SURFI

Water Droplet Spreading

Spontaneous spreading of a droplet on a solid surface is poorly understood from a macroscopic down to a molecular level. In our new publication entitled “Spontaneous Spreading of a Droplet: The Role of Solid Continuity and Advancing Contact Angle” and published in Langmuir, in collaboration with the Stevens Institute of Technology, we investigated the effect of surface topography and wettability on spontaneous spreading of a water droplet. Spreading force was measured for a suspended droplet that minimized interference of kinetic energy in the spontaneous spreading during its contact with solid surfaces of discontinuous (pillar) and continuous (pore) patterns with various shapes and dimensions. Results show that a droplet cannot spread spontaneously on pillared surfaces regardless of their shapes or dimensions due to the solid discontinuity. On the contrary, a droplet on pored surfaces can undergo spontaneous spreading whose force increases with a decrease in the advancing contact angle. Theoretical models based on both the system free energy and capillary force along the contact line validate the direct and universal dependency of the spontaneous spreading force on the advancing contact angle.

Vascular Stent

During the last few years, Zn alloys have been explored by the biomedical community as potential materials for bioabsorbable vascular stents due to their tolerable corrosion rates and tunable mechanical properties. In the new review coming from our research team and entitled “Zinc Alloys for Degradable Vascular Stent Applications” published in Acta Biomaterialia, the authors summarize recent progress made in developing Zn alloys for vascular stenting application. Novel Zn alloys are reviewed regarding their microstructural characteristics, mechanical properties, corrosion behavior and in vivo performance.

Corrosion of Zn-1.3Fe

In our new collaborative study led by Prof. Eli Aghion from Ben-Gurion University of the Negev in Israel and described in the article entitled The Suitability of Zn– 1.3%Fe Alloy as a Biodegradable Implant Material, published in Metals, we evaluated the possibility of using Fe as a relatively cathodic biocompatible alloying element in zinc that can tune the implant degradation rate via microgalvanic effects. The selected Zn–1.3wt %Fe alloy composition produced by gravity casting was examined in vitro and in vivo. The in vitro examination included immersion tests, potentiodynamic polarization and impedance spectroscopy, all in a simulated physiological environment (phosphate-buffered saline, PBS) at 37 °C. For the in vivo study, two cylindrical disks (seven millimeters diameter and two millimeters height) were implanted into the back midline of male Wister rats. The rats were examined post implantation in terms of weight gain and hematological characteristics, including red blood cell (RBC), hemoglobin (HGB) and white blood cell (WBC) levels. Following retrieval, specimens were examined for corrosion rate measurements and histological analysis of subcutaneous tissue in the implant vicinity. In vivo analysis demonstrated that the Zn–1.3%Fe implant avoided harmful systemic effects. The in vivo and in vitro results indicate that the Zn–1.3%Fe alloy corrosion rate is significantly increased compared to pure zinc. The relatively increased degradation of Zn–1.3%Fe was mainly related to microgalvanic effects produced by a secondary Zn11Fe phase.

Water Droplet on a Polymer

In our new study described in article entitled “The most stable state of a droplet on anisotropic patterns: Support for a missing link,” surface tension and capillary forces were measured for water droplets in contact with anisotropic hydrophobic patterns made of microscopic ridges and grooves using a microbalance. Integrated with a CCD camera, the instrument allowed capturing of the synchronous images of a droplet during its spreading, compression, stretching and detachment. These images were used to analyze the evolution of a droplet shape and quantify its base diameter and contact angle in both longitudinal and traverse directions. The experiments confirmed that a water droplet spread preferentially along the longitudinal direction, on top of the ridges, following continuity of solid, and producing asymmetry in the drop shape. Switching the droplet wetting mode from advancing to receding causes the droplet to symmetrize its shape. It was found that the maximum adhesion between the droplet and hydrophobic pattern coincides with droplet base circularity and apparent contact angles of nearly identical values measured in longitudinal and traverse directions. These findings confirm that the most stable configuration for a liquid droplet on rough solid surface appears only when the droplet base is axisymmetric. It is also demonstrated that the Cassie-Baxter equation only pertains to the droplet in the most stable state, where the excess free energy is minimized.

Surface Innovations, the journal

We are glad to share with you the content of the 1st issue of 2018 of Surface Innovations with twelve papers on surface phenomena and innovations.