| Abstract: | We have developed two strategies to develop finger mark images when powdering methods are no longer effective. One strategy is based on flocculation‐on‐demand colloidal chemistry. An example of this strategy is to apply a suspension of titanium dioxide, water, and chitosan polymer onto the finger mark after the surface is pretreated with a dilute solution of sodium dodecyl sulfate. Pigmented titanium dioxide particles get flocculated and deposited to the finger mark surface when the suspension becomes unstable as the cationic polymer (chitosan) electrostatically interacts with the anionic surfactant (sodium dodecyl sulfate) at the interface. The second strategy is based on the differential adsorptions of pigmented particles on background versus the finger mark surface. By choosing proper pigment–surfactant combinations, particles can be oriented such that the surfactant molecules will attract the pigment particles on the finger mark surfaces but not on the background. An example of this strategy is a suspension made of titanium dioxide particles and a fluorocarbon–hydrocarbon amine oxide surfactant. Differential adsorption results in clear finger mark images, which can be lifted off using various sol–gel chemistries. |
