The evaporation of a droplet often leads to the formation of a single ‘coffee ring' pattern due to an outward evaporative flux (Deegan, Nature, 1997), which carries the nonvolatile solute to the edge of the contact line. In this talk, we report the formation of multi-ring DNA precipitate stains (Figure 1) from an evaporating drop with unique stick-slip receding contact-line dynamics. By imaging the formation of a concentrated precursor ring inside the contact line and recording the viscous fingering dynamics prior to the sticking of the contact line, we deduce that the stick-slip dynamics are driven by intra-drop surface trapping and concentration of DNAs by a stagnation micro-flow structure that is quite distinct from the classical outward evaporative flux. DNA bundling to form gel networks and surface aggregation of micro-beads add additional morphology onto the multi-ring template to produce a rich spectrum of surface patterns due to a complex combination of micro-flows, DNA condensation thermodynamics, heterogeneous DNA surface trapping and bead-DNA interaction dynamics within confined domains with complex flows and flow instabilities.

A high-speed fluorescence microscope is used to visualize the morphology and dynamics of the multi-ring formation. As first observed by Takhistov and Chang (Industrial Eng. Chem. Res. 2002), interfacial geometry near the contact-line at the initial periphery precipitate ring exhibits a sharp interfacial dimple, due to wettability of the precipitae, when sufficient evaporation has taken place such that the drop assumes a pan-cake shape. The bottom of this dimple is less than a micron from the substrate and the dimple thins rapidly by intermolecular dewetting forces. The resulting dewetting inward flow meets the outward evaporative flux at a ring of stagnation points some distance inside of the contact line. An annular ring with high concentration of DNA is observed to build up at this stagnation ring within the drop. The less viscous dewetting flow into this more viscous ring is observed to produce clear viscous fingering patterns. A new contact line is formed when the dimple makes contact with the substrate in a rapid rupture event, leaving behind a stain ring at the old contact line. The new contact line recedes rapidly and is repined at the concentrated annulus region. The stagnation ring hence becomes a precursor of the next ring and its separation from the initial contact line determines the ring separation. This stick-slip phenomenon repeats indefinitely to produce evenly spaced ring patterns that form the underlying template for the more complex patterns.

DNA concentration, droplet size and evaporation temperature are found to sensitively affect the spacing and periodicity of the spontaneously emerging multi-rings. A phase diagram is constructed to demarcate the three distinct patterns that appear due to the competition among the dewetting flux, the evaporative flux and the DNA trapping rate.

We have also examined the effects of adding negatively and positively charged silica colloidal particles of varied sizes on the morphology of drying DNA multi-ring stains. Preliminary results show a strong particle size dependence of complex DNA-Silica particle stains: With the presence of large silica beads, periodic multi-rings are significantly disturbed and segregated particle and DNAs aggregates are observed. However, with the presence of small beads, multi-rings are preserved and consist of hybrid silica-DNA complexes. These effects are attributed to the competition among particle assembly, particle-DNA condensation and DNA gel condensation into bundles. The dependence of the particle-DNA condensation on particle size affects the ring spacing as the stagnation flow can only trap uncondensed DNAs. The beads can, in turn, determine the wavelength and the generation number of the successive viscous fingering phenomena. They also leave behind triangular and spoke-like bead aggregates (Figure 2) that are the footprints of the viscous fingers. We are able to correlate the length scales of the spokes to the ring separation in this complex morphology that appears when both beads and uncondensed DNAs are present. Fluorescence correlation spectroscopy (FCS) is employed to further illustrate the interaction between DNA molecules and silica particles.

The rich patterns hence occur when the thermodynamics of different condensation phases are mediated by the confined geometry of the evaporating thin film, the high shear rate of the slipping contact line after the rupture event and the variation DNA concentration gradient both in time and also spatially due to trapping at the contact line and at the stagnation ring. A plethora of complex morphologies are observed on top of the multi-ring template that include triangular fins, radial spokes, hexagonal colloid crystals, coiled DNA bundles and bead-DNA complexes.