Transition from Dendritic to Cell-like Crystalline Structures in Drying Droplets of Fetal Bovine Serum under the Influence of Temperature.

The desiccation of biofluid droplets leads to the formation of complex deposits which are morphologically affected by the environmental conditions, such as temperature. In this work, we examine the effect of substrate temperatures between 20 and 40 °C on the desiccation deposits of fetal bovine serum (FBS) droplets. The final dried deposits consist of different zones: a peripheral protein ring, a zone of protein structures, a protein gel, and a central crystalline zone. We focus on the crystalline zone showing that its morphological and topographical characteristics vary with substrate temperature. The area of the crystalline zone is found to shrink with increasing substrate temperature. Additionally, the morphology of the crystalline structures changes from dendritic at 20 °C to cell-like for substrate temperatures between 25 and 40 °C. Calculation of the thermal and solutal Bénard-Marangoni numbers shows that while thermal effects are negligible when drying takes place at 20 °C, for higher substrate temperatures (25-40 °C), both thermal and solutal convective effects manifest within the drying drops. Thermal effects dominate earlier in the evaporation process leading, we believe, to the development of instabilities and, in turn, to the formation of convective cells in the drying drops. Solutal effects, on the other hand, are dominant toward the end of drying, maintaining circulation within the cells and leading to crystallization of salts in the formed cells. The cell-like structures are considered to form because of the interplay between thermal and solutal convection during drying. Dendritic growth is associated with a thicker fluid layer in the crystalline zone compared to cell-like growth with thinner layers. For cell-like structures, we show that the number of cells increases and the area occupied by each cell decreases with temperature. The average distance between cells decreases linearly with substrate temperature.

Flow transition within an evaporating binary mixture sessile drop.

The flow field along the base of an evaporating ethanol-water droplet and its evolution time was measured by particle image velocimetry. Three stages are revealed, a first stage dominated by multiple vortices, a second transition stage characterized by a remarkable spike in outward flow not previously identified, and a third stage dominated by outward flow identical to that found for pure water. Stage I is thought to be driven by surface tension gradients arising from local concentration variation. The spike in outward flow is explained in terms of a transition corresponding to almost total depletion of ethanol. An exponential decay in vorticity during the transition stage is explained in terms of ethanol diffusion from the bulk to the interface. We speculate on the existence of a zero-concentration wave propagating from the apex to the contact line corresponding to the final total depletion of ethanol.

An in-vitro technique for assessment of thrombogenicity in mechanical prosthetic cardiac valves: evaluation with a range of valve types.

BACKGROUND AND AIM OF THE STUDY: An in-vitro technique has been developed to assess the flow-induced thrombosis of artificial heart valves, using renneted milk as a blood analog. Previous studies have demonstrated similarities between the clotting of blood and milk on both microscopic and macroscopic scales. The study aim was to further validate the milk test by comparing the locations of milk clot to those of thrombus formation on a wide selection of mechanical heart valves. METHODS: Nine different valves were tested in the aortic position of a model heart chamber in the Edinburgh milk rig. These included caged-ball valves (Starr-Edwards silastic ball with bare struts and metal ball with cloth-covered struts), tilting-disc valves (Björk-Shiley Standard, Björk-Shiley Monostrut, Medtronic-Hall, and Ultracor), and bileaflet valves (St. Jude Medical, Edwards-Duromedics, and CarboMedics). Renneted milk was pumped through the valves for 30 min at 2 l/min, 70 bpm pulsatile flow. After each run, valves were photographed for comparison with documented sites of thrombosis. RESULTS: All valves developed milk clot in specific, reproducible locations when run in the aortic position. Milk clot was found on the struts of caged-ball valves and tilting-disc valves, and around the hinge mechanism of the bileaflet valves. This compared favorably to documented cases of thrombosis in vivo. CONCLUSION: Renneted milk may be used to model flow-induced thrombus formation and to predict the thrombogenic sites of mechanical heart valves. Whilst it is not suggested that milk mimics the entire blood coagulation cascade, these results indicate that such behavior may not be necessary for predicting fluid mechanically induced clotting.

Evaluation of an in-vitro thrombosis assessment procedure by application to the Medtronic Parallel and St. Jude Medical valves.

BACKGROUND AND AIM OF THE STUDY: With the failure of animal trials to detect thrombosis in the Medtronic Parallel valve, interest has returned to finding a suitable in-vitro method for preclinical valve assessment. A technique has been developed that uses renneted milk to detect clotting around heart valves. The study aim was to determine whether the milk test could discern differences in clotting between the Parallel valve and the clinically successful St. Jude Medical (SJM) valve. METHODS: The Parallel valve and the SJM valve were tested in the aortic position of a rigid model heart chamber at 2 l/min, 70 bpm pulsatile flow. Initially, the valves were run for 30 min to obtain data for localized clotting. Subsequently, the valves were run for only 5 min to obtain data for the initial stages of clotting, including clot origin. RESULTS: Both valves clotted around their hinge regions in the 30-min tests, and this compared well with documented thrombogenic sites in vivo. The extent of clotting was similar, but clot adherence to the hinge pockets was greater on the Parallel valve than on the SJM valve. After the 5-min runs, no clot was visible on the SJM valve, but clot was consistently found in the hinge recesses of the Parallel valve, adjacent to the pivot. CONCLUSION: A renneted milk test can discern differences between the clotting potentials of different heart valves. The Parallel valve clotted earlier than the SJM valve, indicating that it was more thrombogenic. Early-stage clotting was seen to occur around the pivot in the hinge pockets. The milk test has potential for contributing to in-vitro preclinical assessment techniques.

Order-of-magnitude increase in flow velocity driven by mass conservation during the evaporation of sessile drops.

We report on a dramatic order-of-magnitude increase in flow velocity within pinned evaporating droplets toward the end of their lifetime. The measurements were performed using high-speed microparticle image velocimetry. The study revealed interesting observations about the spatial and temporal evolution of the velocity field. The profile along the radius of the droplet is found to exhibit a maximum toward the three phase contact line with flow oscillations in time in this region. Additional optical measurements allowed further analysis of the observed trends. Analysis of the potential mechanisms responsible for the flow within the droplet demonstrated that these observations can be satisfactorily explained and accounted for by mass conservation within the droplet to compensate for evaporation.