Cracking in films of titanium dioxide nanoparticles with varying interaction strength Alastair G. Mailer ⇑ , Paul S. Clegg School of Physics and Astronomy, The University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, UK article info Article history: Received 8 September 2013 Accepted 14 November 2013 Available online 1 December 2013 Keywords: Titania film formation Crack pattern Gelation abstract We present an experimental investigation into how altering the inter-particle potential between colloids affects the cracking of a ca. 100 lm film deposited on a hard substrate. The colloidal material used was titanium dioxide ðr  30 nmÞ in an aqueous solvent with interaction strength being adjusted through alteration of the pH away from the isoelectric point. Stable suspensions were observed to form as surface charging increased. Drop casting was used to demonstrate the flow properties of the suspensions; doctor- bladed strips of suspension were subsequently imaged to capture the drying dynamics. Alteration of the pH between 3 and 4 resulted in significant changes in cracking patterns, with the normalised crack domain area a ¼ ffiffiffi A p =h increasing for unstable suspensions (pH 4) and the appearance of order in the ini- tial crack pattern for stable suspensions (pH 3). These results can be unified in terms of the barrier to nucleate a new crack. Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction Colloidal films, with thicknesses in the range of 10–500 lm are of technological interest in the areas of energy materials, sensors and catalysis [1,2]. As with larger ceramic bodies, the manufacture of these materials is complicated by various failure modes activated by strong capillary forces induced in the material during drying. Optimizing colloid–colloid and colloid–substrate interac- tions provides a means to influence film behaviour. The fundamen- tal thermodynamic basis of fracture in solid materials was outlined by Griffith [3] in 1920, see also Lawn [4]. Subsequent develop- ments have been reviewed by, for example, Brinker and Scherer [5] and Lewis [6]. Recent work on the drying of latex suspensions [7–9] and aque- ous suspensions of inorganic oxides such as alumina and silica [6,10,11] has shown that the drying process depends on the stress/strain response of the suspended particles. Additionally, the effect of suspension/substrate interactions was established by Chiu et al. [12] by drying suspensions on a bed of mercury, and re- cently revisited by Smith and Sharp to investigate the effect of an elastic substrate [13]. The effect of substrate hydrophobicity and surface chemistry on adhesion and cracking have been investigated by Shorlin et al. [14] and very recently by Jing and Ma [15]. In addi- tion, the importance of capillary forces in the later stages of drying to substrate–film adhesion and delamination has been highlighted by Pauchard [16]. Also pertinent here, the effect of colloidal cluster phases on cracking has recently been presented by Ma and Jing [17]. A previous study on the drying and cracking of charge-stabilised colloidal films focused on modifying the range of the interaction using salt screening effects [10]. The inclusion of salt in this study complicates the interpretation of the final stages of drying (includ- ing cracking phenomena) due to salt precipitation and bridging. A later study looked at the effect of suspension stability for alumina particles (through the modification of pH) and binder addition on the critical cracking thickness [18], but did not note a change to parallel crack formation when moving from a flocculated to a dis- persed casting suspension. Our study looks at the effect of chang- ing the electrostatic repulsion between particles on the cracking process and the resulting crack pattern. The interaction strength is varied by modification of the charge state of the particle surface through alteration of the solvent pH. Titanium dioxide deposited by both drop and tape casting was the material of choice due to its technological importance in soft energy systems [19]. Images and thickness data were gathered during and after drying to eluci- date the effect of electrostatic stabilisation on the flow properties and the drying process. We find that the crack dynamics are differ- ent with more stable suspensions exhibiting more ordered initial crack patterns and smaller normalized domain areas. 2. Methods and materials The colloidal particles used were 99% pure anatase titanium dioxide (Nanostructured and Amorphous Materials, Inc.; stock code 5425HT) with an average particle size of 10 nm. Dilute 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.11.039 ⇑ Corresponding author. E-mail address: a.mailer@physics.org (A.G. Mailer). Journal of Colloid and Interface Science 417 (2014) 317–324 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis