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.
Prof. Jeremy Goldman and Prof. Jaroslaw Drelich are the principal investigators on a new one-year project entitled “Exploratory Research to Suppress Intimal Hyperplasia by Controlling Zinc Implant Biodegradation” awarded with $217,791 by the US National Institute of Health (NIH).
Congratulations to Zhiyong!
Zhiyong just completed his MS program on Zn-Ti biodegradable alloys. He left our team for his first job in California. Drive safely and good luck in your new place Zhiyong!
It is still an open challenge to find a biodegradable metallic material exhibiting sufficient mechanical properties and degradation behavior to serve as an arterial stent. In our new study, and summarized in publication entitled “Novel high-strength, low-alloys Zn-Mg (<0.1 wt% Mg) and their arterial biodegradation” published in Materials Science and Engineering C, Zn-Mg alloys of 0.002 (Zn-002Mg), 0.005 (Zn-005Mg) and 0.08 wt% Mg (Zn-08Mg) content were cast, extruded and drawn to 0.25 mm diameter, and evaluated as potential biodegradable stent materials. Structural analysis confirmed formation of Mg2Zn11 intermetallic in all three alloys with the average grain size decreasing with increasing Mg content. Tensile testing, fractography analysis and micro hardness measurements showed the best integration of strength, ductility and hardness for the Zn-08Mg alloy. Yield strength, tensile strength, and elongation to failure values of >200-300 MPa, >300-400 MPa, and >30% respectively, were recorded for Zn-08Mg. This metal appears to be the first formulated biodegradable material that satisfies benchmark values desirable for endovascular stenting. Unfortunately, the alloy reveals signs of age hardening and strain rate sensitivity, which need to be addressed before using this metal for stenting. The explants of Zn-08Mg alloy residing in the abdominal aorta of adult male Sprague-Dawley rats for 1.5, 3, 4.5, 6 and 11 months demonstrated similar, yet slightly elevated inflammation and neointimal activation for the alloy relative to what was recently reported for pure zinc.
In our new article entitled Direct measurements of adhesion forces for water droplets on smooth and patterned polymers published in the Surface Innovations journal, a microelectronic balance system was employed to measure the force of spreading during water droplet attachment and spreading on polymer surfaces, and the water-polymer adhesion forces (maximum adhesion and pull-off forces) after droplet compression, retreat, and detachment. Equipped with a CCD camera and data acquisition software, the instrument measured directly the forces, monitored droplet-surface separation including distances over which droplet stretched, and collected optical images simultaneously. The images were used to analyze capillary and surface tension forces based on measured droplet shape, surface curvature, droplet base radius, and values of contact angles. The forces measured with the microbalance were compared to calculated capillary/surface tension forces. Nearly excellent agreement between directly measured and calculated forces was verified on polymers with smooth surfaces. Experiments with patterned polymers having pores and pillars revealed that interpretation of forces require a knowledge of a triple contact line characteristic. One relevant parameter, named contact line density, was introduced to surface tension forces to quantify forces measured directly with microbalanace.
Since the invention of froth flotation in 19th century, many laboratories are committed to both fundamental and applied research on collection of particles by dispersed gas bubbles in aqueous solutions of electrolytes, especially those working on processing of natural resources. However, in spite of tremendous progress made in characterization of particles and their surfaces and understanding the particle-bubble interactions, supported with detailed recordings of gas bubble attachments to both bulk specimens and particles, the flotation process remains poorly correlated with the wetting characteristics of particle surfaces. In fact, the contact angles used frequently to describe wettability of mineral surfaces remain among the most controversial, misunderstood and misinterpreted values in mineral processing literature. Contrary to wide-ranging beliefs, in the new contribution by Prof. Drelich (Michigan Tech) and Prof. Marmur (Technion, Israel) argue that neither the methodology of contact angle measurements nor selection of contact angles important to the particle flotation process are properly executed when analyzing flotation process. In their new paper published in Surface Innovations and entitled Meaningful contact angles in flotation systems: critical analysis and recommendations, the authors provide a brief personal prospective on some of the misconceptions on contact angles and the importance of additional fundamental studies in the area of mineral particles flotation.
The research team from the Ben-Gurion University of the Negev and led by Prof. Eli Aghion published, in collaboration with Prof. Goldman and SURFI, a new paper on two new zinc base alloys, Zn-1%Mg and Zn-1%Mg-0.5%Ca, which could serve as structural materials for biodegradable implants. This examination was carried out in in vitro conditions including immersion test, potentiodynamic polarization analysis, electrochemical impedance spectroscopy and stress corrosion cracking assessment in terms of slow strain rate testing. In order to assess the cytotoxicity of the tested alloys, indirect cell viability was preformed using Saos-2 cells. The results obtained demonstrate that both zinc alloys can be considered as adequate candidates for biodegradable implants with a relative advantage to the Zn-1%Mg alloy in terms of its corrosion resistance and SCC performance.
The paper was published in the Journal of Materials Science: Materials in Medicine and is entitled In vitro behavior of biodegradable implants based on Zn-1%Mg and Zn-1%Mg-0.5%Ca alloys.
Surface Innovations Team welcomes a new member, Dr. Ehsan Mostaed. Dr. Mostaed has strong expertise in metallurgy, and his recent research concentrated on formulation and testing of bioabsorbable Zn-based materials. He joins as from Politecnico Di Milano in Italy. In Italy, working with Prof. Maurizio Vedani, Dr. Mostaed formulated and characterized a series of new Zn-based alloys. He will continue exploration of Zn-based alloys at Michigan Tech.
The SURFI Team welcomes a new PhD candidate to our team, Mr. Morteza Shaker. Morteza comes from Iran with a strong background in metallurgy and will work on Zn-based alloys for medical applications under a supervision of both Dr. Drelich and Dr. Kampe.