Evolution Characteristics and Mechanism of Three-Phase Contact Line of a DNA Microdroplet Impacting and Spreading on a Hydrophilic Horizontal Solid Surface

The phenomena and characteristics of droplets impacting and spreading on a horizontal solid surface are key scientific issues in microfluidic technology and surface physics. The movement regularity and mechanism of the three-phase contact line (TCL) of DNA microdroplets spreading on a hydrophilic PMMA smooth surface without splashing or rebounding are investigated by high-speed camera technology. Some significant results are achieved, for example: based on the changes in droplet morphology, the changes of DNA droplet TCL are divided into eight stages, i.e., expanding, pinning, retracting, repinning, microexpanding, micro-oscillating, stabilizing, and retraction drying, which are helpful to understand the deposition of DNA molecules on the substrate surface. Among them, the TCL moves intensely in the stages of expanding and retracting, respectively, following the cubic function and quadratic function; then, TCL remains virtually unchanged since entering the repinning stage until the stabilizing stage, and DNA molecules are deposited on the surface of PMMA, accumulating to form crystals. Furthermore, the relationship between three correlated damped-like oscillations, i.e., droplet top height (h top), TCL position, and dynamic contact angle (DCA) size, is systematically investigated; the h top oscillates first, then the TCL and DCA oscillate after a delay of 5 ms, and the oscillation period increases sequentially, which is 7.55, 7.90, and 8.40 ms, respectively. Finally, the effects of dynamic characteristics and crystallization of DNA molecules on TCL movement and DCA size are expounded. This research provides detailed experimental data and proposes a theoretical model for understanding the dynamic mechanism of DNA droplet spreading on solid surfaces and has been helpful for designing DNA chips and developing micro-/nano-fluidic sensors.