Design Rules for Graphene and Carbon Nanotube Solvents and Dispersants.

The constantly widening industrial applications of carbon-based nanomaterials puts into sharp perspective the lack of true solvents in which the materials spontaneously exfoliate to individual molecules. This work shows that the different geometry of graphene compared to that of carbon nanotubes can change the potency of a molecule to act as a solvent or dispersant. Through analysis of the structure/function relationships, we derive a number of design rules that will aid the identification of the best solvent or dispersant candidates.

High-throughput assessment of mechanical properties of stem cell derived red blood cells, toward cellular downstream processing.

Stem cell products, including manufactured red blood cells, require efficient sorting and purification methods to remove components potentially harmful for clinical application. However, standard approaches for cellular downstream processing rely on the use of specific and expensive labels (e.g. FACS or MACS). Techniques relying on inherent mechanical and physical properties of cells offer high-throughput scalable alternatives but knowledge of the mechanical phenotype is required. Here, we characterized for the first time deformability and size changes in CD34+ cells, and expelled nuclei, during their differentiation process into red blood cells at days 11, 14, 18 and 21, using Real-Time Deformability Cytometry (RT-DC) and Atomic Force Microscopy (AFM). We found significant differences (p < 0.0001; standardised mixed model) between the deformability of nucleated and enucleated cells, while they remain within the same size range. Expelled nuclei are smaller thus could be removed by size-based separation. An average Young's elastic modulus was measured for nucleated cells, enucleated cells and nuclei (day 14) of 1.04 ± 0.47 kPa, 0.53 ± 0.12 kPa and 7.06 ± 4.07 kPa respectively. Our identification and quantification of significant differences (p < 0.0001; ANOVA) in CD34+ cells mechanical properties throughout the differentiation process could enable development of new routes for purification of manufactured red blood cells.

Adsorption and Depletion Regimes of a Nonionic Surfactant in Hydrophilic Mesopores: An Experimental and Simulation Study.

Adsorption and aggregation of nonionic surfactants at oxide surfaces has been studied extensively in the past, but only for concentrations below and near the critical micelle concentration. Here we report an adsorption study of a short-chain surfactant (C6E3) in porous silica glass of different pore sizes (7.5 to 50 nm), covering a wide composition range up to 50 wt % in a temperature range from 20 °C to the LCST. Aggregative adsorption is observed at low concentrations, but the excess concentration of C6E3 in the pores decreases and approaches zero at higher bulk concentrations. Strong depletion of surfactant (corresponding to enrichment of water in the pores) is observed in materials with wide pores at high bulk concentrations. We propose an explanation for the observed pore-size dependence of the azeotropic point. Mesoscale simulations based on dissipative particle dynamics (DPD) were performed to reveal the structural origin of this transition from the adsorption to the depletion regime. The simulated adsorption isotherms reproduce the behavior found in the 7.5 nm pores. The calculated bead density profiles indicate that the repulsive interaction of surfactant head groups causes a depletion of surfactant in the region around the corona of the surface micelles.

Interactions between nanofibers in fiber-surfactant suspensions: theory of corresponding distances.

We present the theory of corresponding distances for interactions mediated by soft nanostructures in fibrous materials. Based on the fundamental understanding of the mechanism that determines the internal structure of the soft component, our theory allows us to predict the entire force field mediated by the soft component for any angle and distance between the fibers from a single simulation or a single experiment. This replaces hundreds of simulations by just one which enables the routine computation of complete fiber-soft-fiber force fields by high-level methods, such as atomistic simulations, and thereby amounts to a true step advancement for soft nanotechnology.

Torsional forces mediated by surfactant aggregates on carbon nanotube junctions.

Surfactant-mediated interactions acting along the line of shortest contact between carbon nanotubes have been investigated by many authors, but the surfactant-mediated torsion that arises in the case of angled tubes have so far been ignored. Here we show for the first time that a strong torsional force originates from the central surfactant aggregate that forms at the crossing between nonparallel nanotubes. Our dissipative particle dynamics simulations demonstrate that this torque pulls the tubes into a parallel arrangement. The torque increases strongly with decreasing angle between the tubes. This trend is due to the growth of the central aggregate which not only provides more molecules able to mediate the force but also increases the "lever arm" on which the force acts. Together with the strong surfactant-mediated attraction acting along the line of shortest contact between the tubes, the torsion increases the difficulty of nanotube dispersion, but could have a positive effect on carbon nanotube materials in which the adsorbed surfactant micelles are intended to bind the tubes together.

Angular dependence of surfactant-mediated forces between carbon nanotubes.

We employ dissipative particle dynamics to examine surfactant-mediated forces between two carbon nanotubes. Calculations are performed varying both the distance and the angle between the nanotubes. For small distances, a repulsive region is observed, followed by an overall attractive interval with strong oscillations in the force. Decreasing the angle between the tubes leads to a steady increase in the force, but the relative dependence on the separation distance is preserved. We find that the force scales linearly with the size of the overlap area between the tubes. This allows us to express the angle dependence by a simple equation, whereas the distance dependence is represented by a master curve. For the parallel case, the behavior is significantly different.

The science of dispersing carbon nanotubes with surfactants.

Self-assembled structures adsorbed on carbon nanotubes and other nanofibres offer a plethora of opportunities to endow them with new functions and to integrate them into devices and materials. At the same time they are key to solve the greatest problem in carbon nanotube utilisation--debundling and individualisation. Success will inevitably require an understanding of the underlying structure-function relationship of the adsorbed surfactant layer. Computer simulations are ideally suited to develop this understanding as they enable us to study the structure-function relationship in great detail. Combining the results from mesoscale and atomistic simulations we begin to develop this understanding and derive a number of recommendations for optimal dispersion design.

Elasticity of human embryonic stem cells as determined by atomic force microscopy.

The expansive growth and differentiation potential of human embryonic stem cells (hESCs) make them a promising source of cells for regenerative medicine. However, this promise is off set by the propensity for spontaneous or uncontrolled differentiation to result in heterogeneous cell populations. Cell elasticity has recently been shown to characterize particular cell phenotypes, with undifferentiated and differentiated cells sometimes showing significant differences in their elasticities. In this study, we determined the Young's modulus of hESCs by atomic force microscopy using a pyramidal tip. Using this method we are able to take point measurements of elasticity at multiple locations on a single cell, allowing local variations due to cell structure to be identified. We found considerable differences in the elasticity of the analyzed hESCs, reflected by a broad range of Young's modulus (0.05-10 kPa). This surprisingly high variation suggests that elasticity could serve as the basis of a simple and efficient large scale purification/separation technique to discriminate subpopulations of hESCs.

Surfactant-induced forces between carbon nanotubes.

Dissipative particle dynamics simulations are employed to study surfactant-mediated forces between a pair of perpendicular carbon nanotubes (CNTs) coated by surfactants which form spherical micelles in bulk solution and on the tubes. Two force regimes are observed: at small tube/tube distances the force is attractive, whereas it is repulsive at larger distances. The attractive regime is dominated by a central micelle binding the tubes, while in the repulsive regime the contact region is depleted. The two regimes are separated by a discontinuous transition. The repulsive regime is critical for stabilizing CNT suspensions. Viewing rebundling as a thermally activated process, a connection between the repulsive force and the rebundling rate is established. We find that a larger hydrophilic surfactant headgroup creates a stronger and longer ranged tube/tube force, which reduces the rebundling rate significantly. The longer range originates directly from the further reaching head corona of the adsorbed surfactant layer. The larger magnitude of the force appears to be related to the axial compression force the adsorbed phase can sustain. This compression force appears to be the most critical factor for suspension design.

The differences in surfactant adsorption on carbon nanotubes and their bundles.

Dissipative particle dynamics simulations of a mesoscale model are performed to investigate the concentration dependence of surfactant adsorption on small-diameter carbon nanotubes and their bundles. Adsorption is found to follow fundamentally different mechanisms in the two cases because of the heterogeneity of the bundle surface and the difference in diameter of bundles compared to that of individual tubes. Whereas aggregation dominates adsorption on individual tubes, on bundles it is largely a Langmuir-type process. High adsorption energy sites on the outer surface of bundles, where surfactant molecules can interact with two tubes simultaneously, dominate at low coverage. They also cause adsorption on bundles to become significant well before adsorption on individual tubes starts. The difference in the adsorption mechanisms leads to a crossover point at higher concentrations, where the adsorbed amount per surface area on individual tubes becomes larger than that for the bundles.

Structural forces from directed self-assembly.

We investigate a candidate structure for the bottom-up design of nanocomposite materials. At a pair of crossing carbon nanotubes, surfactants self-assemble into a micelle-like aggregate incorporating the two tubes. The aggregate forms as long as the gap between the tubes is smaller than the core diameter of a bulk micelle. Moreover, the absorbed surfactant aggregate generates an effective force between the tubes. The dependence of this force on the distance between the tubes is complex and includes structural components, such as layering, and a large attractive region at larger distances. This attraction appears to be entropic in nature and to originate from confinement of the surfactant head groups.

Directed self-assembly of surfactants in carbon nanotube materials.

The self-assembly of surfactant molecules on crossing carbon nanotubes has been investigated using a bead-spring model and implicit solvent dissipative particle dynamics simulations. Adsorption is directed to the nanotube crossing by its higher hydrophobic potential which is due to the presence of two surfaces. As a consequence of the tendency of surfactant molecules to self-assemble into micelles, the adsorbed molecules form a "central aggregate" at the crossing, thus, confining the molecules to the immediate vicinity of the crossing. Adsorption on the remaining nanotube surface becomes significant only at higher surfactant concentrations, where the molecules self-assemble to hemimicelles which grow continuously to full micelles upon increase of the (bulk) surfactant concentration. Our results allow two conclusions for the rational design of nanostructured materials: (i) the size of the central aggregate can not be much larger than that of a bulk micelle and (ii) control of the adsorbed structures is conveniently possible via the (bulk) surfactant concentration.

An analysis of maximum vehicle G forces and brain injury in motorsports crashes.

PURPOSE: Traumatic brain injury from automobile crashes is a major source of trauma deaths. The investigation of crashes to understand factors of occupant injuries is an established practice. Our objective was to evaluate the association between vehicle G forces (G) sustained on impact and brain injury in motor sports crashes. METHODS: We analyzed data regarding Indy Racing League (IRL) car crashes from 1996 to 2003 and compared the likelihood of head injury in those drivers who were in a vehicle that sustained an impact of > or =50 G versus those with a lesser impact. The mean maximal G for those with head injury was compared with those without head injury. RESULTS: We analyzed 374 crashes. A driver in a crash with an impact of > or =50 G developed a head injury 16.0% (30/188) versus 1.6% (3/186) in those of <50 G (P < 0.001). The mean peak G for those with head injury was 79.6 (SD 28.5) versus 50.6 (SD 28.0) in those with no head injury (P < 0.001). CONCLUSION: Findings were that IRL car crashes with peak vehicle G > or = 50 were associated with the development of traumatic brain injuries.

Mesoscale modeling of complex binary fluid mixtures: towards an atomistic foundation of effective potentials.

This paper is devoted to equilibrium molecular-dynamics (MD) simulations of a fully atomistic model of binary mixtures of water (component 1) and ethanol (component 2). We investigate ways to extract from these simulations effective, pairwise additive potentials suitable to describe the interactions between coarse-grained molecules (i.e., beads) in corresponding mesoscale dissipative particle-dynamics simulations. The fully atomistic model employed in MD simulations is mapped onto an implicit water model, where the internal degrees of freedom of ethanol and all the degrees of freedom of water are integrated out. This gives us an effective one-component system consisting only of ethanol beads. The effective interaction potential between a pair of ethanol beads, Phi(R), is approximated at three levels of sophistication. At the lowest one, we approximate Phi(R) by the potential of mean force between the centers of mass of two ethanol beads calculated in the fully atomistic MD simulations; at the second level, we take Phi(R) to be the potential linked to total and direct correlation functions in the hypernetted-chain closure of the Ornstein-Zernike equation. At the third level we approximate Phi(R) numerically by improving it iteratively through the Boltzmann inversion scheme. Our results indicate that the level-one approach works only at the lowest (8 wt %) concentration; the level-two approach works only up to intermediate ethanol concentrations (ca. 50 wt %). Only the Boltzmann inversion scheme works for all, up to the highest concentration considered (70 wt %).

Cosurfactant and cosolvent effects on surfactant self-assembly in supercritical carbon dioxide.

The impact of alcohol additives on the self-assembly of surfactants in supercritical carbon dioxide is investigated using lattice Monte Carlo simulations. We observe that all studied (model) alcohols reduce the critical micelle concentration. The reduction is stronger the longer the hydrocarbon chain of the alcohol, and the higher the alcohol concentration. Short-chain alcohols are found to concentrate in the surfactant layer of the aggregates, replacing surfactant molecules and leading to a strong decrease of the aggregation number and a large increase of the number of aggregates. On the other hand, only a small number of alcohol molecules with longer chain length are found in the aggregates, leading to a slight increase in the aggregation number. However, structural properties such as size and density profiles of aggregates at the same aggregation number are not influenced markedly. Consequently, short-chain alcohols act as cosurfactants, directly influencing the properties of the aggregates, while alcohols with longer hydrocarbon chains work as cosolvents, altering the properties of the solvent. However, the transition between both extremes is gradual.