Constant Contact Line Research Articles

Multicomponent Droplet Evaporation on Chemical Micro-Patterned Surfaces

The evaporation and dynamics of a multicomponent droplet on a heated chemical patterned surface were presented. Comparing to the evaporation process of a multicomponent droplet on a homogenous surface, it is found that the chemical patterned surface can not only enhance evaporation by elongating the contact line, but also change the evaporation process from three regimes for the homogenous surface including constant contact line (CCL) regime, constant contact angle (CCA) regime and mix mode (MM) to two regimes, i.e. constant contact line (CCL) and moving contact line (MCL) regimes. The mechanism of contact line stepwise movement in MCL regimes in the microscopic range is investigated in detail. In addition, an improved local force model on the contact line was employed for analyzing the critical receding contact angles on homogenous and patterned surfaces. The analysis results agree well for both surfaces, and confirm that the transition from CCL to MCL regimes indicated droplet composition changes from multicomponent to monocomponent, providing an important metric to predict and control the dynamic behavior and composition of a multicomponent droplet using a patterned surface.

Octagon to Square Wetting Area Transition of Water-Ethanol Droplets on a Micropyramid Substrate by Increasing Ethanol Concentration.

The wettability and evaporation of water-ethanol binary droplets on the substrate with micropyramid cavities are studied by controlling the initial ethanol concentrations. The droplets form octagonal initial wetting areas on the substrate. As the ethanol concentration increases, the side ratio of the initial wetting octagon increases from 1.5 at 0% ethanol concentration to 3.5 at 30% ethanol concentration. The increasing side ratio indicates that the wetting area transforms from an octagon to a square if we consider the octagon to be a square with its four corners cut. The droplets experience a pinning-depinning transition during evaporation. The pure water sessile droplet evaporation demonstrates three stages from the constant contact line (CCL) stage, and then the constant contact angle (CCA) stage, to the mixed stage. An additional mixed stage is found between the CCL and CCA stages in the evaporation of water-ethanol binary droplets due to the anisotropic depinning along the two different axes of symmetry of the octagonal wetting area. Droplet depinning occurs earlier on the patterned surface as the ethanol concentration increases.

Mechanism study on shape evolution and wetting transition of droplets during evaporation on superhydrophobic surfaces

A model about the shape evolution and wetting transition of a droplet on superhydrophobic surfaces during evaporation was founded through analyzing the driving force and resistance on the three phase contact line (TPCL) of the droplet. The driving force to shift the TPCL was derived by calculating the gradient of interface free energy and the resistance was based on adhesion. The calculation results show that the volume reduction process of a droplet will undergo several stages. The first one is the constant contact line (CCL) stage with the resistance on TPCL larger than driving force. The second period is the constant contact angle (CCA) stage when the driving force is greater than the resistance. Later, the TPCL will move down along micro pillars until the substrate is wetted and the wetting transition is completed. Then, the drop will experience another CCL period before entering into the mixed mode stage. The evolution of droplet on micro/nano hierarchical surfaces is similar to that on micro structure surfaces, but with shorter CCL stage and without obvious contact angle reduction after the wetting transition because only micro structure is wetted. Finally, the reasonable agreement between the model and experimental results has been illustrated.

Evaporation of a nanodroplet on a rough substrate

The wettability and roughness of a substrate are crucial to the evolution of the contact angle and three-phase contact line in the evaporation of sessile droplets. In this paper, by performing molecular dynamics simulations for droplet evaporation at the nanoscale, we show that the wettability is more important than the roughness. For a smooth substrate, the evaporation behavior of a nanodroplet is similar to that at the macroscopic scale. This similarity is also observed in the case of a rough hydrophilic substrate. However, for a rough hydrophobic substrate, both the constant contact angle and contact line pinning appear in turn during evaporation. This suggests that the roughness of the hydrophobic substrate is useful for the evaporation technique in self-assembly at the nanoscale.

Evaporation-Driven Bioassays in Suspended Droplets.

The microtiter plate has been an essential tool for diagnostics, high-throughput screening, and biological assays. We present an alternative platform to perform bioassays in a microplate format that exploits evaporation to drive assay reactions. Our method consists of droplets suspended on plastic pillars; reactions occur in these droplets instead of the wells. The pillars are fabricated by milling, and the rough surface created by this fabrication method pins the droplet to a constant contact line during the assay and also acts as a hydrophobic surface. Upon evaporation, natural convection arising from Marangoni currents mixes solutions in the droplet, which speeds up assay reactions, decreases assay times, and increases limits of detection. As a proof of concept we implemented two colorimetric assays to detect glucose and proteins in only 1.5 μL, without any external devices for mixing and with a digital microscope as a readout mechanism. Our platform is an ideal alternative to the microtiter plate, works with different volumes, is compatible with commercially available reagent dispensers and plate-readers, and could have broad applications in diagnostics and high-throughput screening.

Enhancement of dynamic wetting properties by direct fabrication on robust micro–micro hierarchical polymer surfaces

Understanding evaporation phenomena on hierarchical surfaces is of crucial importance for the design of robust superhydrophobic polymer structures for various applications. This fabrication method enables precise control of the dimensions to elucidate the dynamic wetting behavior affected by geometric parameters. That behavior exhibits three distinct evaporation modes: a constant contact line (CCL), a constant contact angle (CCA), and mixed mode during the droplet evaporation. The droplet evaporation results show that the sticky CCL mode and the Cassie–Wenzel transition can be prevented by engineering hierarchy integration. Moreover, the CCL–CCA transition point time scale exhibits remarkable dependence on surface dimensions such as the area fraction and solid–liquid contact line. Finally, the fabricated hierarchical structures indicate remarkable superhydrophobic properties, static contact angle above 160° and low sliding angle under 10°, with good durability in terms of aging effect and mechanical robustness for 2 months.

Evaporation dynamics of nanodroplets and their anomalous stability on rough substrates

Nanodroplets sitting on substrates in an open system are usually assumed to be thermodynamically unstable, and will eventually either evaporate or grow. However, as a counterpart of nanodroplets, nanobubbles located at the solid-liquid interface were recently demonstrated by numerous experiments to be unexpectedly stable. The accumulated evidence for the existence of stable nanobubbles poses a question of whether nanodroplets are stable. In this work we revisit the stability of nanodroplets upon smooth and rough substrates, concentrating on their evaporation dynamics. On smooth substrates, the droplets evaporate generally in the constant contact angle (CCA) mode, with a contact angle nonmonotonously depending on the fluid-substrate interaction, while on rough substrates, the droplets evaporate in the constant contact line (CCL) mode or the CCL-CCA mixed mode. Our results indeed predict the existence of stable nanodroplets on rough substrates: In situations where the contact line is pinned and the vapor is supersaturated but at a low level of supersaturation, nanodroplets are found to be anomalously stable. The stability of nanodroplets can be interpreted within the framework of the classical nucleation theory.

Evaporation of Droplets on Superhydrophobic Surfaces: Surface Roughness and Small Droplet Size Effects

Evaporation of a sessile droplet is a complex, nonequilibrium phenomenon. Although evaporating droplets upon superhydrophobic surfaces have been known to exhibit distinctive evaporation modes such as a constant contact line (CCL), a constant contact angle (CCA), or both, our fundamental understanding of the effects of surface roughness on the wetting transition remains elusive. We show that the onset time for the CCL-CCA transition and the critical base size at the Cassie-Wenzel transition exhibit remarkable dependence on the surface roughness. Through global interfacial energy analysis we reveal that, when the size of the evaporating droplet becomes comparable to the surface roughness, the line tension at the triple line becomes important in the prediction of the critical base size. Last, we show that both the CCL evaporation mode and the Cassie-Wenzel transition can be effectively inhibited by engineering a surface with hierarchical roughness.