Publication1

Primary and secondary cracks often coexist in a dried colloidal deposit. While, primary cracks have been the focus of investigation in most of the theoretical and experimental studies, understanding the formation and time evolution of secondary cracks is scarce. In addition, secondary cracks at times are undesirable especially when used as a template in the nano-lithography. Therefore, a control over their formation and suppression is of high importance. We consider the drying of sessile drops containing colloidal ellipsoids on a solid substrate. The resulting coffee-ring-like deposit of ellipsoids is found to accompany primary cracks in the circumferential direction and secondary cracks in the radial direction. A systematic study is performed to understand the nucleation and propagation of cracks. Our experiments explicitly showed that the primary cracks always nucleate at the top surface of the deposit and then penetrate the interior of the deposit. However, in contrast, the secondary cracks always form at the bottom surface of the deposit, which is in contact with the solid substrate. The secondary cracks are found to form only when the deposit thickness exceeds a critical value. In addition, we discuss methods to completely suppress the formation of secondary cracks by (i) tuning the aspect ratio of the ellipsoidal particles, and (ii) incorporating a small fraction of spherical particles in the drying drop of ellipsoids prior to the evaporation experiments.