|dc.description.abstract||Early diagnosis and treatment are crucial issues in medicine. Disease leads to alterations in the properties of biological fluids, which in turn, lead to changes in the final desiccation patterns arising from droplet evaporation on surfaces. Hence, investigating the drying patterns of bio-droplets could provide the means for rapid medical diagnosis. The aim of this thesis is to probe the effect of various parameters on the evaporative behaviour and final desiccation patterns of micro-litre bio-fluid droplets.
Initially, the evaporation of aqueous saline drops is studied, as ions are present in human serum and urine. The study reveals the development of a crystallisation-driven flow at the final drying stages, manifested by strong jets of flow towards the growing crystals and accompanied by the existence of vortices in each side of the growing crystals. This flow is attributed to the interplay between continuity and Marangoni convection.
The effect of substrate temperature on the evaporation and dried patterns of Foetal Bovine Serum (FBS) drops is then investigated and found to affect the number of cracks and the mechanism of crack formation, as well as the crystallisation patterns in desiccation deposits. Mechanisms affecting crack formation are examined, indicating that for high substrate temperatures protein denaturation must be taken into account. Additionally, the morphology of crystallisation patterns is probably related to thin film phenomena occurring at the final stages of drying.
The addition of salts (NaCl or CaCl2) to FBS drops is probed, leading to various morphological features that depend on the salt type and initial ionic strength. NaCl is found to cause faster dehydration compared to CaCl2. Interestingly, the deposits occurring from evaporation of FBS drops, that consist of high CaCl2 concentrations, are sensitive to relative humidity (RH) changes and manifest the ability to deliquesce (upon increase of the RH) and re-crystallise (upon decrease of the RH). The addition
of NaCl to FBS-CaCl2 mixtures promotes droplet dehydration and exhibits distinct evaporative dynamics.
The effect of urea concentration on the dried patterns of aqueous saline drops is subsequently examined under polarised light, revealing that urea selectively deposits upon the already formed salt crystals towards the end of drying. The final crystalline patterns initially show up as isotropic structures, but birefringence manifests shortly after pattern formation, indicating the anisotropy caused by urea deposition.||en