Rebound suppression of a droplet impacting on an oscillating horizontal surface

The role of oscillation in ellipsoidal drop impact on a solid surface

Ellipsoidal shapes of drops can significantly modify the impact dynamics and suppress the rebound by inducing symmetry breaking in the mass and momentum distributions compared to the axisymmetric dynamics of typical drops. However, the previous works have assumed that the drop oscillation at the moment of impact only slightly affects the post-dynamics although the oscillation must be involved in the spreading. Here, we study the impact dynamics of the oscillating ellipsoidal drops on non-wetting surfaces as a function of the ellipticity, oscillation phase, and Weber number (We) experimentally and numerically. The spreading dynamics show notable hysteretic features in the maximal spreading diameters at the four regions of the oscillation phase. The hysteresis appears more prominently in prolate drops than in oblate drops and becomes remarkably suppressed at the four phases as We increases. Momentum analysis shows that the phases for shaping the drops spherically can drive higher asymmetry in the horizontal momenta than the other phases for shaping the drops ellipsoidally. The momentum asymmetry in the horizontal axes indicates that the oscillation phase as well as the ellipticity can play an important role in altering the hydrodynamics and reducing the bounce magnitude.

Whole Contact Line Pinning for Droplets Impacting on a Hydrophobic Surface Due to Hydrophilic TiO2 Nanoparticle Addition

Controlling droplet deposition on a hydrophobic surface has received much attention due to its wide applications. Addition of certain elements into a working droplet is a feasible way to improve drop deposition, which, however, often leads to a significant change in droplet spreading properties. In this work, we show that adding a small amount of hydrophilic TiO₂ nanoparticles without any surfactant can significantly suppress the droplet rebound and even generate a whole contact line pinning on the hydrophobic surface. The whole contact line pinning is positively related to the Weber number (i.e., impact velocity) and suspension concentration. Specifically, when the suspension concentration exceeds a critical value, the pinning and droplet deposition occur in the same We range. A mechanism is proposed to explain the observed unique pinning and depinning behaviors, according to which the agglomerated TiO₂ particles depositing at the triple line can change the wettability of the local surface, which leads to pinning, while the disturbance of capillary oscillation leads to depinning. Interestingly, a long-time whole contact line pinning for more than a second was observed under certain conditions. This work can be of value for many practical applications such as pesticide deposition and spray cooling.

New insights into unusual droplets: from mediating the wettability to manipulating the locomotion modes

The ability to manipulate droplets can be utilized to develop various smart sensors or actuators, endowing them with fascinating applications for drug delivery, detection of target analytes, environmental monitoring, intelligent control, and so on. However, the stimuli-responsive superhydrophobic/superhydrophilic materials for normal water droplets cannot satisfy the requirements from some certain circumstances, i.e., liquid lenses and biosensors (detection of various additives in water/blood droplets). Stimuli-responsive wetting/dewetting behaviors of exceptional droplets are open issues and are attracting much attention from across the world. In this perspective article, the unconventional droplets are divided into three categories: ionic or surfactant additives in water droplets, oil droplets, and bubble droplets. We first introduce several classical wettability models of droplets and some methods to achieve wettability transition. The unusual droplet motion is also introduced in detail. There are four main types of locomotion modes, which are vertical rebound motion, lateral motion, self-propulsion motion, and anisotropic wettability controlled sliding behavior. The driving mechanism for the droplet motion is briefly introduced as well. Some approaches to achieve this manipulation goal, such as light irradiation, electronic, magnetic, acid-base, thermal, and mechanical ways will be taken into consideration. Finally, the current researches on unconventional droplets extending to polymer droplets and liquid metal droplets on the surface of special wettability materials are summarized and the prospect of unconventional droplet research directions in the field of on-demand transport application will be proposed.

Normal and oblique droplet impingement dynamics on moving dry walls

Industrial applications that depend on jetting-based technology, such as painting or additive layered manufacturing, involve sequential deposition of droplets onto a moving surface. Spreading and receding dynamics of these impinging drops depend on the momentum transferred by the moving wall to the droplet liquid, which in turn governs the geometric precision and surface finish of the printed outcome. In this work, the impingement dynamics of microdroplets on a flat, smooth, and moving solid surface is computed using a phase-field-based lattice Boltzmann method. Moreover, the motion of the three-phase moving contact line is captured using a geometry-based contact angle formulation. First, we investigate the influence of various process and materials parameters such as wall velocity, droplet viscosity, surface tension, and wettability on the impact behavior of drops. The surface wettability significantly affects the droplet morphology; an elongated tail like structure forms on the rear end of the droplet which becomes sharper as the moving surface becomes more hydrophobic. Furthermore, we examine the underlying flow physics of the symmetry breaking during the spreading and recoiling phases. For a given contact angle, an increase in wall velocity is found to expedite droplet spreading. In addition, for the first time we explore the oblique droplet impingement dynamics on moving dry walls in this work. It is observed that wall momentum affects the structure of the leading edge during the inline impact situations, whereas the moving surface controls the delay in flow reversal inside the droplet for opposing impact scenarios.

Rebounding suppression of droplet impact on hot surfaces: effect of surface temperature and concaveness

When a droplet impinges on a hot surface it is crucial to increase the contact time or decrease the rebounding distance if the heat transfer between the droplet and the surface is important. This will be more sensitive when the temperature regime is above the Leidenfrost values. The focus of the present experimental study is on the maximum height of drop bouncing after impinging on flat and semi-cylindrical concave surfaces, in particular in terms of surface temperature. It is shown that the behavior of the lamella during the spreading to its maximum diameter has a considerable impact on the maximum height of the drop bouncing. For different impact Weber numbers the map of thermal versus inertia effects is extracted for both the flat and concave surfaces for rebound and thermal atomization mid-regimes. It was shown that the thermal atomization mid-regime was eliminated in the case of drop impact on the concave surface in a wide range of impact Weber numbers and surface temperatures. The variations in rebounding and maximum heights at different regions of the maps are quantified and discussed. The morphology of drop impact on the concave surface was captured and the influence of its asymmetric deformation on extensive suppression of drop bouncing was discussed. Finally, the amount of dissipated energy due to drop deformation was obtained based on an energy balance analysis.

Reducing the Bounce Height during Truncated Spherical Drop Impact on a Solid Surface

Controlling drop dynamics on solid surfaces is an important challenge. In many strategies for efficient drop deposition, drop dynamics is generally assumed to be axisymmetrical. We demonstrate shape-dependent impact dynamics that can considerably modify the dynamics by deforming the drop into a truncated spherical shape at the impact moment. Experimental and numerical studies show the exceptional rim dynamics that lead to reduced bounce heights compared with spherical drops. We investigate the impact dynamics of truncated spherical drops as a function of the truncation depth, surface wettability, and impact velocity numerically. The bounce height of the truncated drop reduces by 56% below spherical drops. To elucidate the mechanism for the reduction in the bounce height, we conduct the horizontal and vertical momentum analyses of truncated drops. The truncated drop impact can potentially open up new opportunities for enhancing drop deposition in practical applications, such as surface coating and spray cooling.