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Dive into the research topics where Robin H. A. Ras is active.

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Featured researches published by Robin H. A. Ras.


Nanoscale | 2011

Fluorescent silver nanoclusters

Isabel Díez; Robin H. A. Ras

Silver nanoclusters are a class of fluorophores with attractive features, including brightness, photostability and subnanometer size. In this review we overview the different scaffolds that are used as stabilizer for silver nanoclusters (e.g. polymers, dendrimers, DNA oligomers, cryogenic noble gas matrixes, inorganic glasses, zeolites and nanoparticles), and we briefly discuss the recent advances.


Advanced Materials | 2011

Mechanically Durable Superhydrophobic Surfaces

Tuukka Verho; Chris Bower; Piers Andrew; Sami Franssila; Olli Ikkala; Robin H. A. Ras

Development of durable non-wetting surfaces is hindered by the fragility of the microscopic roughness features that are necessary for superhydrophobicity. Mechanical wear on superhydrophobic surfaces usually shows as increased sticking of water, leading to loss of non-wettability. Increased wear resistance has been demonstrated by exploiting hierarchical roughness where nanoscale roughness is protected to some degree by large scale features, and avoiding the use of hydrophilic bulk materials is shown to help prevent the formation of hydrophilic defects as a result of wear. Additionally, self-healing hydrophobic layers and roughness patterns have been suggested and demonstrated. Nevertheless, mechanical contact not only causes damage to roughness patterns but also surface contamination, which shortens the lifetime of superhydrophobic surfaces in spite of the self-cleaning effect. The use of photocatalytic effect and reduced electric resistance have been suggested to prevent the accumulation of surface contaminants. Resistance to organic contaminants is more challenging, however, oleophobic surface patterns which are non-wetting to organic liquids have been demonstrated. While the fragility of superhydrophobic surfaces currently limits their applicability, development of mechanically durable surfaces will enable a wide range of new applications in the future.


ACS Applied Materials & Interfaces | 2011

Hydrophobic Nanocellulose Aerogels as Floating, Sustainable, Reusable, and Recyclable Oil Absorbents

Juuso T. Korhonen; Marjo Kettunen; Robin H. A. Ras; Olli Ikkala

Highly porous nanocellulose aerogels can be prepared by vacuum freeze-drying from microfibrillated cellulose hydrogels. Here we show that by functionalizing the native cellulose nanofibrils of the aerogel with a hydrophobic but oleophilic coating, such as titanium dioxide, a selectively oil-absorbing material capable of floating on water is achieved. Because of the low density and the ability to absorb nonpolar liquids and oils up to nearly all of its initial volume, the surface modified aerogels allow to collect organic contaminants from the water surface. The materials can be reused after washing, recycled, or incinerated with the absorbed oil. The cellulose is renewable and titanium dioxide is not environmentally hazardous, thus promoting potential in environmental applications.


Angewandte Chemie | 2009

Color Tunability and Electrochemiluminescence of Silver Nanoclusters

Isabel Díez; Matti Pusa; Sakari Kulmala; Hua Jiang; Andreas Walther; Anja S. Goldmann; Axel H. E. Müller; Olli Ikkala; Robin H. A. Ras

Colorful clusters: Silver nanoclusters consisting of only a few atoms exhibit large chemical-environment-responsive shifts of their optical absorption and emission bands, that is, large solvatochromism (see picture). The photophysical characteristics and electrochemiluminescence of the Ag clusters give them remarkable advantages over larger nanoparticles in applications such as molecular sensing.


Langmuir | 2011

Superhydrophobic and Superoleophobic Nanocellulose Aerogel Membranes as Bioinspired Cargo Carriers on Water and Oil

Hua Jin; Marjo Kettunen; Ari Laiho; Hanna Pynnönen; Jouni Paltakari; Abraham Marmur; Olli Ikkala; Robin H. A. Ras

We demonstrate that superhydrophobic and superoleophobic nanocellulose aerogels, consisting of fibrillar networks and aggregates with structures at different length scales, support considerable load on a water surface and also on oils as inspired by floatation of insects on water due to their superhydrophobic legs. The aerogel is capable of supporting a weight nearly 3 orders of magnitude larger than the weight of the aerogel itself. The load support is achieved by surface tension acting at different length scales: at the macroscopic scale along the perimeter of the carrier, and at the microscopic scale along the cellulose nanofibers by preventing soaking of the aerogel thus ensuring buoyancy. Furthermore, we demonstrate high-adhesive pinning of water and oil droplets, gas permeability, light reflection at the plastron in water and oil, and viscous drag reduction of the fluorinated aerogel in contact with oil. We foresee applications including buoyant, gas permeable, dirt-repellent coatings for miniature sensors and other devices floating on generic liquid surfaces.


ACS Nano | 2011

Inorganic Hollow Nanotube Aerogels by Atomic Layer Deposition onto Native Nanocellulose Templates

Juuso T. Korhonen; Panu Hiekkataipale; Jari Malm; Maarit Karppinen; Olli Ikkala; Robin H. A. Ras

Hollow nano-objects have raised interest in applications such as sensing, encapsulation, and drug-release. Here we report on a new class of porous materials, namely inorganic nanotube aerogels that, unlike other aerogels, have a framework consisting of inorganic hollow nanotubes. First we show a preparation method for titanium dioxide, zinc oxide, and aluminum oxide nanotube aerogels based on atomic layer deposition (ALD) on biological nanofibrillar aerogel templates, that is, nanofibrillated cellulose (NFC), also called microfibrillated cellulose (MFC) or nanocellulose. The aerogel templates are prepared from nanocellulose hydrogels either by freeze-drying in liquid nitrogen or liquid propane or by supercritical drying, and they consist of a highly porous percolating network of cellulose nanofibrils. They can be prepared as films on substrates or as freestanding objects. We show that, in contrast to freeze-drying, supercritical drying produces nanocellulose aerogels without major interfibrillar aggregation even in thick films. Uniform oxide layers are readily deposited by ALD onto the fibrils leading to organic-inorganic core-shell nanofibers. We further demonstrate that calcination at 450 °C removes the organic core leading to purely inorganic self-supporting aerogels consisting of hollow nanotubular networks. They can also be dispersed by grinding, for example, in ethanol to create a slurry of inorganic hollow nanotubes, which in turn can be deposited to form a porous film. Finally we demonstrate the use of a titanium dioxide nanotube network as a resistive humidity sensor with a fast response.


Science | 2013

Switchable Static and Dynamic Self-Assembly of Magnetic Droplets on Superhydrophobic Surfaces

Jaakko V. I. Timonen; Mika Latikka; Ludwik Leibler; Robin H. A. Ras; Olli Ikkala

Magnetic Self-Assembly During self-assembly, objects spontaneously assemble into larger ordered patterns as observed, for example, in the phase segregation of block copolymers or the assembly of micrometer-sized objects and components in electronics. In dynamic self-assembly, the ordered patterns require an external energy source, but still form because of intrinsic interactions within the system. Timonen et al. (p. 253; see the Perspective by Hermans et al.) studied the organization of magnetic droplets, in the form of a ferrofluid, placed on a low-friction surface. A time-varying magnetic field transformed the statically arranged droplets into a dynamic pattern. Magnetic droplets oscillate between static and dynamic self-assembly patterns in a magnetic field. [Also see Perspective by Hermans et al.] Self-assembly is a process in which interacting bodies are autonomously driven into ordered structures. Static structures such as crystals often form through simple energy minimization, whereas dynamic ones require continuous energy input to grow and sustain. Dynamic systems are ubiquitous in nature and biology but have proven challenging to understand and engineer. Here, we bridge the gap from static to dynamic self-assembly by introducing a model system based on ferrofluid droplets on superhydrophobic surfaces. The droplets self-assemble under a static external magnetic field into simple patterns that can be switched to complicated dynamic dissipative structures by applying a time-varying magnetic field. The transition between the static and dynamic patterns involves kinetic trapping and shows complexity that can be directly visualized.


Proceedings of the National Academy of Sciences of the United States of America | 2012

Reversible switching between superhydrophobic states on a hierarchically structured surface

Tuukka Verho; Juuso T. Korhonen; Lauri Sainiemi; Ville Jokinen; Chris Bower; Kristian Franze; Sami Franssila; Pierce Andrew; Olli Ikkala; Robin H. A. Ras

Nature offers exciting examples for functional wetting properties based on superhydrophobicity, such as the self-cleaning surfaces on plant leaves and trapped air on immersed insect surfaces allowing underwater breathing. They inspire biomimetic approaches in science and technology. Superhydrophobicity relies on the Cassie wetting state where air is trapped within the surface topography. Pressure can trigger an irreversible transition from the Cassie state to the Wenzel state with no trapped air—this transition is usually detrimental for nonwetting functionality and is to be avoided. Here we present a new type of reversible, localized and instantaneous transition between two Cassie wetting states, enabled by two-level (dual-scale) topography of a superhydrophobic surface, that allows writing, erasing, rewriting and storing of optically displayed information in plastrons related to different length scales.


Science | 2016

Moving superhydrophobic surfaces toward real-world applications

Xuelin Tian; Tuukka Verho; Robin H. A. Ras

Standardized wear and durability testing is needed to advance the best materials Superhydrophobic surfaces have received rapidly increasing research interest since the late 1990s because of their tremendous application potential in areas such as self-cleaning and anti-icing surfaces, drag reduction, and enhanced heat transfer (1–3). A surface is considered superhydrophobic if a water droplet beads up (with contact angles >150°), and moreover, if the droplet can slide away from the surface readily (i.e., it has small contact angle hysteresis). Two essential features are generally required for superhydrophobicity: a micro- or nanostructured surface texture and a nonpolar surface chemistry, to help trap a thin air layer that reduces attractive interactions between the solid surface and the liquid (4, 5). However, such surface textures are highly susceptible to mechanical wear, and abrasion may also alter surface chemistry. Both processes can lead to loss of liquid repellency, which makes mechanical durability a central concern for practical applications (6, 7). Identifying the most promising avenues to mechanically robust superhydrophobic materials calls for standardized characterization methods.


Advanced Materials | 2011

Superhydrophobic Tracks for Low-Friction, Guided Transport of Water Droplets

Henrikki Mertaniemi; Ville Jokinen; Lauri Sainiemi; Sami Franssila; Abraham Marmur; Olli Ikkala; Robin H. A. Ras

anti-fogging, [ 6 ] anti-icing, [ 7 ] buoyancy [ 8 ] and drag reduction. [ 9 ] By defi nition, a surface is superhydrophobic if the contact angle between a water drop and the surface at the solid/liquid/air interface is larger than 150 ° , and the contact angle hysteresis is small, i.e., drops readily slide or roll off when the surface is tilted slightly. [ 10–12 ] Here we explore the feasibility of using superhydrophobicity for guided transport of water droplets. We demonstrate a simple yet effi cient approach for droplet transport, in which the droplet is moving on a superhydrophobic surface, using gravity or electrostatic forces as the driving force for droplet transportation and using tracks with vertical walls as gravitational potential barriers to design trajectories. Although the slope of the platform is as small as a few degrees, the drops move at a considerable speed up to 14 cm s − 1 , even in highly curved trajectories. We further demonstrate splitting of a droplet using a superhydrophobic knife and drop-size selection using superhydrophobic tracks. These concepts may fi nd applications in droplet microfl uidics and lab-on-a-chip systems where single droplets with potential analytes are manipulated. [ 13–16 ]

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Isabel Díez

Helsinki University of Technology

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Kari Rissanen

University of Jyväskylä

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