Experimental Correlation for Splashing Condition of Droplets on Solid Substrates

Droplet splashing behaviors of water-ethanol binary mixture liquids on roughened solid surfaces were experimentally observed in order to investigate the effects of surface tension, viscosity, and wettability/surface roughness on the splashing occurrence. The range of the droplet volumes was from 1.7 μL to 32.6 μL. The ranges of the surface tension and the viscosity were from 21.1 mN/m to 71.9 mN/m, and from 1 mPas to 2.91 mPas, respectively. The surface roughness range was from 0.03 μm to 1.25 μm for Ra. The present experimental data were evaluated on the basis of the existing models. Resulting from these experiments, a simple model using the Ohnesorge number evaluated by the capillary length was proposed and the accuracy of the predicted critical values such as the critical Weber and Reynolds numbers were discussed. The result indicated that the liquid properties and the quantification of the surface condition such as surface roughness are important factors for the prediction of the splashing behavior.

Transitions of bouncing and coalescence in binary droplet collisions

In droplet impacts, transitions between coalescence and bouncing are determined by complex interplays of multiple mechanisms dominating at various length scales. Here we investigate the mechanisms and governing parameters comprehensively by experiments and scaling analyses, providing a unified framework for understanding and predicting the outcomes when using different fluids. Specifically, while bouncing had not been observed in head-on collisions of water drops under atmospheric conditions, it was found in our experiments to appear on increasing the droplet diameter sufficiently. Contrarily, while bouncing was always observed in head-on impacts of alkane drops, we found it to disappear on decreasing the diameter sufficiently. The variations are related to gas draining dynamics in the inter-droplet film and suggest an easier means for controlling bouncing as compared to alternating the ambient pressure usually sought. The scaling analysis further shows that for a given Weber number, enlarging droplet diameter or fluid viscosities, or lowering surface tension contributes to a larger characteristic minimum thickness of the gas film, thus enhancing bouncing. The key dimensionless group $(O{h_{g,l}},\;O{h_l},\;{A^\ast })$ is identified, referred to as the two-phase Ohnesorge number, the Ohnesorge number of liquid and the Hamaker constant, respectively. Our thickness-based model indicates that as ${h^{\prime}_{m,c}} > 21.1{h_{cr}}$ , where ${h^{\prime}_{m,c}}$ is the maximum value of the characteristic minimum film thickness $({h_{m,c}})$ and ${h_{cr}}$ is the critical thickness, bouncing occurs in both head-on and off-centre collisions. That is, when $1.2O{h_{g,l}}/(1 - 2O{h_l}) > \sqrt[3]{{{A^\ast }}}$ , a fully developed bouncing regime occurs, thereby yielding a lower coalescence efficiency. The transitional Weber number is found universally to be 4.

Impingement, coalescence and mixing of micro-droplets on a solid surface

The coalescence of an impinging droplet colliding with a sessile droplet at an angle(θi) is investigated by numerical simulation. The range of θi is 0° - 60° and the surface wettability are set as hydrophilic or hydrophobic, and both of them can affect the droplet mergence behavior. By using a modified mixing function, the dimensionless total mixing time τm can be calculated. The results show that there is no clear effect of θi on τm on a hydrophobic surface, while τm increases as θi increases on the hydrophilic surface. With the Weber number(We) ranging from 5.65 to 22.7 and the Ohnesorge number(Oh) ranging from 0.136 to 0.214, we find τm hardly changes with We and Oh. By dividing the mergence and mixing process in a convection and a diffusion stage, we find that the diffusion is much larger than the convection time.

Sequential Improvement from Cosolvents Ink Formulation to Vacuum Annealing for Ink-Jet Printed Quantum-Dot Light-Emitting Diodes

Optimization of ink-jet printing conditions of quantum-dot (QD) ink by cosolvent process and improvement of quantum-dot light-emitting diodes (QLEDs) characteristics assisted by vacuum annealing were analyzed in this research. A cosolvent process of hexane and ortho-dichlorobenzene (oDCB) was optimized at the ratio of 1:2, and ink-jetting properties were analyzed using the Ohnesorge number based on the parameters of viscosity and surface tension. However, we found that these cosolvents systems cause an increase in the boiling point and a decrease in the vapor pressure, which influence the annealing characteristics of the QD emission layer (EML). Therefore, we investigated QLEDs’ performance depending on the annealing condition for ink-jet printed QD EML prepared using cosolvents systems of hexane and oDCB. We enhanced the quality of QD EML and device performance of QLEDs by a vacuum annealing process, which was used to prevent exposure to moisture and oxygen and to promote effective evaporation of solvent in QD EML. As a result, the characteristics of QLEDs formed using ink-jet printed QD EML annealed under vacuum environment increased luminescence (L), current efficiency (CE), external quantum efficiency (EQE), and lifetime (LT50) by 30.51%, 33.7%, 21.70%, and 181.97%, respectively, compared to QLEDs annealed under air environment.

Effects of Jetting Parameters and Sodium Silicate-Based Binder on Droplet Formation

Binder jetting additive manufacturing is one of the 3D printing technologies currently used to manufacture 3D geometries. In this process, a liquid binder agent is ejected to a desired position of a substrate. The binder’s properties and the jetting condition used for form droplets can affect the formability of the geometries. Herein, we optimized the solid content and jetting condition of a sodium silicate-based inorganic binder, for 3D printing. To observe the range of single droplet formation, the behavior of the discharged droplets was analyzed by Z value, which is the inverse of the Ohnesorge number. As the solid content increased, a higher driving voltage was required to form the droplets to overcome viscous dissipation. For 40S(Z = 4.33) with a content of 40 wt%, the droplet tail from the nozzle was stretched further. The droplets of 25S(Z = 15.09) with a content of 25 wt% were accompanied by satellite droplets. The jetting condition was optimized for 25S, which was capable of ejection at various driving voltages. Stable single droplets were formed at a driving voltage of 20 V and a dwell time of 4 μs. In addition, when ethylene glycol and glycerol were added into 25S as a humectant, stable droplets were formed under the optimum jetting condition, and each droplets was in the range of 2.70 < Z < 15.09.

Experimental study on the formation and break-up of fluid bubbles

The study of fluid surfaces plays an important role in understanding the interfaces encountered in biological systems, as it allows for the investigation of the basic characteristics such as the formation, stability and permeability. Moreover, the adhesion and the fusion of biological membranes can be better understood by the experimental investigations of drops and bubbles formation in controlled dynamical processes. These studies have the potential to generate novel and value information for medical applications in the diagnosis and therapy using microfluidic-based biosensors and controlled drug-delivery micro-devices. In this paper, the dynamics of fluid interfaces have been studied experimentally and a method for determining the surface/interfacial tension is proposed. The analysis started with the investigation of the soap bubble formation and break-up. The rupture was triggered manually, by pinching the tip with a needle. The burst was recorded with high-speed cameras and the burst speed was determined. Furthermore, the thickness of the fluid membrane was approximated and the surface tension was calculated using the Culick-Taylor's law. The obtained values for the surface tension were in the same order of magnitude with that from the literature, thus, considering that the employed method can lead to adequate results. Subsequently, a set-up was created to automatically generate fluid bubbles, at different imposed flow rates. The spontaneous burst was analyzed for three different liquids: soap solution, vegetable oil and polyacrylamide. The phenomenon is characterized by the Ohnesorge number, which takes into account the influence of viscous forces in relation to the inertial and surface tension forces. For the soap bubbles, the obtained thickness of the membrane was in the range of (300-500) nm. The calculated surface tension was found to be 0.038 N/m. In the case of automatically generated fluid bubbles, the lowest Ohnesorge number was obtained for soap bubbles and the highest for oil bubbles. Moreover, soap bubbles had the highest break-up speed, while vegetable oil and polyacrylamide had lower and similar break-up speeds. The experimental study described in this paper is an alternative method for the identification of material parameters, such as density and surface tension, in a dynamical process. Numerical simulations are reported from the viewpoint of servo time constant performance.

Universality in coalescence of polymeric fluids

Unification of coalescence dynamics for polymeric fluids through relaxation time λ, Ohnesorge number Oh, and polymer concentration c.