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ABSTRACT: We here demonstrate the experimental realization of inanimate micro-swimmer
complexes showing emergent self-propulsion at low Reynolds number Re < 10^(-4)
in quasi 2D colloidal systems. Guided by the substrate, self-organized
propulsion occurs for up to 25min with typical velocities of 1-3 \mu m/s, while
none of the involved constituents shows self-propulsion on its own.
11/2012;
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ABSTRACT: Growth of heterogeneously nucleated, wall based crystals plays a major role in determining the micro-structure during melt casting. This issue is here addressed using a model system of charged colloidal spheres in deionized aqueous suspension observed by Bragg microscopy which is a combination of light scattering and microscopy. We examine the evolution of the three-dimensional size, shape, and orientation of twin domains in monolithic crystals growing from two opposing planar walls into a meta-stable (shear-) melt. At each wall crystal orientation and twinning emerges during nucleation with small domains. During growth these widen and merge. From image analysis we observe the lateral coarsening velocities to follow a power law behaviour L(XY) ∝ t(1∕2) as long as the vertical growth continues at constant speed. Lateral coarsening terminates upon intersection of the two solids and hardly any further ripening is seen. Initial lateral coarsening velocities show a Wilson Frenkel type dependence on the melt meta-stability.
The Journal of chemical physics 09/2012; 137(9):094906. · 3.09 Impact Factor
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ABSTRACT: We investigated the crystallization scenario of highly cross linked polystyrene particles dispersed in the good solvent 2-ethylnaphtalene and their mixtures with non-adsorbing low molecular weight polysterene polymer using time resolved static light scattering. The samples were prepared slightly below the melting volume fraction of the polymer free system. For the polymer free samples, we obtained polycrystalline solids via crystallization scenario known from hard sphere suspensions with little competition of wall crystal formation. Addition of non-adsorbing low molecular weight polystyrene polymer leads to a considerably slowing down of the bulk crystallization kinetics. We observed a delay of the precursor to crystal conversion for the bulk crystallization while the induction times for the wall nucleation are reduced. The increased polymer concentration thus shifts the balance between the two competing crystallization pathways giving the possibility to tune the relative amount of wall based crystals.
The Journal of chemical physics 06/2012; 136(23):234906. · 3.09 Impact Factor
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ABSTRACT: Understanding the process that drives an undercooled fluid to the crystal state is still a challenging issue for condensed matter physics and plays a key role in designing new materials. The crystallization kinetics and the resulting polycrystalline morphology are given by a complex interplay of crystal nucleation, growth, and ripening. A great deal of progress has been made in recent years using colloidal suspensions as model systems in the study of crystallization. Close analogies to atomic systems are observed which can be exploited to address questions not accessible in atomic solidification. Here we present systematic measurements of the crystallization kinetics of a charged colloidal model system adding small amounts of seeds using time resolved scattering techniques. Large seeds show strong sedimentation under gravity even on the time scale of the crystallization process. To avoid this problem we performed our measurements under microgravity during parabolic flights. We report how the seed size and structure affect crystal nucleation and growth as functions of metastability giving the possibility to modify the crystallization process and the resulting microstructure of the polycrystal.
Physical Review E 05/2011; 83(5 Pt 1):051405. · 2.26 Impact Factor
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ABSTRACT: We report on the crystallization kinetics in an entropically attractive colloidal system using a combination of time resolved scattering methods and microscopy. Hard sphere particles are polystyrene microgels swollen in a good solvent (radius a=380 nm, starting volume fraction 0.534) with the short ranged attractions induced by the presence of short polymer chains (radius of gyration r g=3 nm, starting volume fraction 0.0224). After crystallization, stacking faulted face centered cubic crystals coexist with about 5% of melt remaining in the grain boundaries. From the Bragg scattering signal we infer the amount of crystalline material, the average crystallite size and the number density of crystals as a function of time. This allows to discriminate an early stage of conversion, followed by an extended coarsening stage. The small angle scattering (SALS) appears only long after completed conversion and exhibits Furukawa scaling for all times. Additional microscopic experiments reveal that the grain boundaries have a reduced Bragg scattering power but possess an increased refractive index. Fits of the Furukawa function indicate that the dimensionality of the scatterers decreases from 2.25 at short times to 1.65 at late times and the characteristic length scale is slightly larger than the average crystallite size. Together this suggests the SALS signal is due scattering from a foam like grain boundary network as a whole.
Physical Review E 05/2010; 81(5 Pt 1):051401. · 2.26 Impact Factor
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ABSTRACT: In titration experiments with NaOH, we have determined the full phase diagram of charged colloidal spheres in dependence on the particle density n, the particle effective charge Z(eff) and the concentration of screening electrolyte c using microscopy, light and ultrasmall angle x-ray scattering (USAXS). For sufficiently large n, the system crystallizes upon increasing Z(eff) at constant c and melts upon increasing c at only slightly altered Z(eff). In contrast to earlier work, equilibrium phase boundaries are consistent with a universal melting line prediction from computer simulation, if the elasticity effective charge is used. This charge accounts for both counterion condensation and many-body effects.
The Journal of chemical physics 04/2010; 132(13):131102. · 3.09 Impact Factor
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ABSTRACT: We review recent work on the phase behaviour of binary charged sphere mixtures as a function of particle concentration and composition. Both size ratios G and charge ratios L are varied over a wide range. Unlike the case for hard spheres, the long-ranged Coulomb interaction stabilizes the crystal phase at low particle concentrations and shifts the occurrence of amorphous solids to particle concentrations considerably larger than the freezing concentration. Depending on G and L, we observe upper azeotrope, spindle, lower azeotrope and eutectic types of phase diagrams, all known well from metal systems. Most solids are of body centred cubic structure. Occasionally stoichiometric compounds are formed at large particle concentrations. For very low G, entropic effects dominate and induce a fluid–fluid phase separation. Since for charged spheres the charge ratio L is also decisive for the type of phase diagram, future experiments with charge variable silica spheres are suggested.
Journal of Physics Condensed Matter 11/2009; · 2.55 Impact Factor
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ABSTRACT: We studied the competition between heterogeneous and homogeneous nucleation of an aqueous suspension of charged colloidal spheres close to the container walls. Samples of equilibrium crystalline structure were shear-melted and the metastable melt left to solidify after the cessation of shear. The crystallization kinetics was monitored using time-resolved scattering techniques: at low particle number densities n we applied an improved static light scattering method while at large particle concentrations ultra-small-angle x-ray scattering was applied for the first time. Our results show some unexpected behavior: the heterogeneous nucleation at the container walls is delayed in comparison to the homogeneous bulk nucleation and its rate density appears surprisingly slightly smaller, demonstrating the complexity of the observed crystallization process.
Journal of Physics Condensed Matter 10/2009; 21(46):464115. · 2.55 Impact Factor
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ABSTRACT: We report on the phase behavior of an aqueous binary charged sphere suspension under exhaustively de-ionized conditions as a function of number fraction of small particles p and total number density n. The mixture of size ratio Gamma=0.557 displays a complex phase diagram. Formation of bcc crystals with no compositional order dominates. We observe a region of drastically decreased crystal stability at 0.55<p<0.95 with the minimum located at p=0.8+/-0.05 at densities above n=26 microm(-3). A peaked region of enhanced stability is observed at p=0.4. Further light scattering experiments were conducted to characterize the crystallization time scales, the density profiles, and the composition of formed phases. For 0.82>p>0.95 crystal formation is partially assisted by gravity, i.e., gravitational separation of the two species precedes crystal formation for samples in the coexistence range. In the composition range corresponding to the decreased crystal stability only lower bounds of the freezing and melting line are obtained, but the general shape of the phase diagram is retained. At p=0.93 and n=43 microm(-3) two different crystalline phases coexist in the bulk, while at p=0.4 additional Bragg peaks appear in the static light scattering experiments. This strongly suggests that we observe an eutectic in the region of decreased stability, while the enhanced stability at p=0.4 seems to correlate with compound formation.
The Journal of chemical physics 10/2009; 131(13):134501. · 3.09 Impact Factor
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ABSTRACT: We report on the phase behavior of a binary charged sphere mixture of size ratio Gamma=0.68 and charge ratio Lambda approximately 1 as a function of composition p and number density n . For p=0.1-0.3 we observe freezing at densities well below the freezing densities of the pure components. At all compositions our data indicate the formation of substitutional alloy crystals of body centered cubic structure. No indications for compound formation were observed. Rather, our findings point at the first observation of an upper azeotrope. Measurements of the crystallization kinetics reveal a combined density and composition dependence of growth velocities and nucleation rate densities, with small but significant anomalies at p*=0.2 . These correlated deviations can be rationalized within classical theories of solidification and suggest an increased similarity between melt and solid for this particular composition.
Physical Review E 08/2009; 80(2 Pt 1):021407. · 2.26 Impact Factor
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ABSTRACT: Highly cross-linked polystyrene microgel colloids dispersed in an index and density matching solvent provide a system with hard-sphere-like interactions, where gravity effects are effectively minimized. They are a suitable target for time-resolved observations of solidification in purely repulsive systems. We have investigated the crystallization kinetics at increasing undercooling using time resolved light scattering. Crystallization starts always with the formation of compressed, structurally heterogeneous precursor domains. In the coexistence region the precursors, after being converted into true crystallites, start growing fast by assimilating particles from the melt. The resulting polycrystalline material consists of high quality crystals and seems not to undergo long time-scale rearrangements. As the particle concentration grows, the higher undercooling and reduced particle mobility increasingly compromise the conversion-growth process. The growth of crystallites relies then on much slower ripeninglike processes, while refining of the crystal structure is detected up to the longest observed times.
The Journal of chemical physics 03/2009; 130(8):084502. · 3.09 Impact Factor
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ABSTRACT: We report on the crystal growth scenario in gravity-matched, polydisperse hard-sphere-like colloids at increasing particle concentration. In the fluid-crystal coexistence region, the crystal size as a function of time shows two separate regimes corresponding to crystal growth and crystal ripening. At higher supersaturation the crystal size grows according to the same power law through the whole experimental window of a few days: crystal growth and ripening merge together. We show that our observations cannot be explained by considering the slowing down of single-particle dynamics due to increasing volume fraction. We suggest that size fractionation occurring at the crystal-fluid interface is the dominant mechanism.
Physical Review E 02/2009; 79(1 Pt 1):010601. · 2.26 Impact Factor
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ABSTRACT: We report on a novel and flexible experiment to investigate the non-equilibrium melting behaviour of model crystals made from charged colloidal spheres. In a slit geometry polycrystalline material formed in a low salt region is driven by hydrostatic pressure up an evolving gradient in salt concentration and melts at large salt concentration. Depending on particle and initial salt concentration, driving velocity and the local salt concentration complex morphologic evolution is observed. Crystal-melt interface positions and the melting velocity are obtained quantitatively from time resolved Bragg- and polarization microscopic measurements. A simple theoretical model predicts the interface to first advance, then for balanced drift and melting velocities to become stationary at a salt concentration larger than the equilibrium melting concentration. It also describes the relaxation of the interface to its equilibrium position in a stationary gradient after stopping the drive in different manners. We further discuss the influence of the gradient strength on the resulting interface morphology and a shear induced morphologic transition from polycrystalline to oriented single crystalline material before melting.
12/2008;
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ABSTRACT: Colloidal model systems have been used for over three decades for investigating liquids, crystals, and glasses. Colloidal crystal superstructures have been observed in binary systems of repulsive spheres as well as oppositely charged sphere systems showing structures well known from atomic solids. In this work we study the structural transition of colloidal crystals under confinement. In addition to the known sequence of crystalline structures, crystal superstructures with dodecagonal and hexagonal symmetry are observed in one component systems. These structures have no atomic counterpart.
Physical Review E 06/2008; 77(6 Pt 1):061401. · 2.26 Impact Factor
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ABSTRACT: We have characterized a close-packed structure of confined charged colloidal spheres, which has been recently discovered. Using different microscopy experiments, the vertically arranged hexagonal planes of n - hcp perpendicular are found to continuously evolve from the horizontally oriented stacks of n hexagonal planes (nDelta) following the maximum packing criterion, but discontinuously transform to a stack of n+1 square planes [(n+1)[SHAPE OF A SQUARE]]. Large mechanically stable domains with threefold twin structures are regularly observed in the suspended state at packing fractions between 0.4 and 0.58.
Physical Review E 12/2007; 76(5 Pt 1):050402. · 2.26 Impact Factor
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ABSTRACT: We present a comprehensive study of the solidification scenario in suspensions of colloidal hard spheres for three polydispersities between 4.8% and 5.8%, over a range of volume fractions from near freezing to near the glass transition. From these results, we identify four stages in the crystallization process: (i) an induction stage where large numbers of precursor structures are observed, (ii) a conversion stage as precursors are converted to close packed structures, (iii) a nucleation stage, and (iv) a ripening stage. It is found that the behavior is qualitatively different for volume fractions below or above the melting volume fraction. The main effect of increasing polydispersity is to increase the duration of the induction stage, due to the requirement for local fractionation of particles of larger or smaller than average size. Near the glass transition, the nucleation process is entirely frustrated, and the sample is locked into a compressed crystal precursor structure. Interestingly, neither polydispersity nor volume fraction significantly influences the precursor stage, suggesting that the crystal precursors are present in all solidifying samples. We speculate that these precursors are related to the dynamical heterogeneities observed in a number of dynamical studies.
The Journal of Chemical Physics 09/2007; 127(8):084505. · 3.33 Impact Factor
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ABSTRACT: We have studied the nucleation kinetics of charged colloidal model systems under salt free conditions crystallizing in bcc structure covering a wide range of particle number densities 18 microm(-3) < or =n< or =66.3 microm(-3). We employed direct video-microscopic observation of individual nucleation events to obtain time resolved nucleation rate densities. Polarization microscopy and static light scattering on the resulting solids in combination with Avrami theory is used to determine the steady state nucleation rate at high undercoolings. The final nucleation rate densities J from different methods are observed to be consistent with each other. By increasing the difference in the chemical potential between melt and crystal Delta mu about one order of magnitude J increases from 10(9)m(-3)s(-1) to 10(17)m(-3)s(-1) over approximately seven orders of magnitude. The data can be well analyzed and interpreted using classical nucleation theory (CNT) leading to a linearly increasing melt-crystal surface tension. Surprisingly, the reduced surface tension is about one order of magnitude larger compared to other system (metals; hard sphere colloids). The critical radius of the crystal nuclei is decreasing down to a very small value of 1.5 coordination shells. The determined kinetic prefactors are up to 10 orders of magnitude smaller than the prefactor calculated by CNT.
Physical Review E 05/2007; 75(5 Pt 1):051405. · 2.26 Impact Factor
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ABSTRACT: We present hard-sphere crystallization kinetics for three samples with small differences in polydispersity. We show that an increase in polydispersity of 1% is sufficient to cause dramatic changes in the crystallization kinetics: crystallization is delayed by almost one decade in time and quantitative and qualitative changes in the crystallization scenario are observed. Surprisingly the nucleation rate density is enhanced by almost a factor of 10. We interpret these results in terms of polydispersity limited growth, where local fractionation processes lead to a delayed but faster nucleation.
Physical Review E 01/2007; 74(6 Pt 1):060401. · 2.26 Impact Factor
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ABSTRACT: The shear modulus G of charged colloidal crystals was measured at several constant particle densities n and varying salt concentrations c up to the melting salt concentration cM using torsional resonance spectroscopy. Far from the phase boundary the samples are polycrystalline and the shear modulus stays roughly constant as a function of c. Upon approaching the melting transition an increasing amount of wall based crystal material is formed surrounding a shrinking polycrystalline core and G drops nearly linearly. When the transition is complete G again stays constant. The morphologic transitions may be scaled upon a single master curve. For the polycrystalline morphology, the elastic data are evaluated in terms of a pairwise additive screened Coulomb interaction yielding a particle effective charge Z(G)*. Under de-ionized conditions Z(0,G)* is independent of n and significantly lower than expected from charge renormalization theory. With increasing salt concentration Z(G)* increases. The increase becomes more pronounced at larger n. By extrapolation we further obtain the melting line effective elasticity charge Z(M,G)*. Z(M,G)* shows a steplike increase with increasing nM and cM to values consistent with charge renormalization theory. Interestingly, the increase coincides semi-quantitatively with the one expected from the universal melting line for charged spheres, thus facilitating a consistent description of phase behavior and elasticity over an extended range of the phase diagram.
The Journal of Chemical Physics 12/2006; 125(19):194714. · 3.33 Impact Factor
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ABSTRACT: The Debye-Hückel-Potential in combination with an effective or renormalized charge is a widely and often successfully used
concept to describe the interaction in charged colloidal model systems and the resulting suspension properties. In particular
the phase behaviour can be described in dependence of the parameters particle number density, salt concentration and effective
charge. We performed simultaneous measurements of the phase behaviour, the shear modulus and the low frequency conductivity
of deionised aqueous suspensions of highly charged colloidal spheres. From the shear modulus the interaction potential at
the nearest neighbour distance in terms of a Debye-Hückel potential can be determined with an effective charge Z*G as free parameter. Conductivity measures the number of freely moving small ions Z*σ and thus relates to the ion condensation process in the electric double layer under conditions of finite macroion concentrations.
We present the first experimental access of the pair energy of interaction in charged colloidal suspensions which describes
both the elastic properties and the fluid crystalline phase behaviour. This means that a consistent description of the suspension
properties is obtained, when Z*G is taken from the elasticity measurement.
08/2006: pages 88-94;
