Introduction to Electrowetting
Electrowetting is a physical phenomenon that involves the manipulation of a liquid droplet’s contact angle on a solid surface through the application of an external electric field. By altering the surface tension at the liquid–solid interface, electrowetting allows precise control over the shape, movement, and spread of droplets on both hydrophobic and hydrophilic surfaces. This principle forms the basis of a wide range of applications in microfluidics, where it enables the actuation and transport of minute fluid volumes without mechanical components. It is also pivotal in surface chemistry and materials science, particularly in the design of responsive surfaces, digital microfluidic devices, and lab-on-a-chip systems. In forensic science, electrowetting is gaining attention for its potential to enhance biological trace detection, especially in the visualization of latent fingerprints composed of sweat residues. By enabling controlled rehydration, redistribution, or enhancement of biological fluids on surfaces, electrowetting presents a non-destructive and high-resolution approach for improving the detection and analysis of trace evidence offering a promising alternative to traditional chemical or powder-based enhancement methods.
Detection of invisible biological traces in relation to the physicochemical properties of substrates surfaces in forensic casework
Biological Trace Evidence and Its Forensic Value
Biological trace evidence refers to minute quantities of materials such as sweat, saliva, blood, and epithelial cells left on surfaces at crime scenes. Latent fingerprints, primarily composed of sweat, are a crucial source of forensic information not only for their unique ridge patterns but also because they contain DNA and biochemical markers that can link individuals to objects or locations. Detecting and analyzing these traces is challenging due to their small size and susceptibility to contamination or degradation. Traditional enhancement methods may alter or damage the biological components, making it harder to perform downstream genetic or chemical analyses. Therefore, there is a critical need for non-destructive techniques that can both reveal and preserve these delicate biological traces for comprehensive forensic investigation.
Electrowetting for Fingerprint Sweat Enhancement
Electrowetting technology enables precise control over the behavior of fingerprint sweat films by applying an electric field to manipulate the wettability and spreading of these microscopic liquid residues on surfaces. This process can increase the contrast and visibility of latent fingerprints by redistributing sweat deposits in a controlled manner, making ridge details clearer without physically touching or chemically altering the print. Unlike traditional powder or chemical enhancement methods which may be abrasive or introduce substances that can interfere with subsequent DNA or biochemical analyses electrowetting offers a non-destructive and reversible approach. This innovative technique holds the potential to complement or even outperform conventional methods by preserving the integrity of the biological material while providing enhanced visualization, thereby improving both the quality and reliability of forensic fingerprint detection.
Longitudinal extraction of intracellular contents from live cells
Surface Chemistry and Material Compatibility
The effectiveness of electrowetting in enhancing fingerprint sweat traces largely depends on the chemical and physical properties of the surface on which the residue is deposited. Surfaces can be broadly classified as hydrophobic (water-repellent) or hydrophilic (water-attracting), each interacting differently with sweat films. Electrowetting modifies the surface wettability by applying an electric field, which reduces the contact angle of liquid droplets, causing them to spread more readily on hydrophobic surfaces or contract on hydrophilic ones. This precise control over surface tension allows for the targeted redistribution of sweat components, enhancing the contrast and definition of latent prints. The electrostatic manipulation induced by the electric field facilitates the movement and reorganization of biological fluids at the micro-scale, enabling better visualization of fingerprint patterns without damaging the sample.