7 research outputs found
On the Oblique Impact Dynamics of Drops on Superhydrophobic Surfaces. Part II: Restitution Coefficient and Contact Time
Oblique water drop impacts were performed on a superhydrophobic surface at normal Weber numbers in the range of 3 < Wen < 80 and at angles of incidence in the range of 0 < AOI < 60°. While holding Wen constant, we varied the AOI to investigate how the oblique nature of the impact affects the sliding length and spreading diameter of impacting drops. Our sliding length measurements indicate that drops impacting at Wen < 10 retain essentially full mobility on the surface, whereas the sliding of higher-Wen impacts is inhibited by drag forces. We attribute this trend to increased penetration into air-trapping surface features occurring in higher-Wen impacts, which results in more adhesion between the liquid and solid. Regarding the spreading of drops on SHP surfaces, the dimensionless maximum spread diameter (D*max) increases not only with Wen but also with the angle of incidence such that more oblique drop impacts stretch to a wider maximum diameter. We attribute this behavior to adhesion forces, which act to stretch the drop as it slides tangentially across the surface in oblique impacts. On the basis of this theory, we derived a model predicting D*max for any Wen and AOI. The model’s predictions are highly accurate, successfully predicting D*max for our entire experimental space. Finally, by placing the camera above the sample, we observed that oblique drop impacts spread into an elliptical shape, and we present a model predicting the maximum spread area
On the Oblique Impact Dynamics of Drops on Superhydrophobic Surfaces. Part I: Sliding Length and Maximum Spreading Diameter
Oblique water drop impacts were performed on a superhydrophobic surface at normal Weber numbers in the range of 3 < Wen < 80 and at angles of incidence in the range of 0 < AOI < 60°. While holding Wen constant, we varied the AOI to investigate how the oblique nature of the impact affects the sliding length and spreading diameter of impacting drops. Our sliding length measurements indicate that drops impacting at Wen < 10 retain essentially full mobility on the surface, whereas the sliding of higher-Wen impacts is inhibited by drag forces. We attribute this trend to increased penetration into air-trapping surface features occurring in higher-Wen impacts, which results in more adhesion between the liquid and solid. Regarding the spreading of drops on SHP surfaces, the dimensionless maximum spread diameter (D*max) increases not only with Wen but also with the angle of incidence such that more oblique drop impacts stretch to a wider maximum diameter. We attribute this behavior to adhesion forces, which act to stretch the drop as it slides tangentially across the surface in oblique impacts. On the basis of this theory, we derived a model predicting D*max for any Wen and AOI. The model’s predictions are highly accurate, successfully predicting D*max for our entire experimental space. Finally, by placing the camera above the sample, we observed that oblique drop impacts spread into an elliptical shape, and we present a model predicting the maximum spread area
Injuries to the eye
Oblique drop impacts were performed
at high speeds (up to 27 m/s,
We > 9000) with millimetric water droplets, and a linear model
was
applied to define the oblique splashing threshold. Six different sample
surfaces were tested: two substrate materials of different inherent
surface wettability (PTFE and aluminum), each prepared with three
different surface finishes (smooth, rough, and textured to support
superhydrophobicity). Our choice of surfaces has allowed us to make
several novel comparisons. Considering the inherent surface wettability,
we discovered that PTFE, as the more hydrophobic surface, exhibits
lower splashing thresholds than the hydrophilic surface of aluminum
of comparable roughness. Furthermore, comparing oblique impacts on
smooth and textured surfaces, we found that asymmetrical spreading
and splashing behaviors occurred under a wide range of experimental
conditions on our smooth surfaces; however, impacts occurring on textured
surfaces were much more symmetrical, and one-sided splashing occurred
only under very specific conditions. We attribute this difference
to the air-trapping nature of textured superhydrophobic surfaces,
which lowers the drag between the spreading lamella and the surface.
The reduced drag affects oblique drop impacts by diminishing the effect
of the tangential component of the impact velocity, causing the impact
behavior to be governed almost exclusively by the normal velocity.
Finally, by comparing oblique impacts on superhydrophobic surfaces
at different impact angles, we discovered that although the pinning
transition between rebounding and partial rebounding is governed primarily
by the normal impact velocity, there is also a weak dependence on
the tangential velocity. As a result, pinning is inhibited in oblique
impacts. This led to the observation of a new behavior in highly oblique
impacts on our superhydrophobic surfaces, which we named the stretched
rebound, where the droplet is extended into an elongated pancake shape
and rebounds while still outstretched, without exhibiting a recession
phase
A Practical Comparison of Beam Shuttering Technologies for Pulsed Laser Micromachining Applications
In this report we investigate the performance of various beam shutter technologies when applied to femtosecond laser micromachining. Three different shutter options are considered: a mechanical blade shutter, a bistable rotary solenoid shutter, and an electro-optic modulator (EOM) shutter. We analyzed the behavior of each shutter type during repeated open/close commands (period of 10 ≤ T ≤ 200 ms) using both high-speed videography and practical micromachining experiments. To quantify the performance at varying cycle periods, we introduce a new variable called the compliance that characterizes the average state of the shutter with respect to its intended position. We found that the solenoid shutter responds poorly to sequential commands. The mechanical shutter provides reliable performance for cycled commands as short as T = 40 ms, but begins to lag significantly behind the control signal for T ≤ 20 ms. The EOM shutter provides the most precise and reliable performance, with an opening time of only 0.6 ms and a high compliance with the signal commands, even when cycled very quickly (T = 10 ms). Overall, this study acts as an extensive practical guide for other laser users when considering different shutter options for their laser system and desired application