Cracking in films of titanium dioxide nanoparticles with varying interaction strength

• Cracking in nanoparticulate films is affected by inter-particle interactions.
• For stable nanoparticles, cracks are initially ordered and follow the drying front.
• For unstable nanoparticles, arc-like cracks nucleate at defects.
• The normalized domain area doubles on changing from stable to unstable particles.
• This is understood as the effect of gelation on the activation of crack nucleation.

We present an experimental investigation into how altering the inter-particle potential between colloids affects the cracking of a ca. 100 μμm film deposited on a hard substrate. The colloidal material used was titanium dioxide (r≈30nm) 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 α=A/h increasing for unstable suspensions (pH 4) and the appearance of order in the initial crack pattern for stable suspensions (pH 3). These results can be unified in terms of the barrier to nucleate a new crack.

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