Session H13: Colloid and Polymer Interfaces

Confinement-induced restructuring of colloidal particles

Packing configuration of colloidal particles in the bulk suspension is determined by interparticle interaction and volume fraction. However, many modern technological applications of colloidal suspensions entail applications of surface confinement and significantly modify the packing structure of colloidal particles. The effect of surface confinement on particle packing is yet poorly understood. We have custom designed a compression apparatus that is well equipped with a confocal microscope, allowing us to vary the gap spacing between two flat solid substrates confining colloidal suspension and image the 3-D microstructure of colloidal particles simultaneously. We observe enhanced layering of confined colloidal particles and a well-ordered 2D crystal structure of each layer formed after quasi-static compression. Surprisingly, we have also observed disordered layers of colloidal particles and the jamming of multi-layers against one another after rapid quench. These observations suggest the intriguing confinement-induced restructuring of dense colloidal suspensions, and provide a unifying perspective from which to pack the particles more efficiently than in bulk. In preliminary experiments, we have also studied the effect of surface chemistry and topography on the packing configuration of confined colloidal particles.   

Wetting Behavior of Nitrous Oxide Near its Critical Point

A liquid is said to ``not wet'' a surface if its beads on it and thus exhibits a nonzero contact angle. However, if the liquid spreads out uniformly on the surface, with a contact angle of zero, then it is said to ``wet'' the surface. Given any liquid whatsoever that initially does not wet a surface, the Cahn theory [1] predicts that the liquid will eventually wet the surface as we increase the temperature and follow along the liquid-vapor coexistence line. For liquids on solid surfaces, the wetting transition, from non-wetting to wetting, is predicted to be first order and occur sharply at a wetting temperature Tw [2]. Tw can be calculated from our knowledge of interatomic potentials that characterize the adatom-adatom attraction and the adatom-surface attraction. The Cahn theory has been verified for non-polar liquids on solid surfaces but so far there exist no experiments for polar liquids. Nitrous Oxide is polar and has a critical point slightly above room temperature. We will report on initial experiments to verify the Cahn theory for this system. [1] J. W. Cahn, J. Phys. Chem. 66, 3667 (1977). [2] M. Gatica, X. Zhao, J. K. Johnson and M. W. Cole, J. Phys. Chem. B 108, 11704 (2004) and References Therein.   

Effects of contact line hysteresis and surface tension on contact line pinning of inclined droplets.

The stability and pinning of drops on inclined surfaces is of fundamental and technological interest. Contact line dynamics depends on the properties of the liquid, the solid surface and the manner in which line motion is driven. Achieving reproducible final contact line and drop shapes is important in biotechnology applications where drops are used as mini-reactors, such as in structural genomics where sessile drops are used for protein and virus crystallization. We will describe experiments investigating pinning and deformation of drops containing pure liquids, proteins and colloids, placed on substrates with a range of surface treatments. Proteins affect contact angle hysteresis at low concentrations, and surface tension at high concentration, leading to two different regimes of pinning. Theoretical approaches that can be useful in understanding our results will also be presented.   

Contact Line Pinning by Microfabricated Patterns

Interactions of the liquid-solid contact line with micrometer-sized features on solid surfaces can provide a method for controlling contact line stability and shape. We have investigated how the size, shape and chemical properties of regular features affect the contact angle and its hysteresis. Microfabrication techniques are used to create 4-50 micrometer-size patterns on otherwise homogeneous flat surfaces, and the relative importance of geometric and chemical factors are determined. We discuss applications to protein crystallization, and in particular show that the volumes, shape stability and shape reproducibility of 10-100 microliter aqueous drops containing proteins and surfactants can be greatly increased. Surface patterning provides greater reproducibility in crystallization outcomes and greater flexibility in the design of crystallization experiments.   

The kinetic coefficient of hard-sphere crystal-melt interfaces from molecular-dynamics simulations

The kinetic coefficient for a crystal melt interface, $\mu$, is the ratio of the interface growth velocity to the undercooling $(T_M - T)$, where $T_M$ is the melting point. In this work we determine the kinetic coeffiecient for the hard-sphere system by analyzing capillary fluctuations in interface position using molecular dynamics (MD) simulation [Hoyt \emph{et al}, Mat. Sci. Eng. $\bf R~41$, 121-163 (2003)]. We report the kinetic coefficient for the three interfaces: (100), (110), and (111). Our results for $\mu_{100}, \mu_{110}$, and $\mu_{111}$ are 1.15(4) $(k_B/(m T_M))^{1/2}$, 0.85(6) $(k_B/(m T_M))^{1/2}$, and 0.57(8) $(k_B/(m T_M))^{1/2}$, respectively, which gives the relation $\mu_{100} > \mu_{110} > \mu_{111}$. This ordering is consistent with the recent results of MD simulations for a variety of metals. The anisotropy ratios $\mu_{100}/\mu_{110}$, and $\mu_{100}/\mu_{111}$ are 1.35(11), and 2.0(3), respectively. We compare our results to those of classical density functional theory (DFT) of [Mikheev and Chernov, J. Cryst. Growth $\bf 112$, 591-596 (1991)].   

Quasielastic neutron scattering study of the dynamics of 1,3-diphenylpropane grafted to the pore surface of MCM-41

An initial study of the dynamics of 1,3-diphenylpropane (DPP, $\equiv $Si-O-C$_{6}$H$_{4}$(CH$_{2})_{3}$C$_{6}$H$_{5})$ attached to the surface of the mesoporous silica MCM-41 has been carried out using quasielastic neutron scattering. Measurements of the elastic intensity were carried out in the temperature range 50-380 K and indicate a trend in DPP dynamics with changing grafting density and pore size. Full quasielastic scans over an energy range of $\pm $ 17 $\mu $eV were carried out at temperatures of 240 K, 280 K, and 320 K. Initial analysis employed a stretched exponential to model the Fourier transformed data in the time domain. An average relaxation time, defined as $<\tau >$ = ($\tau $/$\beta )\Gamma $(1/$\beta )$ where $\beta $ is the stretched exponent in the model, shows a dependence on temperature, pore size, and DPP grafting density. Molecular dynamics simulations using a model for DPP grafted onto the surface of MCM-41 was compared with the experimental results.   

Experimental two-point microrheology of two-dimensional systems

Microrheology is often used to determine the bulk viscoelasticity of materials such as polymer solutions, by tracking the diffusion of tracer particles. For heterogeneous materials, where single particle microrheology breaks down, a modified technique known as two-point microrheology has been established where motions of particles with varying spatial separations are correlated. In 3-d systems, this correlated motion decays as 1/R, and can be used to identify the long wavelength modes in the system, and therefore the bulk viscoelasticity. For 2-d systems, theory has predicted a logarithmic decay, but to date, this prediction has gone untested. We look at the correlated motions of colloidal particles in two types of quasi 2-d systems (protein molecules at an air-water interface, and thin soap films), and observe a transition from 3-d to 2-d, by continuously varying the Boussinesq number, B$_{0}$ = ($\eta _{s}$/$\eta $a) from 0 to $\infty $ , where $\eta _{s}$ is the surface viscosity, $\eta $ is the bulk viscosity, and a is the size of the colloidal particle. The experimental results are compared to theory, and consequences for microrheology at interfaces are discussed.   

Thermal response of polymer coated colloidal diffusion near surfaces

Thermally responsive poly(/N/-isopropylacrylamide) (PNIPAAm) tethered silica particles represent a potential building block for new materials. For this research, Surface-initiated Atom Transfer Radical Polymerization (ATRP) was chosen for its ability to generate well defined, homogeneous polymer brushes on silica nanoparticles and surfaces. Phase contrast optical microscopy was used along with single particle tracking methods to characterize the motion of these particles in suspension as well as close to brush modified surfaces. Temperature dependence experiments reveal that increased temperature decreases the radius of the particles. Additionally, heating may trigger a transition from ``sticky'' surfaces to ``slippery'' surfaces. Below LCST, swelled PNIPAAm chains may interdigitate and slow down particle motion. Thermal hysteresis was also apparent; kinetic trapping of particles resulted in the retention of an immobilized population upon cooling. Furthermore, the analysis of the diffusion dynamics also implies interparticle interactions and particle-surface interactions. Understanding the kinetics of this complex fluid system facilitates the design and control of novel materials.   

Surface patterns on co-assembled fibers from charged, amphiphilic molecules

We analyze local segregation of species in charged multicomponent cylindrical micelles. In particular, we consider co-assemblies of cationic and anionic heterogeneous molecules such as lipids and peptide amphiphiles. Charge heterogeneities can develop at the surface of the fibers due to the competition between the short-range incompatibility of the co-assembled components and the electrostatic interactions. The system can be described by a line tension between domains $\gamma $, which favors growth of the domains of segregated components, and the electrostatic energy of the domains with charge density $\sigma $, which increases rapidly with the size of the segregated domain L. The competition results in the formation of domains with a characteristic size $L_o=(\gamma\varepsilon /\sigma^2)^{1/2}$. In stoichiometric mixtures the constraints of the geometry and the long range correlations lead to lamellar stripes of different pitch. We present results on the different orientations of the lamellar patterns as a function of the ratio between the lamellar size and the cylinder radius. We find the critical salt concentration, which is a function of radius and lamellar size, at which the patterns grow to macroscopic sizes due to the screening of the Coulomb interactions.   

Ion and neutral particle emission from polytetrafluoroethyene under 157-nm irradiation

Polytetrafluoroethylene (PFTE) is an important electronic material. Interactions with 157-nm laser radiation are of interest in the deposition and patterning of thin films of PTFE and similar materials. We have characterized the desorption of neutral and charged species from PTFE surfaces during 157-nm irradiation by time-resolved quadrupole mass spectroscopy. The principal neutral species are (CF$_{2})_{x}$ units. The neutral time-of-flight signals have a fast component with kinetic energies on the order of an eV, consistent with direct photochemical emission. These energetic neutrals are presumably created by scission of the C-C backbone by UV excitation. Slower neutral emissions are consistent with thermal emission from the surface at temperatures of several hundred kelvin above ambient. Much of the slower emission is attributed to thermally activated unzipping of the polymer backbone after photoelectronic scission. The principal positive and negative ions are C$^{+}$ and F$^{-}$. A large number of positive and negative charges are electrostatically coupled and travel together until separated by the electric fields of the mass filter.   

Dual Syringe Electrospinning of FNfds-modified Hyaluronic Acid

We described the fabrication of a unique HA nanofibrous scaffold using dual syringe reactive electrospinning. 3$^{'}$-dithiobis(propanoic dihydrazide)-modified HA (HA-DTPH) and Poly (ethylene glycol)-diacrylate (PEGDA) were selected as the cross-linking system. PEO was blended with HA-DTPH to facilitate the fiber formation. Fibronectin functional domains (FNfds) were incorporated with PEGDA and covalently linked to HA via conjugate addition to improve the cell attachment. The as-spun scaffold was soaked into DI water to remove PEO and yield an FNfds-modified HA-DTPH nanofibrous scaffold. Human dermal fibroblasts CF31 were seeded on FNfds-modified HA-DTPH scaffolds. The CF31 fibroblasts showed a unique extended dendritic morphology which is opposed to the typical flattened morphology of cells on regular 2D geometries. Supported by NSF-MRSEC.   

Single Electron Tunneling to Individual Gold Nano-Particles

Single electron tunneling to naked gold clusters and monolayer protected gold clusters on a thermally grown silicon dioxide film has been performed by single-electron tunneling force microscopy (SETFM) at room temperature.$^{1,2}$ In this technique, single electron tunneling events are detected by electrostatic force measurement. We observe several abrupt steps in the smooth frequency shift curve as the tip approaches a cluster. These steps correspond to single or multiple electron tunneling events to the gold particle. Since the tunneling electron changes the charge on the particle, it is also detected as a change in surface potential. The electron can be manipulated to and from the gold particle by changing the voltage applied to the tip. These tunneling events can be used to determine the electronic structure and charging energy of the individual gold particle. The technique will be described and the single electron tunneling results will be discussed. A theoretical model of the tunneling measurements is in agreement with the measurements. \newline \newline [1] E. Bussmann, D. J. Kim, and C.C. Williams\textit{, Appl. Phys. Lett. }\textbf{85}, 2538 (2004) \newline [2] E. Bussmann, N. Zheng, and C.C. Williams, \textit{Appl. Phys. Lett}. \textbf{86}, 163109 (2005)