Carlos G Lopez | RWTH Aachen University (original) (raw)

Papers by Carlos G Lopez

A comparative examination is presented of materials and approaches for the fabrication of microfl... more A comparative examination is presented of materials and approaches for the fabrication of microfluidic devices for small-angle neutron scattering (SANS). Representative inorganic glasses, metals, and polymer materials and devices are evaluated under typical SANS configurations. Performance criteria include neutron absorption, scattering background and activation, as well as spatial resolution, chemical compatibility and pressure resistance, and also cost, durability and manufacturability. Closed-face polymer photolithography between boron-free glass (or quartz) plates emerges as an attractive approach for rapidly prototyped microfluidic SANS devices, with transmissions up to $98% and background similar to a standard liquid cell (I ' 10 À3 cm À1). For applications requiring higher durability and/or chemical, thermal and pressure resistance, sintered or etched boron-free glass and silicon devices offer superior performance, at the expense of various fabrication requirements, and are increasingly available commercially.

We present steady shear rheology data for sodium polystyrene sulfonate (NaPSS) in semidilute unen... more We present steady shear rheology data for sodium polystyrene sulfonate (NaPSS) in semidilute unentangled salt-free aqueous solution as a function of polymer concentration (c) and degree of polymerisation (N). The measured terminal modulus (G) agrees with the scaling prediction of G k B Tc/N. The specific viscosity varies with the degree of polymerisation as η sp ∝ N 1.24±0.08. The observed dependence differs with the linear relation expected from the Rouse model for ideal chains as predicted by de Gennes' scaling model and subsequent theories. Together with the diffusion data of Oostwal et al., our results suggest that chains may follow non-Gaussian statistics beyond the correlation length (ξ). Small angle neutron scattering data on salt-free semidilute solutions at low polymer concentrations partially support this hypothesis but do not confirm it. The electrostatic Kuhn length of NaPSS salt-free solutions is found to be proportional and slightly larger than the correlation length (L K ,e 1.3ξ). This result agrees well with the scaling model of Dobrynin et al. Radii of gyra-tion (R g) data as a function of polymer concentration reveal that a concentrated regime (R g ∝ c 0) does not occur for NaPSS in the concentration range considered (c 4M). We conclude by comparing the predictions of the scaling model of Dobrynin et al. and Muthukumar's double screening theory with viscosity data for different polyelectrolyte-solvent systems. Dobrynin's model provides a better description of experimental observations. Our findings confirm several aspects of current models for polyelectrolyte solutions but some deviations from theory remain. Published by AIP Publishing.

Reported is an unanticipated mechanism of attractive electrostatic interactions of fully neutrali... more Reported is an unanticipated mechanism of attractive electrostatic interactions of fully neutralized polyacrylic acid (PAA) with like-charged surfactants. Amphiphilic polymer -surfactant complexes with high interfacial activity and a solubilization capacity exceeding that of conventional micelles are formed by bridging with Ca 2+ ions. Incorporation of a protease into such dynamic nanoreactors results in a synergistically enhanced cleaning performance because of the improved solubilization of poorly water-soluble immobilized proteins. Competitive interfacial and intermolecular interactions on different time-and length-scales have been resolved using colorimetric analysis, dynamic tensiometry, light scattering, and molecular dynamic simulations. The discovered bridging association mechanism suggests reengin-eering of surfactant/polymer/enzyme formulations of modern detergents and opens new opportunities in advancing labile delivery systems.

Intermediate filaments are a major structural element in the cytoskeleton of animal cells that me... more Intermediate filaments are a major structural element in the cytoskeleton of animal cells that mechanically integrate other cytoskeletal components and absorb externally applied stress. Their role is likely to be linked to their complex molecular architecture which is the product of a multi-step assembly pathway. Intermediate filaments form tetrameric subunits which assemble in the presence of monovalent salts to form unit length filaments that subsequently elongate by end–to–end annealing. The present work characterises this complex assembly process using reconstituted vimentin intermediate filaments with monovalent salts as an assembly trigger. A multi-scale approach is used, comprising static light scattering, dynamic light scattering and quantitative scanning transmission electron microscopy (STEM) mass measurements. Light scattering reveals the radius of gyration (R g), molecular weight (M w) and diffusion coefficient (D) of the assembling filaments as a function of time and salt concentration (c S) for the given protein concentration of 0.07 g/L. We show that filament formation is accelerated with increasing c S. At low c S (10 mM KCl) no lateral or elongational growth is observed, whereas at c S = 50 – 200 mM, the hydrodynamic cross-sectional radius and the elongation rate increases with c S. R g versus M w plots suggest that the mass per unit length increases with increasing salt content, which is confirmed by STEM mass measurements. A kinetic model based on rate equations for a two step process is able to accurately describe the variation of mass, length and diffusion coefficient of the filaments with time and provides a consistent description of the elongation accelerated by increasing c S .

The rheology of water-soluble polyelectrolytes at intermediate and high concentrations is control... more The rheology of water-soluble polyelectrolytes at intermediate and high concentrations is controlled by entanglement, hydrophobic, and electrostatic interactions, whose influences are difficult to isolate. We investigate the rheology of semidilute solutions of sodium carboxymethyl cellulose (NaCMC) with molecular weight M w ≃ 2.5 × 10 5 g/ mol and varying degree of substitution (DS) as a function of polymer concentration in various solvent media: salt-free water (long-ranged electrostatic interactions), 0.5 M aqueous NaCl (screened electrostatics), and 0.5 M aqueous NaOH (screened electrostatics, diminished hydrophobic interactions) in order to selectively examine the role played by these different interactions. Decreasing DS is found to decrease solubility and induce partial aggregation and eventual gelation. In salt-free and 0.5 M NaCl solution, NaCMC with DS ≃ 1.2 exhibits hydrophilic polyelectrolyte and neutral polymer in good solvent behavior, respectively. Decreasing DS to ≃0.7−0.8 leads to hydrophobic behavior in both media, becoming weak gels at high concentrations. In 0.5 M NaOH (pH = 13.5) the viscosities of solutions with different DS become identical when plotted against the overlap parameter, which we interpret as resulting from the solubilization of unsubstituted cellulose blocks. Small-angle neutron scattering (SANS) data indicate that the polymer conformation is not strongly affected by hydrophobic interactions. By varying DS, ionic strength, and pH, we demonstrate the tuning of NaCMC−solvent interactions, controlling separately the electrostatic and hydrophobic effects on the solution rheology.

The coupling of droplet microfluidics and Small Angle Neutron Scattering (SANS) is demonstrated w... more The coupling of droplet microfluidics and Small Angle Neutron Scattering (SANS) is demonstrated with a range of model systems: isotopic solvent (H2O/D2O) mixtures, surfactant (sodium dodecyl sulfate, SDS) solutions and colloidal (silica) suspensions. Several droplet carrier phases are evaluated and fluorinated oil emerges as a suitable fluid with minimal neutron background scattering (commensurate with air), and excellent interfacial properties. The combined effects of flow dispersion and compositional averaging caused by the neutron beam footprint are evaluated in both continuous and droplet flows and an operational window is established. Systematic droplet-SANS dilution measurements of colloidal silica suspensions enable unprecedented quantification of form and structure factors, osmotic compressibility, enhanced by constrained global data fits. Contrast variation measurements with over 100 data points are readily carried out in 10-20 min timescales, and validated for colloidal silica of two sizes, in both continuous and droplet flows. While droplet microfluidics is established as an attractive platform for SANS, the compositional averaging imposed by large (≈1 cm) beam footprints can, under certain circumstances, make single phase, continuous flow a preferable option for low scattering systems. We propose simple guidelines to assess the suitability of either approach based on well-defined system parameters.

The swelling of thermoresponsive microgels is widely modelled through Flory-Rehner theory, which ... more The swelling of thermoresponsive microgels is widely modelled through Flory-Rehner theory, which combines Flory-Huggins solution thermodynamics with the affine network model of elasticity. While it has been shown that FR theory closely follows experimental results for a range of systems, the large number of free parameters required to fit size vs. temperature data make a proper evaluation of the theory difficult. In order to test the applicability of FR theory to microgel particles, we analyse viscosity and light scattering data for PNIPAM microgels as a function of temperature, cross-linking degree (f) and molar mass. In the collapsed state, the polymer volume fraction is estimated to be φ C 0.44, independent of cross linking degree and molar mass. Fixing φ C , f and the θ temperature to independent estimates, the FR model appears to describe microgel swelling well, particularly for high cross-linking densities. Estimates for the various fit parameters differ from earlier reports by an order of magnitude. A comparison of the χ parameter obtained from FR theory with values for the linear polymer reveals that the agreement between experiment and theory is somewhat fortuitous. Although the FR model can accurately describe experimental data, the accuracy of the obtained fit parameters is significantly poorer.

We investigate the viscosity dependence on concentration and molecular weight of semiflexible pol... more We investigate the viscosity dependence on concentration and molecular weight of semiflexible polyelectrolyte sodium carboxymethylcellulose (NaCMC) in aqueous salt-free and NaCl solutions. Combining new measurements and extensive literature data, we establish relevant power laws and crossovers over a wide range of degree of polymerization (N) as well as polymer (c) and salt (c s ) concentrations. In salt-free solution, the overlap concentration shows the expected c* ∝ N −2 dependence, and the entanglement crossover scales as c e ∝ N −0.6±0.3 , in strong disagreement with scaling theory for which c e ∝ c* is expected, but matching the behavior found for flexible polyelectrolytes. A second crossover, to a steep concentration dependence for specific viscosity (η sp ∝ c 3.5±0.2 ), commonly assigned to the concentrated regime, is shown to follow c** ∝ N −0.6±0.2 (with c**/c e ≃ 6) which thus suggests instead a dynamic crossover, possibly related to entanglement. The scaling of c* and c e in 0.01 and 0.1 M NaCl shows neutral polymer in good solvent behavior, characteristic of highly screened polyelectrolyte solutions. This unified scaling picture enables the estimation of viscosity of ubiquitous NaCMC solutions as a function of N, c, and c s and establishes the behavior expected for a range of semiflexible polyelectrolyte solutions.

We investigate the viscosity dependence on concentration and molecular weight of semiflexible pol... more We investigate the viscosity dependence on
concentration and molecular weight of semiflexible polyelectrolyte
sodium carboxymethylcellulose (NaCMC) in aqueous salt-free and
NaCl solutions. Combining new measurements and extensive
literature data, we establish relevant power laws and crossovers
over a wide range of degree of polymerization (N) as well as
polymer (c) and salt (cs) concentrations. In salt-free solution, the
overlap concentration shows the expected c* ∝ N−2 dependence,
and the entanglement crossover scales as ce ∝ N−0.6±0.3, in strong
disagreement with scaling theory for which ce ∝ c* is expected, but
matching the behavior found for flexible polyelectrolytes. A second crossover, to a steep concentration dependence for specific viscosity (ηsp ∝ c3.5±0.2), commonly assigned to the concentrated regime, is shown to follow c** ∝ N−0.6±0.2 (with c**/ce ≃ 6) which thus suggests instead a dynamic crossover, possibly related to entanglement. The scaling of c* and ce in 0.01 and 0.1 M NaCl shows neutral polymer in good solvent behavior, characteristic of highly screened polyelectrolyte solutions. This unified scaling picture enables the estimation of viscosity of ubiquitous NaCMC solutions as a function of N, c, and cs and establishes the behavior expected for a range of semiflexible polyelectrolyte solutions.

Vimentin intermediate filaments (IFs) are part of a family of proteins that constitute one of the... more Vimentin intermediate filaments (IFs) are part of a family of proteins that constitute one of the three filament systems in the cytoskeleton, a major contributor to cell mechanics. One property that distinguishes IFs from the other cytoskeletal filament types, actin filaments and microtubules, is their highly hierarchical assembly pathway, where a lateral association step is followed by elongation. Here we present an innovative technique to follow the elongation reaction in solution and in situ by time-resolved static and dynamic light scattering, thereby precisely capturing the relevant time and length scales of seconds to minutes and 60–600 nm, respectively. We apply a quantitative model to our data and succeed in consistently describing the entire set of data, including particle mass, radius of gyration, and hydrodynamic radius during longitudinal association. cytoskeleton | intermediate filaments | vimentin | self-assembly | time-resolved light scattering I ntermediate filaments (IFs) constitute one of the three protein filament systems in the cytoskeleton of metazoa. Together with actin filaments and microtubules they form a sophisticated composite network, which has been identified as a main player in cell mechanics (1). By contrast to actin filaments and microtu-bules, which are conserved across cell types and organisms, IFs comprise a large family of proteins, encoded by 70 genes in humans (2), and they are expressed in a cell-type–specific manner. Vimentin is an IF protein expressed in fibroblasts, the eye lens, and cells of mesenchymal origin. The monomers with a molecular weight M w of 53.5 × 10 3 g/mol share their tripartite structure consisting of an α-helical rod flanked by intrinsically disordered " head " and " tail " domains, as shown in Fig. 1A, with all other IFs. These monomers are stable in denaturing conditions , such as 8 M urea, and assemble into coiled-coil dimers and subsequently into antiparallel tetramers with M w = 214 × 10 3 g/mol, a length of 60 nm, and a diameter of 5 nm upon stepwise dialysis into low-salt buffers, such as 2 mM sodium phosphate (3). Thus, in buffer conditions without urea, these tetramers with a mass per unit length M tet L of 3,570 g/(mol·nm) are the smallest subunits and starting precursors for vimentin IF assembly. In vitro the assembly of tetramers into full-length filaments can be initiated by the addition of, e.g., monovalent salts such as potassium chloride (KCl) at concentrations of a few tens of millimolars. It has been shown by time-lapse electron microscopy (4) and more recently by real-time small-angle X-ray scattering (SAXS) in combination with micro-fluidic techniques (5–7) that a lateral assembly step into unit-length filaments (ULFs) consisting of typically eight tetramers (Fig. 1B) is followed by an elongation reaction where ULFs and short filaments join to form micrometer-long filaments (Fig. 1C). However, the exact molecular mechanism of the elongation reaction remains elusive. It is clear that the tail domains are not needed (4). The way that the IF consensus domains at either end of the rod interact, overlapping (8) vs. interdigitating (9), is not clear. Moreover, the extent to which the subdomain flexibility plays a role is still uncertain (10). Importantly, the number of monomers per cross-section may vary, depending on the overall and local buffer conditions (4). Note that in the fully assembled filament, the mass per unit length is 40 × 10 3 g/(mol·nm), when assuming a typical number of eight tetramers per cross-section, due to a partial overlap of the ULFs during the elongation reaction. A ULF is 60 nm long and 17 nm in diameter, and thus SAXS is a highly suitable technique to observe the lateral assembly from tetramers to ULFs (5, 11). Recently fluorescence labeling techniques have been developed for vimentin IFs (12, 13), enabling us to observe the elongational growth of filaments 1 μm long or longer. However, the length scales between ∼100 nm and 500 nm are not easily accessible by SAXS or by visible light microscopy and dynamic data of the assembly process in this range are largely missing. We bridge this gap by applying a combination of time-resolved static light scattering (SLS) and dynamic light scattering (DLS), which accesses exactly the missing length scales and is sensitive in the range of contour lengths between 60 nm and 600 nm. SLS is a very suitable technique to determine the mass and structure of protein aggregates and assemblies in dilute solution with radii of gyration and molar masses in the range of 10–200 nm and 5 × 10 3 –1 × 10 8 g/mol, respectively. DLS further allows us to measure the distribution of relaxation times in solution (14, 15), from which diffusion coefficients can be extracted. An early study of aggregation applying combined SLS and DLS analyzed the initial stage of coagulation of spherical colloids (16). Time-resolved SLS was used to record the consumption of triarylamine Significance The hierarchical assembly pathway of the cytoskeletal protein vimentin may be responsible for the astonishing mechanical properties of the emerging filaments, such as high flexibility and extensibility, and thus play a key role in cellular mechanics. A two-step assembly mechanism, involving a lateral and a subsequent elongational step, has been established; however, the elongational step could not be followed in solution. Here, we show direct in situ observation and modeling of the elon-gation reaction of the filaments on the relevant length and time scales, using time-resolved, multiangle static and dynamic light scattering. We thus achieve sufficient spatio-temporal resolution without the need of labeling, staining, or adsorption to substrates.

We report a small angle neutron scattering (SANS) and rheology study of cellulose derivative poly... more We report a small angle neutron scattering (SANS) and rheology study of cellulose derivative polyelectrolyte sodium carboxymethyl cellulose with a degree of substitution of 1.2. Using SANS, we establish that this polymer is molecu- larly dissolved in water with a locally stiff conformation with a stretching parameter B = 1.06. We determine the cross sec- tional radius of the chain (rp = 3.4 A ̊ ) and the scaling of the correlation length with concentration (n 5 296 c^1/2A ̊ for c in g/L) is found to remain unchanged from the semidilute to concentrated crossover as identified by rheology. Viscosity measurements are found to be in qualitative agreement with scaling theory predictions for flexible polyelectrolytes exhibit-
ing semidilute unentangled and entangled regimes, followed by what appears to be a crossover to neutral polymer concen- tration dependence of viscosity at high concentrations. Yet those higher concentrations, in the concentrated regime defined by rheology, still exhibit a peak in the scattering func- tion that indicates a correlation length that continues to scale as c^1/2.

Chemphyschem : a European journal of chemical physics and physical chemistry, Jan 29, 2015

Rapid precipitation, immersion of a liquid formulation into a nonsolvent, is compared with drop c... more Rapid precipitation, immersion of a liquid formulation into a nonsolvent, is compared with drop casting for fabricating organic solar cells. Blends comprising poly-3-hexylthiophene (P3HT), phenyl-C61-butyric acid methyl ester (PCBM), and chlorobenzene were processed into bulk samples by using two distinct routes: rapid precipitation and drop casting. The resulting structure, phases, and crystallinity were analyzed by using small-angle neutron scattering, X-ray diffraction, differential scanning calorimetry, and muon spin resonance. Rapid precipitation was found to induce a finely structured phase separation between PCBM and P3HT, with 65 wt % crystallinity in the P3HT phase. In contrast, solvent casting resulted in a mixed PCBM/P3HT phase with only 43 wt % P3HT crystallinity. The structural advantages conferred by rapid precipitation were shown to persist following intense thermal treatments.

Scientific reports, Jan 12, 2015

Understanding and engineering the flow-response of complex and non-Newtonian fluids at a molecula... more Understanding and engineering the flow-response of complex and non-Newtonian fluids at a molecular level is a key challenge for their practical utilisation. Here we demonstrate the coupling of microfluidics with small angle neutron scattering (SANS). Microdevices with high neutron transmission (up to 98%), low scattering background (<0.01cm-1), broad solvent compatibility and high pressure tolerance (≈3-15 bar) are rapidly prototyped via frontal photo polymerisation. Scattering from single microchannels of widths down to 60 μm, with beam footprint of 500 μm diameter, was successfully obtained in the scattering vector range 0.01-0.3 Å(-1), corresponding to real space dimensions of . We demonstrate our approach by investigating the molecular re-orientation and alignment underpinning the flow response of two model complex fluids, namely cetyl trimethylammonium chloride/pentanol/D2O and sodium lauryl sulfate/octanol/brine lamellar systems. e assess the applicability and outlook of microfluidic-SANS for high-throughput and flow processing studies, with emphasis of soft matter.

Langmuir, Feb 25, 2014

We report the controlled formation of internally porous polyelectrolyte particles with diameters ... more We report the controlled formation of internally porous polyelectrolyte particles with diameters ranging from tens to hundreds of micrometers through selective solvent extraction using microfluidics. Solvent-resistant microdevices, fabricated by frontal photopolymerization, encapsulate binary polymer (P)/solvent (S1) mixtures by a carrier solvent phase (C) to form plugs with well-defined radii and low polydispersity; the suspension is then brought into contact with a selective extraction solvent (S2) that is miscible with C and S1 but not P, leading to the extraction of S1 from the droplets. The ensuing phase inversion yields polymer capsules with a smooth surface but highly porous internal structure. Depending on the liquid extraction time scale, this stage can be carried out in situ, within the chip, or ex situ, in an external S2 bath. Bimodal polymer plugs are achieved using asymmetrically inverted T junctions. For this demonstration, we form sodium poly(styrenesulfonate) (P) particles using water (S1), hexadecane (C), and methyl ethyl ketone (S2). We measure droplet extraction rates as a function of drop size and polymer concentration and propose a simple scaling model to guide particle formation. We find that the extraction time required to form particles from liquid droplets does not depend on the initial polymer concentration but is rather proportional to the initial droplet size. The resulting particle size follows a linear relationship with the initial droplet size for all polymer concentrations, allowing for the precise control of particle size. The internal particle porous structure exhibits a polymer density gradient ranging from a dense surface skin toward an essentially hollow core. Average particle porosities between 10 and 50% are achieved by varying the initial droplet compositions up to 15 wt % polymer. Such particles have potential applications in functional, optical, and coating materials.

We investigate the viscosity dependence on concentration and molecular weight of semiflexible pol... more We investigate the viscosity dependence on
concentration and molecular weight of semiflexible polyelectrolyte
sodium carboxymethylcellulose (NaCMC) in aqueous salt-free and
NaCl solutions. Combining new measurements and extensive
literature data, we establish relevant power laws and crossovers
over a wide range of degree of polymerization (N) as well as
polymer (c) and salt (cs) concentrations. In salt-free solution, the
overlap concentration shows the expected c* ∝ N−2 dependence,
and the entanglement crossover scales as ce ∝ N−0.6±0.3, in strong
disagreement with scaling theory for which ce ∝ c* is expected, but
matching the behavior found for flexible polyelectrolytes. A second crossover, to a steep concentration dependence for specific viscosity (ηsp ∝ c3.5±0.2), commonly assigned to the concentrated regime, is shown to follow c** ∝ N−0.6±0.2 (with c**/ce ≃ 6) which thus suggests instead a dynamic crossover, possibly related to entanglement. The scaling of c* and ce in 0.01 and 0.1 M NaCl shows neutral polymer in good solvent behavior, characteristic of highly screened polyelectrolyte solutions. This unified scaling picture enables the estimation of viscosity of ubiquitous NaCMC solutions as a function of N, c, and cs and establishes the behavior expected for a range of semiflexible polyelectrolyte solutions.

Chemphyschem : a European journal of chemical physics and physical chemistry, Jan 29, 2015

Scientific reports, Jan 12, 2015

Langmuir, Feb 25, 2014