Impact of surface roughness on the deposition of saliva and fingerprint residue on non-porous substrates

A diverse range of non-porous substrate surfaces routinely encountered in crime scene settings were evaluated for their affinity towards deposited saliva and fingerprint residue. Raman spectroscopy, atomic force microscopy (AFM), field-emission scanning electron microscopy (FE-SEM) and water contact angle measurements were employed to quantitatively and qualitatively evaluate the physicochemical variables impacting the deposition of touch and salivary deposits. It was established that small changes in surface roughness and physicochemical interactions, including hydrophobicity, resulted in observable impacts on the spread of salivary deposits and deposition of fingerprint residue. It was further identified that not only did the surface free energy (SFE) available to the deposit, measured through water contact angles, influence the deposition and spread of the material on the surface, but that the nature of the deposit further impacted these dynamics. This preliminary study has developed a platform to gain a better understanding of the importance of surface roughness as a consideration, when interrogating how forensically relevant deposits such as saliva and fingerprint residue interact with non-porous surfaces. This better understanding is necessary when reflecting on the successfulness of recovering DNA, when considering means to improve recovery of trace samples from crime scenes, and when improving assessment of the likelihood of transfer of a deposit from one substrate to another upon contact.