Tim Still

Suppression of the coffee-ring effect by shape-dependent capillary interactions

When a drop of liquid dries on a solid surface, its suspended particulate matter is deposited in ring-like fashion. This phenomenon, known as the coffee-ring effect, is familiar to anyone who has observed a drop of coffee dry. During the drying process, drop edges become pinned to the substrate, and capillary flow outward from the centre of the drop brings suspended particles to the edge as evaporation proceeds. After evaporation, suspended particles are left highly concentrated along the original drop edge. The coffee-ring effect is manifested in systems with diverse constituents, ranging from large colloids to nanoparticles and individual molecules. In fact—despite the many practical applications for uniform coatings in printing, biology and complex assembly—the ubiquitous nature of the effect has made it difficult to avoid. Here we show experimentally that the shape of the suspended particles is important and can be used to eliminate the coffee-ring effect: ellipsoidal particles are deposited uniformly during evaporation. The anisotropic shape of the particles significantly deforms interfaces, producing strong interparticle capillary interactions. Thus, after the ellipsoids are carried to the air–water interface by the same outward flow that causes the coffee-ring effect for spheres, strong long-ranged interparticle attractions between ellipsoids lead to the formation of loosely packed or arrested structures on the air–water interface. These structures prevent the suspended particles from reaching the drop edge and ensure uniform deposition. Interestingly, under appropriate conditions, suspensions of spheres mixed with a small number of ellipsoids also produce uniform deposition. Thus, particle shape provides a convenient parameter to control the deposition of particles, without modification of particle or solvent chemistry.

High-modulus organic glasses prepared by physical vapor deposition.

Adv. Mater. 2010, 22, 39–42 2010 WILEY-VCH Verlag Gmb T IO N A wide range of packing structures are available to glasses, with more efficient packing leading to materials exhibiting higher moduli. Aging a glass allows for better packing and a higher modulus, but even long aging times increase themodulus by only a few percent; preparing high-modulus materials in this manner is impractical. Using Brillouin light scattering spectroscopy, we show that physical vapor deposition can be used to circumvent this kinetic limitation and produce glasses whose moduli exceed those of the ordinary glass by up to 19%. These high-modulus glasses resist thermal treatment and take at least 10 times longer than the structural relaxation time to transform to the supercooled liquid. The facile production of high-modulus glasses will likely prove useful for fundamental investigations and coating technologies. Unlike their crystalline counterparts, glasses have a nearly limitless array of packing arrangements. As a supercooled liquid is cooled, molecular motions eventually slow to such an extent that equilibrium cannot be maintained. Below this transition temperature Tg, a mechanically stable, non-equilibrium glass is formed. Glasses slowly evolve towards equilibrium (i.e., aging) in a process that optimizes packing and creates higher moduli materials. The structural relaxation time ta dictates the rate at which this process takes place, and due to the steep temperature dependence near Tg, ta is on the order of days only a few degrees below Tg. The aging process thus increases the moduli so slowly that in practice changes of only a few percent are possible. If high-modulus amorphous materials are to be utilized for science and technology, new preparation techniques are needed which circumvent these kinetic restrictions and allow for more optimized amorphous packing. In this work, we show that high-modulus organic glasses can be made efficiently with physical vapor deposition. Using this preparation technique, we can avoid the kinetic limitations of aging and prepare high-modulus glasses in a matter of hours. Enhanced dynamics at the surface of amorphous materials allows for rapid configurational sampling in the top few nanometers. Vapor deposition can build an efficiently packed amorphous material in a layer-by-layer fashion by taking advantage of enhanced surface dynamics and thus is not limited by the slow relaxation kinetics of the bulk. We determine the mechanical properties of such vapor-deposited films using Brillouin light scattering spectroscopy (BLS). Because of the non-destructive nature of BLS, the moduli of the as-deposited glass, the supercooled liquid, and ordinary glass (created by cooling the liquid) can be determined from a single sample. Physical vapor depositions were performed separately on two organic-glass-formingmaterials: indomethacin (IMC, Tg1⁄4 315K) and trisnaphthylbenzene (TNB, Tg1⁄4 348K). These twomolecules are well-known glass-formers, and their glasses have been previously prepared with physical vapor deposition. During deposition, the temperature of the substrate Tsubstrate and the deposition rate are the important control parameters. For this work, Tsubstrate was near 0.85 Tg, i.e., 265K for IMC and room temperature ( 295K) for TNB. The rate of the deposition in all cases was held at 0.2 0.03 nm s 1 until a thickness of 10–15mm was reached. It has previously been shown that these deposition conditions produce glasses with low enthalpy, high density, and high kinetic stability. Tsubstrate was controlled by attaching the SiO2 substrates to a copper temperature stage (Fig. 1a). The deposition rate was controlled by adjusting the temperature of a crucible containing either IMC or TNB. Further details are given in the Experimental section. BLS detects photons that are inelastically scattered by propagating density fluctuations (phonons). The frequency shift between the incident laser and the scattered light is analyzed by a high-resolution tandem Fabry–Perot interferometer (details in Experimental section). The incident laser polarization was chosen to be perpendicular to the scattering plane. Scattered light polarized perpendicular and parallel to the incident polarization was measured in separate experiments, providing access to scattering from longitudinal and transverse phonons, respectively. The scattering from these two polarizations is shown in Figure 1d for vapor-deposited IMC glass at 298 K (lower panel) and supercooled liquid IMC at 336K (upper panel). Stokes and anti-Stokes shifts by the longitudinal (L) and transverse (T) phonons are observed to the left and right of the Rayleigh line region (shaded area), respectively. The vertical lines drawn on the Stokes side of the spectrum illustrate the temperatureindependent scattering of the SiO2 substrate and the temperature-dependent scattering of IMC. The absence of longitudinal phonons in the transverse spectrum indicates no birefringence in the vapor-deposited sample within the sensitivity of BLS. This is fully consistent with the amorphous nature of the stable glass as documented by wide-angle X-ray scattering and observations of crystal growth on very long timescales. The peaks in the BLS spectra provide access to the phase velocities cl,t for the L and T phonons and through this route the moduli can be obtained. Spectra similar to those found in

Effects of particle shape on growth dynamics at edges of evaporating drops of colloidal suspensions.

We study the influence of particle shape on growth processes at the edges of evaporating drops. Aqueous suspensions of colloidal particles evaporate on glass slides, and convective flows during evaporation carry particles from drop center to drop edge, where they accumulate. The resulting particle deposits grow inhomogeneously from the edge in two dimensions, and the deposition front, or growth line, varies spatiotemporally. Measurements of the fluctuations of the deposition front during evaporation enable us to identify distinct growth processes that depend strongly on particle shape. Sphere deposition exhibits a classic Poisson-like growth process; deposition of slightly anisotropic particles, however, belongs to the Kardar-Parisi-Zhang universality class, and deposition of highly anisotropic ellipsoids appears to belong to a third universality class, characterized by Kardar-Parisi-Zhang fluctuations in the presence of quenched disorder.

Synthesis of micrometer-size poly(N-isopropylacrylamide) microgel particles with homogeneous crosslinker density and diameter control

Poly(N-isopropylacrylamide) microgel particles are synthesized using a semi-batch surfactant-free emulsion polymerization method. Particle diameter can be precisely adjusted by controlling the initial conditions, the electrolyte concentration, and the monomer feeding rate and duration. Larger particles are obtained in the presence of small amounts of co-monomer with cationic amino groups that compete against the negative charges arising from the initiator. Monodisperse particles with uniform cross-linker density, homogeneous optical properties, and pronounced thermoresponsivity are readily produced with a wide variety of diameters ranging from several hundred nanometers to a few micrometers. The charge stabilization mechanisms that control particle growth are discussed.

Colloidal systems: a promising material class for tailoring sound propagation at high frequencies

In this paper we report on the phononic properties of mesoscopic colloidal materials. Using high resolution Brillouin light scattering (BLS) the resonance modes of submicron particles as well as the dispersion relations of their ordered assemblies in a liquid matrix are studied. Two different kinds of very recently realized acoustic bandgaps are presented. In colloidal mixtures, the particle acoustic resonances are independent of composition.

The "Music" of Core-Shell Spheres and Hollow Capsules: Influence of the Architecture on the Mechanical Properties at the Nanoscale

We report on the first measurement of elastic vibrational modes in core-shell spheres (silica-poly(methyl methacrylate), SiO2-PMMA) and corresponding spherical hollow capsules (PMMA) with different particle size and shell thickness using Brillouin light scattering, supported by numerical calculations. These localized modes allow access to the mechanical moduli down to a few tens of nanometers. We observe reduced mechanical strength of the porous silica core, and for the core-shell spheres a striking increase of the moduli in both the SiO2 core and the PMMA shell. The peculiar behavior of the vibrational modes in the hollow capsules is attributed to antagonistic dependence on overall size and layer thickness in agreement with theoretical predictions.

Diagnosing hyperuniformity in two-dimensional, disordered, jammed packings of soft spheres

Hyperuniformity characterizes a state of matter for which (scaled) density fluctuations diminish towards zero at the largest length scales. However, the task of determining whether or not an image of an experimental system is hyperuniform is experimentally challenging due to finite-resolution, noise, and sample-size effects that influence characterization measurements. Here we explore these issues, employing video optical microscopy to study hyperuniformity phenomena in disordered two-dimensional jammed packings of soft spheres. Using a combination of experiment and simulation we characterize the possible adverse effects of particle polydispersity, image noise, and finite-size effects on the assignment of hyperuniformity, and we develop a methodology that permits improved diagnosis of hyperuniformity from real-space measurements. The key to this improvement is a simple packing reconstruction algorithm that incorporates particle polydispersity to minimize the free volume. In addition, simulations show that hyperuniformity in finite-sized samples can be ascertained more accurately in direct space than in reciprocal space. Finally, our experimental colloidal packings of soft polymeric spheres are shown to be effectively hyperuniform.

Chiral Structures And Defects Of Lyotropic Chromonic Liquid Crystals Induced By Saddle- Splay Elasticity

An experimental and theoretical study of lyotropic chromonic liquid crystals (LCLCs) confined in cylinders with degenerate planar boundary conditions elucidates LCLC director configurations. When the Frank saddle-splay modulus is more than twice the twist modulus, the ground state adopts an inhomogeneous escaped-twisted configuration. Analysis of the configuration yields a large saddle-splay modulus, which violates Ericksen inequalities but not thermodynamic stability. Lastly, we observe point defects between opposite-handed domains, and we explain a preference for point defects over domain walls.

Rheology of soft colloids across the onset of rigidity: scaling behavior, thermal, and non-thermal responses

We study the rheological behavior of colloidal suspensions composed of soft sub-micron-size hydrogel particles across the liquid-solid transition. The measured stress and strain-rate data, when normalized by thermal stress and time scales, suggest our systems reside in a regime wherein thermal effects are important. In a different vein, critical point scaling predictions for the jamming transition, typical in athermal systems, are tested. Near dynamic arrest, the suspensions exhibit scaling exponents similar to those reported in Nordstrom et al., Phys. Rev. Lett., 2010, 105, 175701. The observation suggests that our system exhibits a glass transition near the onset of rigidity, but it also exhibits a jamming-like scaling further from the transition point. These observations are thought-provoking in light of recent theoretical and simulation findings, which show that suspension rheology across the full range of microgel particle experiments can exhibit both thermal and athermal mechanisms.

Phonons in two-dimensional soft colloidal crystals

The vibrational modes of pristine and polycrystalline monolayer colloidal crystals composed of thermosensitive microgel particles are measured using video microscopy and covariance matrix analysis. At low frequencies, the Debye relation for two-dimensional harmonic crystals is observed in both crystal types; at higher frequencies, evidence for van Hove singularities in the phonon density of states is significantly smeared out by experimental noise and measurement statistics. The effects of these errors are analyzed using numerical simulations. We introduce methods to correct for these limitations, which can be applied to disordered systems as well as crystalline ones, and we show that application of the error correction procedure to the experimental data leads to more pronounced van Hove singularities in the pristine crystal. Finally, quasilocalized low-frequency modes in polycrystalline two-dimensional colloidal crystals are identified and demonstrated to correlate with structural defects such as dislocations, suggesting that quasilocalized low-frequency phonon modes may be used to identify local regions vulnerable to rearrangements in crystalline as well as amorphous solids.