Jaakko V. I. Timonen

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

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.

Multifunctional High-Performance Biofibers Based on Wet-Extrusion of Renewable Native Cellulose Nanofibrils

Fibrous architectures are among the most abundant loadcarrying materials in nature, encompassing molecular level peptide assemblies (e.g., amyloids), supramolecular protein materials (e.g., collagen), colloidal level native cellulose nanofi brils (nanofi brillated cellulose, NFC), through to macroscale spider silk. [ 1 , 2 ] NFC, also denoted as microfi brillated cellulose (MFC), exhibits diameters in the nanometer range and lengths up to several micrometers. These nanofi brils are composed of aligned β D -(1 → 4)glucopyranose polysaccharide chains, which form native cellulose I crystals where the parallel chains are strongly intermolecularly hydrogen bonded. NFC materials can be isolated by chemical/enzymatic and homogenization treatments [ 3 , 4 ] from the cell walls of wood and plants, where they are responsible for structural strength. NFC forms a remarkable emerging class of nature-derived nanomaterials because of its extraordinary mechanical properties, combining high stiffness of up to ca. 140 GPa and expected strength in the GPa range with a lightweight character (density ca. 1.5 g mL − 1 ). These properties rank NFC at the top end of high-performance natural materials, where the stiffness of cellulose I is 2–3 times higher than that of glass fi bers (50–80 GPa) and approaches that of steel (200 GPa). Since NFC is derived from wood or plant sources, it is globally abundant and renewable, and represents a resource that does not interfere with the food chain or require petrochemical components. In addition, related nanofi brils known as bacterial cellulose can be produced biotechnologically. [ 5 ] Consequently, NFC is emerging as one of the most promising sustainable building blocks for future advanced materials. So far the main interest in NFC has been to generate strong and tough nanopapers, nanocomposites upon adding small contents to polymeric matrices, or robust foams and aerogels. [ 4 , 6–16 ]

Free-decay and resonant methods for investigating the fundamental limit of superhydrophobicity

The recently demonstrated extremely water-repellent surfaces with contact angles close to 180° with nearly zero hysteresis approach the fundamental limit of non-wetting. The measurement of the small but non-zero energy dissipation of a droplet moving on such a surface is not feasible with the contemporary methods, although it would be needed for optimized technological applications related to dirt repellency, microfluidics and functional surfaces. Here we show that magnetically controlled freely decaying and resonant oscillations of water droplets doped with superparamagnetic nanoparticles allow quantification of the energy dissipation as a function of normal force. Two dissipative forces are identified at a precision of ~ 10 nN, one related to contact angle hysteresis near the three-phase contact line and the other to viscous dissipation near the droplet-solid interface. The method is adaptable to common optical goniometers and facilitates systematic and quantitative investigations of dynamical superhydrophobicity, defects and inhomogeneities on extremely superhydrophobic surfaces.

From Hot-Injection Synthesis to Heating-Up Synthesis of Cobalt Nanoparticles: Observation of Kinetically Controllable Nucleation†

Monodisperse nanoparticles of well-defined size and shape are required in several emerging applications, which take advantage of their size-dependent properties such as the superparamagnetic limit in the case of magnetic nanoparticles. 2] Accurate tuning of the nanoparticle size and shape requires understanding of the mechanisms involved in particle nucleation and growth. In spite of extensive ongoing research, these mechanisms are still not fully understood owing to their complexity and interplay. Moreover, the current small-scale synthesis methods, such as the hotinjection method, can be difficult to scale to industrially relevant levels. Hence, more suitable methods are sought. Herein, we revisit a widely studied hot-injection synthesis of monodisperse cobalt nanoparticles and show that the particle nucleation differs from what is expected for a hotinjection synthesis. Evidence is given that the particles nucleate several tens of seconds or a few minutes after the injection, depending delicately on how the reaction temperature is controlled after the sudden temperature drop caused by the injection. The delayed nucleation is followed by a period during which the cobalt precursor decomposes endothermically, the temperature drops, carbon monoxide evolves, and the nuclei rapidly grow into mature nanoparticles. Particle growth after the endothermic period is negligible, and we show that the final particle size is determined by the rate of temperature increase after the injection-induced temperature drop. A rapid increase results in a higher peak temperature before the endothermic period and more nuclei, hence smaller particles, in comparison to the case of a slower rate of temperature increase. The contribution of the injection to particle nucleation seems minor, and it is shown that injection can be replaced entirely by an accurately controlled heating up of the solution containing all reagents (including the cobalt precursor) from room temperature to the nucleation temperature. This synthetic method, which is often termed either “non-injection synthesis” 16] or “heating-up synthesis”, 11] results in nanoparticles that are nearly identical to those made by the hot-injection method. We synthesized cobalt nanoparticles by injecting dicobalt octacarbonyl, [Co2(CO)8], dissolved in a small amount of ortho-dichlorobenzene (o-DCB, b.p. 181 8C) into a solution of oleic acid and trioctylphosphine oxide (TOPO) in o-DCB at reflux. The injection led to an immediate temperature drop of several tens of degrees, which is characteristic of the hotinjection method in general. It has been shown that [Co2(CO)8] undergoes partial decarbonylation during the injection to form gaseous carbon monoxide and intermediate cobalt carbonyl species (e.g. tetracobalt dodecacarbonyl, [Co4(CO)12], and cobalt tetracarbonyl, [Co(CO)4]) in the solution phase; these species then further decompose more slowly to cobalt atoms. 27] It has been shown that both maintaining the lower temperature and letting the temperature recover to the reflux temperature after the injection can lead to monodisperse nanoparticles. In this study, we concentrated on the latter approach and studied for the first time in detail the kinetics of the temperature recovery to reflux. The recovery rate can be conveniently controlled by tuning the rate of heat transfer from the heat bath to the reaction medium, for example, by using an oil bath at different temperatures or an electric heating mantle with different heating powers. A typical development in the reaction temperature after the injection is shown in Figure 1 for the hot-injection synthesis HI1 (see the Experimental Section for a complete list of syntheses with details). The temperature dropped from 180 to 143 8C during the injection, after which it started to recover, as heat was being transferred from the heat bath to the reaction medium. In contrast to the expected continuous increase until the reflux temperature was reached, one minute after the injection we observed a characteristic endothermic period during which the reaction temperature dropped despite continuous heating. Interestingly, the peak temperature (174 8C) reached just before the endothermic period was very close to the temperature prior to injection (180 8C). Vigorous evolution of carbon monoxide during the endothermic period indicated decomposition of the cobalt carbonyl species and release of cobalt atoms. Further evolution of carbon monoxide after the endothermic period was negligible, even when the reflux temperature was reached. This observation indicated that nearly all cobalt carbonyl species had decomposed during the endothermic period, which was [*] J. V. I. Timonen, Prof. Dr. O. Ikkala, Dr. R. H. A. Ras Department of Applied Physics, Aalto University (formerly Helsinki University of Technology) P.O. Box 15100, FI-02150 Espoo (Finland) E-mail: jaakko.timonen@tkk.fi robin.ras@tkk.fi Homepage: http://tfy.tkk.fi/molmat/

Photothermally triggered actuation of hybrid materials as a new platform for in vitro cell manipulation

Mechanical forces in the cell’s natural environment have a crucial impact on growth, differentiation and behaviour. Few areas of biology can be understood without taking into account how both individual cells and cell networks sense and transduce physical stresses. However, the field is currently held back by the limitations of the available methods to apply physiologically relevant stress profiles on cells, particularly with sub-cellular resolution, in controlled in vitro experiments. Here we report a new type of active cell culture material that allows highly localized, directional and reversible deformation of the cell growth substrate, with control at scales ranging from the entire surface to the subcellular, and response times on the order of seconds. These capabilities are not matched by any other method, and this versatile material has the potential to bridge the performance gap between the existing single cell micro-manipulation and 2D cell sheet mechanical stimulation techniques.

Oleoplaning droplets on lubricated surfaces

Recently, there has been much interest in using lubricated surfaces to achieve extreme liquid repellency: a foreign droplet immiscible with the underlying lubricant layer was shown to slide off at a small tilt angle <5°. This behaviour was hypothesized to arise from a thin lubricant overlayer film sandwiched between the droplet and solid substrate, but this has not been observed experimentally. Here, using thin-film interference, we are able to visualize the intercalated film under both static and dynamic conditions. We further demonstrate that for a moving droplet, the film thickness follows the Landau–Levich–Derjaguin law. The droplet is therefore oleoplaning—akin to tyres hydroplaning on a wet road—with minimal dissipative force and no contact line pinning. The techniques and insights presented in this study will inform future work on the fundamentals of wetting for lubricated surfaces and enable their rational design. Lubricated surfaces are known to display extreme liquid repellency. Such behaviour is now confirmed to be due to the formation of a film between the surface and the repelled liquid, with a thickness profile following the Landau–Levich–Derjaguin law.

Reversible Supracolloidal Self-Assembly of Cobalt Nanoparticles to Hollow Capsids and Their Superstructures

The synthesis and spontaneous, reversible supracolloidal hydrogen bond-driven self-assembly of cobalt nanoparticles (CoNPs) into hollow shell-like capsids and their directed assembly to higher order superstructures is presented. CoNPs and capsids form in one step upon mixing dicobalt octacarbonyl (Co2 CO8 ) and p-aminobenzoic acid (pABA) in 1,2-dichlorobenzene using heating-up synthesis without additional catalysts or stabilizers. This leads to pABA capped CoNPs (core ca. 5 nm) with a narrow size distribution. They spontaneously assemble into tunable spherical capsids (d≈50-200 nm) with a few-layered shells, as driven by inter-nanoparticle hydrogen bonds thus warranting supracolloidal self-assembly. The capsids can be reversibly disassembled and reassembled by controlling the hydrogen bonds upon heating or solvent exchanges. The superparamagnetic nature of CoNPs allows magnetic-field-directed self-assembly of capsids to capsid chains due to an interplay of induced dipoles and inter-capsid hydrogen bonds. Finally, self-assembly on air-water interface furnishes lightweight colloidal framework films.

A long-lasting concentration cell based on a magnetic electrolyte

A concentration cell is composed of two equivalent half-cells made of the same material but differing in the concentration of reactants. As these concentrations equilibrate, the increase in entropy is converted into a flow of electricity with the voltage output determined by the Nernst equation and proportional to the logarithm of the concentration ratios. However, as diffusion constantly strives to erase all concentration gradients, concentration cells produce only moderate voltages (typically tens of millivolts at room temperature) over relatively short times and, consequently, such devices have not been regarded as promising for energy storage. Here, we report a concentration cell that produces significantly higher voltages (∼ 0.5 V) for over 100 h. The key to our design is that the citric acid molecules involved in the electrode reactions are tethered onto magnetic nanoparticles, and a sharp gradient (10(7)-10(11) anode/cathode concentration ratio) is maintained at one of the electrodes by a permanent magnet external to the cell. Our cell does not result in corrosion of the electrodes, produces no harmful by-products, and can be regenerated by recoating used nanoparticles with fresh citric acid. We show that a series of such centimetre-sized cells produces enough electricity to power small electronic devices (timers and calculators) for several tens of hours. Our results illustrate how redox-active molecules that are, in themselves, non-magnetic can be effectively concentrated by magnetic fields to produce electrical energy.

Cobalt Nanoparticle Langmuir−Schaefer Films on Ethylene Glycol Subphase

The Langmuir-Schaefer (LS) technique was applied to prepare two-dimensional films of tridodecylamine (TDA)-stabilized Co nanoparticles. Ethylene glycol was used as the subphase because the Co nanoparticles spread better on it than on water. Surface pressure-area isotherms provided very little information on the floating films, and Brewster angle microscopy (BAM) was needed to characterize the film formation in situ. In addition to the subphase, various other experimental factors were tested in the LS film preparation, including solvent and presence of free TDA ligands and poly(styrene-b-ethylene oxide) (PS-b-PEO) in the nanoparticle dispersion. LS films deposited from dispersions from which the excess TDA ligands had been removed by washing the Co nanoparticles with 2-propanol consisted of hexagonally organized particles in rafts that were organized in necklace structures. The addition of PS-b-PEO to the deposition dispersion resulted in small nanoparticle rafts evenly distributed over the substrate surface. The best Co-nanoparticle-PS-b-PEO films were obtained with a mass ratio of 20:1 between Co (9 nm) and block copolymer (38 200 g/mol, PEO content 22 mass %). These films were successfully transferred onto Formvar-coated TEM grids and characterized by transmission electron microscopy (TEM) and a superconducting quantum interference device (SQUID) magnetometer. At room temperature the films showed superparamagnetic behavior with a saturation magnetization M(s) of 100 emu/g (Co). Our work indicates that it is possible to obtain thin superparamagnetic LS films of TDA-stabilized Co nanoparticles. This is an important result as the TDA-stabilized Co nanoparticles show a very good resistance to corrosion.

Magnetofluidic Tweezing of Nonmagnetic Colloids

Magnetofluidic tweezing based on negative magnetophoresis and microfabricated core-shell magnetic microtips allows controlled on-demand assembly of colloids and microparticles into various static and dynamic structures such as colloidal crystals (as shown for 3.2 μm silica particles).