Controlling the contact angle of biological sessile drops for study of their desiccated cracking patterns

Current exploration of cracking patterns of desiccated biological sessile drops as a new approach of scientific research is progressing rapidly. It has been proposed that biological fluids are naturally capable of storing information. Cracking patterns of desiccated biological sessile drops have the potential to provide a facile means to study the links between compositions of biofluids, their structures and their functions. This potential is, however, limited by our current inability to control the influences of non-pathological factors on cracking patterns. Among the non-pathological factors, the initial sessile drop contact angle has a strong influence on cracking patterns through affecting the material transport and stress distributions within the drop. In this work, we developed a method to control the initial drop contact angle on a glass surface to enable the investigation of the contact angle-induced pattern changes in a biological sessile drop. Human blood was selected as the biofluid in this study, because of its richness in cracking patterns. It has been found that the increase in the initial contact angle enlarges the orthoradial cracks close to the drop edge and compresses the width of the peripheral region. We have also concluded that the number of cracks in the central region of the desiccated pattern can be correlated with the drop contact angle. This work also provides a novel protocol for fabricating standardized substrates for studies of desiccation patterns of biological and other complex colloidal fluids.