The fluid dynamics of droplet impacts on inclined surfaces with application to forensic blood-spatter analysis
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Bloodstain pattern analysis is used in the investigation of a crime scene to infer the impact velocity and size of an impacting droplet and from these, the droplet’s point and cause of origin. The final pattern is the result of complex fluid dynamic processes involved in the impact and spreading of a blood drop on a surface with variable surface properties such as wettability and porosity. An experiment has been designed to study these processes and the resulting patterns for the case of a single Newtonian droplet impacting an inclined surface with variable roughness and wetting properties. An experimental apparatus, including a droplet generator, has been designed to produce droplets on-demand, and that impact an interchangeable surface. In addition, a blood-simulant liquid has been developed as a replacement for performing tests with real blood. With this apparatus and blood simulant, fluid dynamics concepts, such as contact line motion and wetting behavior are examined in the context of parameters of interest to the forensics community. These include eccentricity, spread factor and the number of spines formed on impact. The effect of varying dimensionless parameters including Reynolds number, Weber number and Laplace number, the angle of impact and surface properties is examined. Correlations are developed for predicting conditions at the point of impact from those observed later, as would be available to a forensics examiner, and the accuracy of the predictions developed in this thesis are evaluated.