Influence of the relative humidity on the blood drip stain formation on jeans fabrics

The impact of whole human blood drops on jeans fabrics is studied as a function of the impact velocity U₀, room relative humidity RH, and spacing S between the fabric and backing substrate. Experiments are performed with blood drops of the same initial diameter of D₀ = 3 ± 0.08 mm and temperature of 37^∘C. Whole human blood is collected from the same donor. The impact dynamics of blood drops is described as a function of U₀ and S, and it is shown that the spacing has an effect on the splashing limit and the surface area of the drip stains. At RH = 30%, the drip stains (parent stains) after impact do not wick the fabrics. Moreover, the area of the drip stains increases with the impact velocity until a critical value (U₀ = 3.3 ± 0.1 m∕s), where it becomes constant. A modified correlation is proposed to predict the drip stains area. At RH = 70%, the drip stains after impact wick the jeans fabrics. The area of the drip stains after impact is dependent on the impact velocity while the final area after wicking is not. Further, the contribution of the wicking, to the formation of the drip stains, decreases with the impact velocity. These findings show the importance of taking into account RH in future research work and in the analysis of the drip stains at crime scenes.