Effect Of Surface Wettability On The Impact Dynamics Of Blood Droplets

After the blood hits the surface,it will form a specific blood drying Pattern through evaporation.Through BPA(Bloodstain Pattern Analysis),specific information can be extracted to reconstruct the scene,which has a wide application in the field of forensic exploration.Existing blood stain analysis mainly focuses on the chemical and biological mechanisms of blood stain pattern formation during evaporation,such as the difference in blood stain pattern morphology between healthy blood and diseased blood after evaporation.However,the influence mechanism of blood droplet impact dynamic behavior before evaporation on blood dry pattern morphology remains unclear.According to the current research status,the effects of different wettability surfaces on the dynamic behavior of blood droplet and glycerin-water droplet impact were studied through systematic experiments.The main contents of this paper are as follows:(1)In this paper,the dynamic behavior of blood droplet and non-Newtonian fluid glycerin-aqueous solution impingement on hydrophilic surface,hydrophobic surface and superhydrophobic surface is studied experimentally,and the impingement phenomenon,maximum spread coefficient and oscillation period of blood droplet impingement process are analyzed with the evolution of Weber number.The experimental results show that the dynamic behavior of blood and glycerol-aqueous solution is similar in the spreading stage,but glycerol-aqueous solution has larger spreading radius and contact radius on the hydrophilic surface than blood.The damping coefficients and spring constants of the two liquids on different wettability surfaces were predicted by the existing models.It was found that the amplitude of blood on the hydrophilic and hydrophobic surface was significantly smaller than that of glycerin-aqueous solution,which was attributed to the non-Newtonian fluid property of blood: the shear viscosity decreased with the increase of shear rate.(2)This paper calculates the pattern morphologies of blood droplets on different wettability surfaces after drying,and studies the relationship between the area of the dried blood pattern and the contact area when blood hits the solid surface during the spreading process.It is found that the contact line of blood does not change during the evaporation process of the hydrophilic surface,but the contact line will shrink obviously on the hydrophobic surface.This suggests that surface wettability affects the evaporation process of blood;The pattern morphologies of blood droplets drying on hydrophilic and hydrophobic surfaces at different speeds were analyzed.It was found that with the increase of impact velocity,the radius of blood patterns also increased,and there was a great difference in the drying morphologies on hydrophilic and hydrophobic surfaces,indicating that surface wettability and impact velocity would affect the formation of blood patterns.The results show that the spreading stage of blood and glycerol-aqueous solution after impact on different surfaces at different speeds is very similar,indicating that the short spreading stage of blood droplet after impact on the surface is a low viscosity Newtonian fluid.However,the subsequent dynamic behavior is quite different,indicating that the viscoelasticity of the liquid will significantly affect the retraction and shock stage of the droplet after spreading.Surface wettability and impact velocity will affect the evaporation process of blood droplets on the surface,and finally form different morphologies of blood patterns on the surface.