The anisotropic droplet formulation is generalized from hydrophilic to superhydrophobic surfaces. An experimental method to calibrate the ellipsoidal droplet volume on both hydrophilic and hydrophobic surfaces is presented. A broad range of contact angles (CAs) is produced on the copper and stainless-steel surfaces using femtosecond laser patterning. The effects of line spacing between the laser scanning on the formation of anisotropic CAs are discussed. The comparative study of the evolution of anisotropic CAs and droplet’s spreading dynamics are studied on both surfaces. According to the triple contact line (TCL) theory, CAs are determined by the TCL between droplet and surface rather than the contact area. We presented the mathematical formalism and the experimental validity of the TCL theory on ellipsoidal droplets over a broad range of CAs, from as low as 37°–172°. This work experimentally validated the TCL theory over a broad range of CAs with good confidence.
Antireflective surfaces that are efficient over a broad
spectral
range with extremely low total reflectivity have abundant potential
applications in defense, space, and industries. We successfully fabricated
three-dimensional micro-to-nano hierarchical structures on a thin
sheet of stainless steel (SS), 100 μm in thickness, by interference-based
ultrafast laser patterning for near-perfect absorption. The Sagnac-based
interferometer reduces the footprint of the laser on the substrate
to smaller than the diffraction limit. In this work, we achieved a
groove cut of 13 μm size using a 1030 nm ultrafast laser. We
achieved an average total reflectivity of 1.2% over the spectral range
of 400–2000 nm and an average specular reflectivity of 0.02%
over a broad range of wavelengths of UV–visible–NIR
(400–2000 nm) on thin low-weight SS sheets. Further, the average
total reflectivity in the spectral region of 400 to 650 nm was 1.1%
only. The average groove depth from the top of the surface was 8.5
μm, and most of the material was intact in this process. As
these structures are modifications of the same material, they have
high mechanical strength and thermal stability and also exhibit self-cleaning
ability. The patterned surface is insensitive to incident polarization
and has a wide range of acceptance to the incidence cone angle of
±60°. These are the best anti-reflectivity values on an
SS surface fabricated via laser surface patterning.
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