5 research outputs found
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
Colored Poly(vinyl chloride) by Femtosecond Laser Machining
Colored
poly(vinyl chloride) (PVC) was fabricated by femtosecond
laser micromachining without the addition of chemical colorants, eliminating
the concern of leaching dyes and pigments. We determined that the
changes in surface chemistry and surface topography both contribute
to the observed yellow, brown, and black color formation. X-ray photoelectron
spectroscopy (XPS) on the machined samples showed that conjugated
double bonds are liable for the yellow and brown colors, whereas the
presence of oxidized carbon and surface topography contribute to the
black color. Fourier transform infrared spectroscopy (FTIR) indicated
that laser irradiation altered the material’s properties only
near the surface, which left the bulk properties unaltered. Furthermore,
chemical resistance tests showed that some of the samples were able
to withstand the influence of aggressive chemicals and their color
did not fade. Finally, we showed that the fabrication of colored PVC
highly depends on its ablation energy threshold which is affected
by the laser pulse duration and wavelength
Internal and External Flow over Laser-Textured Superhydrophobic Polytetrafluoroethylene (PTFE)
In this work, internal and external
flows over superhydrophobic (SH) polytetrafluoroethylene (PTFE) were
studied. The SH surface was fabricated by a one-step femtosecond laser
micromachining process. The drag reduction ability of the textured
surface was studied experimentally both in microscale and macroscale
internal flows. The slip length, which indicates drag reduction in
fluid flow, was determined in microscale fluid flow with a cone-and-plate
rheometer, whereas a pressure channel setup was used for macroscale
flow experiments. The textured PTFE surface reduced drag in both experiments
yielding comparable slip lengths. Moreover, the experimentally obtained
slip lengths correspond well to the result obtained applying a semianalytical
model, which considers the solid fraction of the textured surface.
In addition to the internal flow studies, we fabricated SH PTFE spheres
to test their drag reduction abilities in an external flow experiment,
where the terminal velocities of the falling spheres were measured.
These experiments were conducted at three different Reynolds numbers
in both viscous and inertial flow regimes with pure glycerol, a 30%
glycerol solution, and water. Surprisingly, the drag on the SH spheres
was higher than the measured drag on the non-SH spheres. We hypothesize
that the increase in form drag outweighs the decrease in friction
drag on the SH sphere. Thus, the overall drag increased. These experiments
demonstrate that a superhydrophobic surface that reduces drag in internal
flow might not reduce drag in external flow
Reducing Ice Adhesion on Nonsmooth Metallic Surfaces: Wettability and Topography Effects
The
effects of ice formation and accretion on external surfaces
range from being mildly annoying to potentially life-threatening.
Ice-shedding materials, which lower the adhesion strength of ice to
its surface, have recently received renewed research attention as
a means to circumvent the problem of icing. In this work, we investigate
how surface wettability and surface topography influence the ice adhesion
strength on three different surfaces: (i) superhydrophobic laser-inscribed
square pillars on copper, (ii) stainless steel 316 Dutch-weave meshes,
and (iii) multiwalled carbon nanotube-covered steel meshes. The finest
stainless steel mesh displayed the best performance with a 93% decrease
in ice adhesion relative to polished stainless steel, while the superhydrophobic
square pillars exhibited an increase in ice adhesion by up to 67%
relative to polished copper. Comparisons of dynamic contact angles
revealed little correlation between surface wettability and ice adhesion.
On the other hand, by considering the ice formation process and the
fracture mechanics at the ice–substrate interface, we found
that two competing mechanisms governing ice adhesion strength arise
on nonplanar surfaces: (i) mechanical interlocking of the ice within
the surface features that enhances adhesion, and (ii) formation of
microcracks that act as interfacial stress concentrators, which reduce
adhesion. Our analysis provides insight toward new approaches for
the design of ice-releasing materials through the use of surface topographies
that promote interfacial crack propagation
Icephobic Behavior of UV-Cured Polymer Networks Incorporated into Slippery Lubricant-Infused Porous Surfaces: Improving SLIPS Durability
Ice
accretion causes damage on power generation infrastructure, leading
to mechanical failure. Icephobic materials are being researched so
that ice buildup on these surfaces will be shed before the weight
of the ice causes catastrophic damage. Lubricated materials have imposed
the lowest-recorded forces of ice adhesion, and therefore lubricated
materials are considered the state-of-the-art in this area. Slippery
lubricant-infused porous surfaces (SLIPS) are one type of such materials.
SLIPS are initially very effective at repelling ice, but the trapped
fluid layer that affords their icephobic properties is easily depleted
by repeated icing/deicing cycles, even after one deicing event. UV-cured
siloxane resins were infused into SLIPS to observe effects on icephobicity
and durability. These UV-cured polymer networks enhanced both the
icephobicity and longevity of the SLIPS; values of ice adhesion below
10 kPa were recorded, and appreciable icephobicity was maintained
up to 10 icing/deicing cycles