Journal of Colloid and Interface Science 313 (2007) 160–168 www.elsevier.com/locate/jcis Avoiding “mud” cracks during drying of thin films from aqueous colloidal suspensions E. Santanach Carreras 1 , F. Chabert, D.E. Dunstan, G.V. Franks ∗ Department of Chemical & Biomolecular Engineering, The University of Melbourne, Vic 3010, Australia Received 4 December 2006; accepted 26 March 2007 Available online 24 April 2007 Abstract The critical cracking thickness of films obtained by drying aqueous alumina suspensions has been investigated. The effects of solution chemistry, binder and binder crosslinking were studied. Films formed from flocculated and dispersed suspensions are compared. The influence of the addition of the polymeric binder, poly(vinyl alcohol) (PVA) was also investigated. In addition, in some of the dispersed suspensions the PVA was covalently crosslinked. The critical cracking thickness is found to be 3 times greater for the films obtained from dispersed suspensions than for the films obtained from flocculated suspensions. The superior mechanical properties are primarily due to the higher final solids concentration in the films obtained from dispersed suspensions. Addition of PVA leads to an increase of the critical cracking thickness by a factor of two for both dispersed and flocculated systems. When the PVA is crosslinked, the mechanical properties of the gel during drying are improved and the critical cracking thickness is increased 10 fold with respect to the suspensions with uncrosslinked PVA.  2007 Elsevier Inc. All rights reserved. Keywords: Thin film; Cracking; Drying; Suspension; Alumina; PVA; Crosslinking 1. Introduction When a coating of a colloidal suspension is applied to a non-porous rigid substrate and allowed to dry, its volume re- duces due to the evaporation of the solvent. Constrained by the rigid substrate, this reduction in volume generates stresses in the drying material. If these stresses exceed the strength of the material, they will be released in the form of cracks. This prob- lem is of great importance in numerous industrial fields for in- stance, paints, paper coatings, fabrication of ceramic substrates for electronic applications, and drying concrete. The colloids, the suspending media, and the interactions among the different components of the suspension vary depending upon the applica- tion; nevertheless, the underlying physics of the drying process and the development of the stresses leading to these cracks re- main unchanged. * Corresponding author. E-mail address: gvfranks@unimelb.edu.au (G.V. Franks). 1 Current address: Department of Physics and Division of Applied Science, Harvard University, USA. Following is a brief summary of the three stages of the dry- ing process as described by many investigators [1–6]. Initially a constant drying rate stage occurs in which consolidation of the particles in the suspension and evaporation take place at the same constant rate. Indeed, a layer of liquid covers the film at first, the liquid in the pores does not play a role in the evapora- tion, and stress build-up is negligible. The air–liquid interface is macroscopically flat during the first stage of drying. The length of time of this stage is proportional to the initial volume fraction of solvent in the suspension [3]. Eventually the particle volume fraction in the drying film increases to the gelpoint (φ g ) and a three-dimensional particle network forms [6]. At this point in the drying, the particle network develops a non-zero compres- sive yield stress which increases as drying continues because the volume fraction of the suspension increases. The particle network is able to resist consolidation if the applied isostatic compressive consolidation pressure is less than the compres- sive yield stress. The liquid–air interface attempts to penetrate into the top of the powder bed. In order to do so, it must form a curved meniscus between neighboring particles. The curved menisci between particles create an isostatic consolidation pres- 0021-9797/$ – see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.03.076