Contact time of impacting droplets on a superhydrophobic surface with tunable curvature and groove orientation

Regulating the impact dynamics of water droplets on a solid surface is of great significance for some practical applications. In this study, the droplet impingement on a flexible superhydrophobic surface arrayed with micro-scale grooves is investigated experimentally. The surface was curved into cylindrical shapes with certain curvatures from two orthogonal directions, where axial and circumferential grooves were formed, respectively. The effects of curvature diameter and Weber number, as well as the orientation of grooves on droplet spreading and retracting dynamics were discussed and explained. Results show that the circumferential grooves promote the spreading of droplet in the azimuthal direction, where the droplet rebounds from the surface with a stretched shape. This mechanism further reduces the contact time of impacting droplets on the superhydrophobic surface compared to the other curving mode.

“Endoscopy Salon” for Controlling Respiratory Droplet Spreading During Endoscopic Procedure

Introduction Preventing droplet dispersal is an important issue for decreasing the coronavirus 2019 (COVID-19) transmission rate; numerous personal protective equipment (PPE) devices have been recently developed for this. Objective To evaluate the effectiveness of a novel PPE device to prevent droplet spread during nasal endoscopic and fiber optic laryngoscopic examination and postuse equipment cleaning technique. Methods The “endoscopy salon” was created with a hooded salon hair dryer, plastic sheath, and silicone nipple. Comparison fluorescence dye dispersal from simulating forceful coughing with and without using the “endoscopy salon” was conducted to assess the droplet spread control. The effects of heat produced in the “endoscopy salon” and disinfection cleaning were also evaluated. Results Fluorescent dye droplet spread from a mannequin's mouth without using the “endoscopy salon” to care providers' clothes and the floor surrounding mannequin, whereas no dye droplets spread out when using the “endoscopy salon”. The maximal temperature observed in the hair dryer was 56.3°C. During the cleaning process, when a plastic bag was attached to the hair dryer's hood to create a closed system, the temperature increased to 79.8 ± 3.1 °C. These temperatures eliminated four test organism cultures during equipment disinfection. Conclusion This novel “endoscopy salon” device prevented respiratory droplet spread and eliminated infectious organisms during postuse equipment cleaning.

Liquid Droplet Impact on a Sonically Excited Thin Membrane

Impacting droplet characteristics on hydrophobic surfaces can be altered by introducing surface oscillations. Impacting water droplet contact duration, spreading, retraction, and rebounding behaviors are examined at various sonic excitation frequencies of the hydrophobic membrane. Membrane oscillation and droplet behavior are analyzed by utilizing a high-speed camera. The restitution coefficient and membrane dynamics are formulated and the findings are compared with those of the experiments. It is found that the mode of membrane oscillation changes as the sonic excitation frequency is changed. The droplet spreading and retraction rates reduce while rebound height and restitution coefficient increase at a sonic excitation frequency of 75 Hz. However, further increase of the excitation frequency results in reduced rebound height because of increased energy dissipation on the impacted surface. The droplet contact (transition time) duration reduces as the excitation frequency increases. Increasing droplet Weber number enhances the droplet contact period on the membrane, which becomes more apparent at low frequencies of sonic excitation.

Direct Fabrication of Micron-Thickness PVA-CNT Patterned Films by Integrating Micro-Pen Writing of PVA Films and Drop-on-Demand Printing of CNT Micropatterns

The direct fabrication of micron-thickness patterned electronics consisting of patterned PVA films and CNT micropatterns still faces considerable challenges. Here, we demonstrated the integrated fabrication of PVA films of micron-thickness and CNT-based patterns by utilising micro-pen writing and drop-on-demand printing in sequence. Patterned PVA films of 1–5 μm in thickness were written first using proper micro-pen writing parameters, including the writing gap, the substrate moving velocity, and the working pressure. Then, CNT droplets were printed on PVA films that were cured at 55–65 °C for 3–15 min, resulting in neat CNT patterns. In addition, an inertia-pseudopartial wetting spreading model was established to release the dynamics of the droplet spreading process over thin viscoelastic films. Uniform and dense CNT lines with a porosity of 2.2% were printed on PVA substrates that were preprocessed at 55 °C for 9 min using a staggered overwriting method with the proper number of layers. Finally, we demonstrated the feasibility of this hybrid printing method by printing a patterned PVA-CNT film and a micro-ribbon. This study provides a valid method for directly fabricating micron-thickness PVA-CNT electronics. The proposed method can also provide guidance on the direct writing of other high-molecular polymer materials and printing inks of other nanosuspensions.

Observation of contact angle hysteresis due to inhomogeneous electric fields

Static contact angle hysteresis determines droplet stickiness on surfaces, and is widely attributed to surface roughness and chemical contamination. In the latter case, chemical defects create free-energy barriers that prevent the contact line motion. Electrowetting studies have demonstrated the similar ability of electric fields to alter the surface free-energy landscape. Yet, the increase of apparent static contact angle hysteresis by electric fields remains unseen. Here, we report the observation of electrowetting hysteresis on micro-striped electrodes. Unlike most experiments with stripes, the droplet spreading on the substrate is experimentally found to be isotropic, which allows deriving a simple theoretical model of the contact angle hysteresis depending the applied voltage. This electrowetting hysteresis enables the continuous and dynamic control of contact angle hysteresis, not only for fundamental studies but also to manufacture sticky-on-demand surfaces for sample collection.