Understanding of the role of dilution on evaporative deposition patterns of blood droplets over hydrophilic and hydrophobic substrates.

Blood is a complex colloidal suspension which carries myriads of information about human health. Understanding the evaporation dynamics and its consequent deposition patterns have direct relevance in disease detection. We report evaporation dynamics of whole and diluted blood droplets over hydrophilic (glass) and hydrophobic (PDMS, polydimethylsiloxane) substrates. Our experiments show that blood drops evaporating on a hydrophilic substrate exhibit radial and orthoradial cracks in the coronal region and random cracks in the central region. Using Griffith's energy criterion, we show that crack formation takes place when the capillary pressure and the resulting compressive stress inside the evaporating droplet exceeds critical stress which depends on the elastic modulus, interfacial energy, and the particle concentration of the system. The width of the coronal region (w), the film thickness (h) at the contact line, and the crack pitch (p) decrease with increasing blood dilution. In the dilution range of 2.0-0.8% HCT (hematocrit), the transition from the cracking to the non-cracking regime is observed, which can be attributed to inadequate compressive stress available even after the evaporation of the blood droplet is completed. For the hydrophobic substrate, buckling instead of cracking is observed for the whole blood droplets, which can be attributed to the distinct wetting and evaporation kinetics. The buckling of the blood drop on a hydrophobic surface is attributed to the competition between capillary pressure originated due to the formation of an elastic network of RBCs (red blood cells) and the menisci formed between adjacent RBCs, and the critical buckling pressure. With increasing blood dilution, a transition from buckling (between 21 and 42% HCT) to cracking (between 21 and 2.0% HCT) of the droplets, and eventually to the non-cracking regime (between 2.0 and 0.8% HCT) is observed. Our study unravels the interesting attributes about one of the important physico-chemical factors (i.e. % HCT) that affect the evaporation of blood droplets and the resulting deposition patterns on substrates with different hydrophobicity.