such research is varied and finds an increasingly important role in biotechnology and na- noscience. Research on dielectrophoresis (DEP) is focussed on the sorting, trapping and manipulation of live cells (Cheng et al., 2007) and DNA (Tuukkanen et al., 2005). The frequency-dependent cell membrane polarizability can be used to separate an incoming cell suspension into streams of live and dead cells and cells of different genotype in flow-through devices. Self assembly of particles in a di- rected AC field typically combines DEP with dipolar chaining force resulting in particle chains, crystals, and micro- or nanowires (Gong et al., 2002; Richetti et al., 1984; Ristenpart et al., 2004) and the in- terfacing of particle assemblies with on-chip electronic circuits (Velev and Kaler, 1999). Cell-particle assemblies formed on a chip may be used as biosensors to electrically detect changes in cell impedance caused by toxins or changes in environment, artificial tissues for microsurgery, advanced vaccines and drugs, smart biomaterials or chemical sensors (Gupta et al., 2008). Anisotropic Janus particles in the presence of AC fields show frequency dependent self-assembly that can be used for the fabrication of materials with directional electrical and heat transfer and parallel waveguides (Gangwal et al., 2008). When colloidal Laponite® films desiccate in the presence of alter- nating fields, their crack patterns display a signature curving perpen- dicular to the direction of the applied field, quite unlike normal de- siccation cracks or desiccation cracks in the presence of DC field (Khatun et al., 2012; Khatun, 2013a; Sircar et al., 2016). The me- chanism of the curving cracks is complex and involves stress https://doi.org/10.1016/j.clay.2018.01.031 Received 8 November 2017; Received in revised form 24 January 2018; Accepted 24 January 2018 * Corresponding author. E-mail address: tapati_dutta@sxccal.edu (T. Dutta). Applied Clay Science 156 (2018) 69–76 Available online 03 February 2018 0169-1317/ © 2018 Elsevier B.V. All rights reserved. T