Athanasios Milionis

Spatially controlled surface energy traps on superhydrophobic surfaces.

Water wetting and adhesion control on polymeric patterns are achieved by tuning the configuration of their surfaces structural characteristics from single to dual and triple length-scale. In particular, surfaces with combined micro-, submicrometer-,and nanoroughness are developed, using photolithographically structured SU-8 micro-pillars as substrates for the consecutive spray deposition of polytetrafluoroethylene (PTFE) submicrometer particles and hydrophobically capped iron oxide colloidal nanoparticles. The PTFE particles alone or in combination with the nanoparticles render the SU-8 micropillars superhydrophobic. The water adhesion behaviour of the sprayed pillars is more complex since they can be tuned gradually from totally adhesive to completely non adhesive. The influence of the hierarchical geometrical features of the functionalized surfaces on this behaviour is discussed within the frame of the theory. Specially designed surfaces using the described technique are presented for selective drop deposition and evaporation. This simple method for liquid adhesion control on superhydrophobic surfaces can find various applications in the field of microfluidics, sensors, biotechnology, antifouling materials, etc.

Control of the water adhesion on hydrophobic micropillars by spray coating technique

We present an alternative approach for controlling the water adhesion on solid superhydrophobic surfaces by varying their coverage with a spray coating technique. In particular, micro-, submicro-, and nanorough surfaces were developed starting from photolithographically tailored SU-8 micropillars that were used as substrates for spraying first poly(tetrafluoroethylene) submicrometer particles and subsequently iron oxide nanoparticles. The sprayed particles serve to induce surface submicrometer and nanoscale roughness, rendering the SU-8 patterns superhydrophobic (apparent contact angle values of more than 150°), and also to tune the water adhesion between extreme states, turning the surfaces from “non-sticky” to “sticky” while preserving their superhydrophobicity. The influence of the chemical properties and of the geometrical characteristics of the functionalized surfaces on the wetting properties is discussed within the frame of the theory. This simple method can find various applications in the fabrication of microfluidic devices, smart surfaces, and biotechnological and antifouling materials.

Self-Cleaning Organic/Inorganic Photo-Sensors

We present the fabrication of a multifunctional, hybrid organic-inorganic micropatterned device, which is capable to act as a stable photosensor and, at the same time, displaying inherent superhydrophobic self-cleaning wetting characteristics. In this framework several arrays of epoxy photoresist square micropillars have been fabricated on n-doped crystalline silicon substrates and subsequently coated with a poly(3-hexylthiophene-2,5-diyl) (P3HT) layer, giving rise to an array of organic/inorganic p-n junctions. Their photoconductivity has been measured under a solar light simulator at different illumination intensities. The current-voltage (I-V) curves show high rectifying characteristics, which are found to be directly correlated with the illumination intensity. The photoresponse occurs in extremely short times (within few tens of milliseconds range). The influence of the interpillar distance on the I-V characteristics of the sensors is also discussed. Moreover, the static and dynamic wetting properties of these organic/inorganic photosensors can be easily tuned by changing the pattern geometry. Measured static water contact angles range from 125° to 164°, as the distance between the pillars is increased from 14 to 120 μm while the contact angle hysteresis decreases from 36° down to 2°.

Atmospheric Ice Adhesion on Water-Repellent Coatings: Wetting and Surface Topology Effects

Recent studies have shown the potential of water-repellent surfaces such as superhydrophobic surfaces in delaying ice accretion and reducing ice adhesion. However, conflicting trends in superhydrophobic ice adhesion strength were reported by previous studies. Hence, this investigation was performed to study the ice adhesion strength of hydrophobic and superhydrophobic coatings under realistic atmospheric icing conditions, i.e., supercooled spray of 20 μm mean volume diameter (MVD) droplets in a freezing (-20 °C), thermally homogeneous environment. The ice was released in a tensile direction by underside air pressure in a Mode-1 ice fracture condition. Results showed a strong effect of water repellency (increased contact and receding angles) on ice adhesion strength for hydrophobic surfaces. However, the extreme water repellency of nanocomposite superhydrophobic surfaces did not provide further adhesion strength reductions. Rather, ice adhesion strength for superhydrophobic surfaces depended primarily on the surface topology spatial parameter of autocorrelation length (Sal), whereby surface features in close proximities associated with a higher capillary pressure were better able to resist droplet penetration. Effects from other surface height parameters (e.g., arithmetic mean roughness, kurtosis, and skewness) were secondary.

Microscopic Receding Contact Line Dynamics on Pillar and Irregular Superhydrophobic Surfaces

Receding angles have been shown to have great significance when designing a superhydrophobic surface for applications involving self-cleaning. Although apparent receding angles under dynamic conditions have been well studied, the microscopic receding contact line dynamics are not well understood. Therefore, experiments were performed to measure these dynamics on textured square pillar and irregular superhydrophobic surfaces at micron length scales and at micro-second temporal scales. Results revealed a consistent “slide-snap” motion of the microscopic receding line as compared to the “stick-slip” dynamics reported in previous studies. Interface angles between 40–60° were measured for the pre-snap receding lines on all pillar surfaces. Similar “slide-snap” dynamics were also observed on an irregular nanocomposite surface. However, the sharper features of the surface asperities resulted in a higher pre-snap receding line interface angle (~90°).

Tuning of the characteristics of Au nanoparticles produced by solid target laser ablation into water by changing the irradiation parameters.

We report the production of Au nanoparticles with different average sizes and size distributions, by laser ablation of a solid Au target into pure deionized water. Tuning laser parameters such as pulse duration, energy, and wavelength is possible to tune the size and the size distributions of the produced nanoparticles into the liquid. We demonstrate the possibility of production of highly monodispersed colloidal solutions, in which the average nanoparticle size ranges from 3 to 10 nm, using laser pulses of ns duration. Laser ablation using fs laser pulses can also produce very small nanoparticles, although a small population of bigger nanoparticles is always present. Low and high‐resolution transmission electron microscopy (TEM), in combination with UV–Vis spectroscopy have been employed for the characterization of our samples. Microsc. Res. Tech. 73:937–943, 2010.

Combination of Lithography and Coating Methods for Surface Wetting Control

In recent years, many different lithographic approaches have been applied in order to fabricate micro and nano-structures which are successfully used for the formation of surfaces with special wetting properties [1,2]. In fact, the modification of the surface roughness in the micron, sub-micron and nano-scale with or without chemical treatment, results in surfaces with controlled wetting properties exhibiting the extreme limits (e.g. superhydrophobic, superhy‐ drophilic surfaces) [3-5]. In this work, we examine different approaches in order to achieve such patterned surfaces with tunable wetting characteristics. In particular, we fabricate microrough substrates by using different lithographic techniques. As an additional step, these substrates are coated with different types of sub-micrometer particles or nanoparticles (NPs) (organic or inorganic) in order to achieve the desired chemical modification and to induce submicron or nano-roughness to the surfaces. Specifically, the coatings consist of polyte‐ fluoroethylene (PTFE) sub-micrometer particles and iron oxide colloidal NPs and they are applied by using triboelectric (fricition induced) or spray deposition. Apart from their interesting wetting properties from the theoretical point of view, such type of surfaces can be used in various applications such as in biological scaffolds, microfluidics, lab-on-a-chip devices and aerospace vehicles [6-9].

Laser-Based Lithography for Polymeric Nanocomposite Structures

In recent years, the synthesis of polymeric nanocomposites has gained much interest in the scientific community thanks to their unique capability to combine the properties of the host polymer matrices, such as toughness, elasticity, processability, solubility, thermal stability, etc, with those of inorganic nanoparticles (NPs), such as hardness, chemical resistance, optical and electronic properties. Among a variety of nanofillers, semiconductor and metallic NPs are extensively studied and used, because of their unique properties especially in the nanoscale. In this work we deal with polymeric nanocomposites incorporating various nanofillers, each one of them having extremely attractive properties for technological applications. In particular we focus on titanium dioxide (TiO2) NPs due to their unique reversible wettability, iron oxide (Fe2O3) NPs due to their superparamagnetic nature, gold (Au) NPs due to their enhanced electronic conductivity, and cadmium sulphide (CdS) NPs due to their tuned photoemission in the quantum size regime. Nanocomposite materials are usually prepared by simple blending of the nanosized inorganic components into polymer solutions. Nevertheless, this method quite often leads to formation of aggregates or macroscopic phase separation, since the control of the dispersion and distribution of the nanofillers within the organic matrices is impossible. To obtain a higher compatibility between the filler and the host polymeric material, and achieve coatings with high content of inorganic particles, the use of polymerization techniques is widely adopted (Fouassier 1995, Decker et al 2005, Lee et al 2006, Wang & Ni 2008). From this point of view, several methods can be considered, depending on the type of monomers and nanomaterials, such as bulk polymerization, photoinitiated polymerization, emulsion polymerization, in situ thermal polymerization, or copolymerization in solution. Among them, the use of ultraviolet (UV) light in combination with proper photoinitiators to produce polymeric nanocomposite films is one of the most rapid and effective method, the main advantage being the creation of well defined patterned structures. Patterning of nanocomposites using UV polymerization, the so-called UV photolithography is ideal for the direct incorporation of nanocomposites into specific parts of systems and devices. Photolithographically prepared nanocomposite structures can be used for the selective deposition of molecules, which have specific affinity to the photopolymerized