Dynamic Wetting by Viscous Liquids: Effects of Softness, Wettability and Curvature of the Substrate and Influence of External Electric Fields

The wetting of solid surfaces by liquids is commonly observed in nature, and it is also a key to a number of industrial applications and biological processes. In the past two centuries, most studies about wetting were devoted mainly to equilibrium situations and thus to static measurements. However, in most cases the dynamic wetting is more relevant and it has received less attension. The goal of this thesis is to study the effects of softness, wettability and curvature of the substrate and influence of external electric fields on dynamic wetting of viscous liquids. The thesis contains two main parts. The first part focuses on the early dynamic wetting of simple liquids on two types of surfaces that show different complexity: flat viscoelastic substrates and highly curved solid microparticles. On the viscoelastic substrates, a novel wetting stage dominated by inertia was found. The dynamics in this stage is characterized by the wetting radius, r=K't^α, following a power law similarly as on rigid surfaces, with the exponent α only depending on surface wettability. After the inertial wetting stage, spreading slows down and enters another stage dominated by the viscoelasticity of the substrate. The transition between inertial and viscoelastic stage is controlled by the surface “softness”. A simple theory was developed with Prof. Martin E.R. Shanahan to explain these findings. An early inertial wetting stage was also observed during the snap-in process, i.e. the wetting, of single colloidal particles into large water drops. The snap-in time is dependent on the capillary force and on inertia, but is independent on surface wettability. In contrast, the snap-in force is larger for hydrophilic and smaller for hydrophobic particles. A scaling model was proposed to describe the snap-in or early wetting of individual colloids. The second part of the thesis is devoted to study the dynamic wetting of rigid flat surfaces by simple and viscous liquids. First, the early spreading of drops of aqueous electrolyte solutions on various wettable surfaces driven by electrostatic forces, which was termed “electrospreading”, was investigated. It was found that early electrospreading is only dominated by inertia and electrostatics. The wetting dynamics is not only dependent on surface wettability and applied electric potential, but also on the concentration of the electrolyte solutions. The electrostatic energy stored in the electric double layer near the solid-liquid interface served as an additional energy for driving drop spreading. Based on molecular dynamics simulation done by Dr. Chunli Li, a simple scaling model was presented to describe the wetting dynamics. Finally, a systematic study of dynamic wetting of various wettable surfaces by viscous liquids was carried out. Both surface wettability and liquid viscosity influence the inertial stage of wetting as well as the viscous stage. During the inertial wetting stage, the effective mass of the spreading drop is affected by surface wettability and liquid viscosity. This results in a slower spreading speed on hydrophobic surfaces, or of highly viscous liquids. Viscous wetting did not take place on all substrates, but only on those surfaces with equilibrium contact angles smaller than a critical value, which depended again on liquid viscosity. A scaling law was proposed to explain these experimental observations

A standing Leidenfrost drop with Sufi-whirling

The mobility of Leidenfrost drop has been exploited for the manipulation of drop motions. In the classical model, the Leidenfrost drop was levitated by a vapor cushion, in the absence of touch to the surface. Here we report a standing Leidenfrost state on a heated hydrophobic surface where drop stands on the surface with partial adhesion and further self-rotates like Sufi-whirling. To elucidate this new phenomenon, we imaged the evolution of the partial adhesion, the inner circulation, and the ellipsoidal rotation of the drop. The stable partial adhesion is accompanied by thermal and mechanical equilibrium, and further drives the development of the drop rotation.Comment: 16 pages, 4 figure

Printing surface charge as a new paradigm to program droplet transport

Directed, long-range and self-propelled transport of droplets on solid surfaces, especially on water repellent surfaces, is crucial for many applications from water harvesting to bio-analytical devices. One appealing strategy to achieve the preferential transport is to passively control the surface wetting gradients, topological or chemical, to break the asymmetric contact line and overcome the resistance force. Despite extensive progress, the directional droplet transport is limited to small transport velocity and short transport distance due to the fundamental trade-off: rapid transport of droplet demands a large wetting gradient, whereas long-range transport necessitates a relatively small wetting gradient. Here, we report a radically new strategy that resolves the bottleneck through the creation of an unexplored gradient in surface charge density (SCD). By leveraging on a facile droplet printing on superamphiphobic surfaces as well as the fundamental understanding of the mechanisms underpinning the creation of the preferential SCD, we demonstrate the self-propulsion of droplets with a record-high velocity over an ultra-long distance without the need for additional energy input. Such a Leidenfrost-like droplet transport, manifested at ambient condition, is also genetic, which can occur on a variety of substrates such as flexible and vertically placed surfaces. Moreover, distinct from conventional physical and chemical gradients, the new dimension of gradient in SCD can be programmed in a rewritable fashion. We envision that our work enriches and extends our capability in the manipulation of droplet transport and would find numerous potential applications otherwise impossible.Comment: 11 pages, 4 figure

Impact dynamics of Newtonian and viscoelastic droplets on heated surfaces at low Weber number

The mechanism of contact between liquid droplets and hot surfaces is important and attracts many researches recently. Herein, we experimentally investigate the contact of Newtonian and viscoelastic droplets between gradually heated surfaces at low Weber numbers, which has not been explored. To present a detailed analysis of contact line dynamics, experiments were performed across a surface temperature ranging boiling conditions to above Leidenfrost temperature (100°C−300°C), while various impact phenomena with increasing surface temperature have been observed for Newtonian and viscoelastic droplets. We demonstrate that the polymer additives significantly affect the dynamic contact angle and contact radius on heated hydrophilic surface. In the Leidenfrost regime, the increased velocity causes considerable reduction on contact time of water droplets, especially at the breaks-up mode. However, it does not influence the contact time of impinging polymer droplet. Aqueous polymer droplets with high molecular weight and high concentration suppress secondary atomization, splashing and break-up, but promote droplet foaming and the generation of viscoelastic filaments. The results illustrate how the polymer additives, surface temperature and impact velocity affect the impact outcomes, and original impact phase diagrams are proposed finally