Inkjet-printed line morphologies and temperature control of the coffee ring effect

We have studied inkjet-printed drops of a conductive polymer. We show how varying drop spacing and temperature lead to several different printed line morphologies and offer a simple geometric explanation for these various forms. Also, by controlling the evaporation profile of drying drops and lines, we demonstrate control of the coffee ring effect by which solute is transferred to the rim. Under appropriate conditions, we are able to enhance or eliminate the coffee ring effect in our drying features.     

Evaporation of sessile droplets of dilute aqueous solutions containing sodium n-alkylates from polymer surfaces: influences of alkyl length and concentration of solute

The evaporation of sessile droplets placed on polymer surfaces was studied by microscopic observation of the changes in shape of aqueous solution droplets in which the alkyl lengths and the initial concentrations of sodium n-alkylates were varied. Although the initial contact angles of the droplets were not significantly different, the evaporation process varied significantly with the alkyl length of the sodium n-alkylate employed. For the sodium dodecanoate (C 12), showing the highest surface activity, the concentration was found to have a significant effect on the evaporation process of the droplets. In the evaporation of water droplets, variations in the three distinct stages were caused by the different concentration of solutes distributed near or at the air/water interface. It is revealed that the concentration of droplet solute near the air/water interface requires not only solvent evaporation but also some affinity of the solute for the interface. The initial C 12 concentration-dependence of the evaporation of C 12 solution droplets is discussed with particular emphasis on the sudden spreading or sudden contraction of the contact area near the end of evaporation. It is suggested that the cluster formation by C 12 molecules at the air/liquid interface during the evaporation causes Marangoni instability in an evaporating droplet, and the clusters are expected to move dynamically, depending on the droplet concentration of C 12, from the droplet center to the contact line and vice versa, showing Marangoni flow along the air/water interface.     

Edge excitation red shift and energy migration in quinine bisulphate dication

Photoinduced excited state dynamical processes in quinine sulphate dication (QSD) have been studied over a wide range of solute concentrations using steady state and nanosecond time-resolved fluorescence spectroscopic techniques. The edge excitation red shift (EERS) of emission maximum, emission wavelength dependence of fluorescence lifetimes and the time dependence of emission maximum are known to occur due to the solvent relaxation process. With increase in solute concentration, the emission spectrum shifts towards the lower frequencies accompanied with decrease in fluorescence intensity, however, absorption spectrum remains unchanged. A decrease in EERS, fluorescence lifetimes, time dependent fluorescence Stokes shift (TDFSS), fluorescence polarization and the solvent relaxation time (τ_r) is observed with the increase in solute concentration. The process of energy migration among the QSD ions along with solvent relaxation has been found responsible for the above experimental findings.     

Effect of soluble polymer binder on particle distribution in a drying particulate coating

Soluble polymer is frequently added to inorganic particle suspensions to provide mechanical strength and adhesiveness to particulate coatings. To engineer coating microstructure, it is essential to understand how drying conditions and dispersion composition influence particle and polymer distribution in a drying coating. Here, a 1D model revealing the transient concentration profiles of particles and soluble polymer in a drying suspension is proposed. Sedimentation, evaporation and diffusion govern particle movement with the presence of soluble polymer influencing the evaporation rate and solution viscosity. Results are summarized in drying regime maps that predict particle accumulation at the free surface or near the substrate as conditions vary. Calculations and experiments based on a model system of poly(vinyl alcohol) (PVA), silica particles and water reveal that the addition of PVA slows the sedimentation and diffusion of the particles during drying such that accumulation of particles at the free surface is more likely.     

Towards a modellisation of the solvation energy in multi-component solvents. The interesting case of a charged solute imbedded in a polymer-containing electrolyte solution

In this paper we propose a mean-field theory to calculate the solvation free energy of a charged solute imbedded in a complex multi-component solvent. We considered a solvent made up of a mixture of small (electrolyte solution) and large (polymer) components. The presence of macromolecules ensures reduced mixing entropy among the different solvent components, an effect due to polymer connectivity. The reduced entropy favours strong preferential distribution of a particular solvent even in the presence of weak preferential solute–solvent interactions. In addition, two energy terms must be considered: (a) the interaction between the solute electrostatic potential and the electrolyte solution and (b) the formation of a polymer–solute interface. Because of the different dielectric permittivity of the solvent components, the electrolyte and polymer distribution functions are strongly coupled: ions, indeed, are more solvated in regions of higher local dielectric permittivity arising from the inhomogeneous mixing of solvent and polymer. We combined together the different energy terms in the framework of the de Gennes free energy functional for polymer solutions along with a generalised Poisson–Boltzmann equation developed for inhomogeneous dielectric media. Moreover, the preferential electrolyte solvation in regions of greater polarity was considered by an extension of the Born equation. Setting the polymer dielectric permittivity smaller than the solvent one and making null the specific polymer–solute interactions, we calculated enhanced electrolyte concentration and reduced polymer concentration near the solute surface on raising the solute surface charge density. The theory shows also the breakdown of the widely used separation between electrostatic and surface tension-dependent contributions to solvation energy when non-ideal mixed solvents are considered. In fact, according to the model, the surface tension of such mixed solvents strongly depends on the solute surface charge density: at high potentials the interfacial tension may increase rather than decrease on raising the polymer volume fraction. The theoretical results have been compared with experimental data on polymer+electrolyte solution surface tension and with solubility data of colloidal particles. The comparison evidences the complex behaviour of multi-component solvents going well beyond the trivial weighted average of the dielectric permittivity and surface tension of the isolated chemical components. Deviations from the simple behaviour predicted by an average picture of multi-component solvents could be understood by developing more sophisticated, but still simple, approaches like that proposed in this paper.Contribution to the Jacopo Tomasi Honorary Issue. This paper is dedicated to Jacopo Tomasi. I learned much of the difficult art of transforming complex problems into simple models after reading his early works on solvation energy.     

Modeling the drying of ink-jet-printed structures and experimental verification

This article presents a numerical model that was developed for the drying of ink-jet-printed polymer solutions after filling the pixels in a polymer LED display. The model extends earlier work presented in the literature while still maintaining a practical approach in limiting the number of input parameters needed. Despite some rigorous assumptions, the model is in fair agreement with experimental data from a pre-pilot ink-jet printing line. Comparison inside a single pixel is shown, as well as a general trend in which the amount of polymer that is transported out of the central part of the pixel decreases with the rate of viscosity increase as a function of polymer concentration. Moreover, the effect of a varying solute diffusion coefficient is studied.     

Complex macromolecular dynamics:: I. Estimation technique for time-resolved emission anisotropy ratio of chromophores incorporated into polymer chains

Time-resolved fluorescence emission anisotropy ratios of carbazolyl groups incorporated into polystyrene chains in polyethyleneoxide(PEO)/1,2-dichloroethane mixtures have been measured by the single photon counting method. The fluorescence depolarization method is very excellent to clarify various dynamical modes of polymer chains, and many theoretical and experimental researches have so far been reported in the field of polymer chain dynamics. However there are few reports about the dynamics on the polymer side chain, because the dynamical mechanism of the polymer side chain is very complicated. In this report we tried to analyze the dynamical modes of the polymer side chains by the fluorescence depolarization method. Five dynamical modes of a polymer chain based on the Wöessner model were estimated by our original analytical technique `χ²-map method'. The value of each mode of a polymer side chain was discussed above the overlap concentration (C^*) of PEO and the micro-environments were clarified in the vicinity of the chromophore attached to the polymer side chain.     

Dissipative particle dynamics simulations of polymer-protected nanoparticle self-assembly

Dissipative particle dynamics simulations were used to study the effects of mixing time, solute solubility, solute and diblock copolymer concentrations, and copolymer block length on the rapid coprecipitation of polymer-protected nanoparticles. The simulations were aimed at modeling Flash NanoPrecipitation, a process in which hydrophobic solutes and amphiphilic block copolymers are dissolved in a water-miscible organic solvent and then rapidly mixed with water to produce composite nanoparticles. A previously developed model by Spaeth et al. [J. Chem. Phys. 134, 164902 (2011)] was used. The model was parameterized to reproduce equilibrium and transport properties of the solvent, hydrophobic solute, and diblock copolymer. Anti-solvent mixing was modeled using time-dependent solvent-solute and solvent-copolymer interactions. We find that particle size increases with mixing time, due to the difference in solute and polymer solubilities. Increasing the solubility of the solute leads to larger nanoparticles for unfavorable solute-polymer interactions and to smaller nanoparticles for favorable solute-polymer interactions. A decrease in overall solute and polymer concentration produces smaller nanoparticles, because the difference in the diffusion coefficients of a single polymer and of larger clusters becomes more important to their relative rates of collisions under more dilute conditions. An increase in the solute-polymer ratio produces larger nanoparticles, since a collection of large particles has less surface area than a collection of small particles with the same total volume. An increase in the hydrophilic block length of the polymer leads to smaller nanoparticles, due to an enhanced ability of each polymer to shield the nanoparticle core. For unfavorable solute-polymer interactions, the nanoparticle size increases with hydrophobic block length. However, for favorable solute-polymer interactions, nanoparticle size exhibits a local minimum with respect to the hydrophobic block length. Our results provide insights on ways in which experimentally controllable parameters of the Flash NanoPrecipitation process can be used to influence aggregate size and composition during self-assembly.     

Ultrafast dynamical localization of photoexcited states in conformationally disordered poly(p-phenylenevinylene)

We consider two types of ultrafast dynamical localization of photoexcited states in conformationally disordered poly(p-phenylenevinylene). First, we discuss nonadiabatic interconversion from higher energy extended exciton states to lower energy more localized local exciton ground states. Second, we calculate the dynamics of local exciton ground states on their Born-Oppenheimer potential energy surfaces. We show that within the first C-C bond oscillation following photoexcitation (～35 fs) the exciton becomes self-trapped and localized over approximately eight monomers. This process is associated with a Calderia-Leggett type loss of phase coherence owing to the coupling of the polymer to a dissipative environment. Subsequent torsional relaxation (on a time scale of approximately picoseconds) has little effect on the localization. We conclude from this that the initial torsional disorder determines the spatial distribution and localization length of vertical excitations but that electron-phonon coupling is largely responsible for the localization length of self-trapped excitons. We next consider the effect of dynamical localization on fluorescence depolarization. We show that exciting higher energy states causes a larger fluorescence depolarization, because these states have a larger initial delocalization. Using the observation that fluorescence depolarization is a function of excitation wavelength and polymer conformation, we show how the models of exciton localization discussed here can be experimentally investigated.     

The behavior of fluids near solutes: a density functional theory and computer simulation study

The density distribution of solvent near a solute particle is studied using density functional theory and Monte Carlo simulation. The fluid atoms interact with each other via a hard sphere plus Yukawa potential, and interact with the solute via a hard sphere potential. For small solute sizes, the solvent displays liquidlike ordering near the particle. When the solute become larger, a drying transition is observed at state points near the coexistence conditions of the solvent. These predictions are similar to those of a recent theory for the hydrophobic effect by Lum, Chandler, and Weeks [J. Phys. Chem. 103, 4570 (1999)], although a comparison with simulations shows that the theory of this work is quantitatively more accurate. The connection between density functional methods and the LCW approach is also established.     

Convection-diffusion of solutes in dynamic media

We investigate convective-diffusive transport of a solute through a medium with properties that can be externally modulated in space and time. In particular, we focus on the effect of a front—a sharp transition in the convective velocity (v) and diffusivity (D)—on the evolution of the solute concentration profile. Numerical results show that by suitably moving the front during the process an anti-dispersive effect may be realized, in which the solute accumulates in a thin region close to the moving boundary. Our computations take into account the realistic case of a front having a small but finite thickness, and we find that the width of the concentration profile scales as , where Pe is the Péclet number. This is in sharp contrast to the 1/Pe scaling observed for the ideal case of the singular front assumed in previous work. The effect of the thickness of the front and the magnitude of the drop inv andD, on the solute concentration profile has also been studied. These results are relevant in order to implement and optimize protocols that apply an externally controlled moving boundary for the purpose of separation. We also present experimental results characterizing solute transport across a stationary front, expected to display many features needed in a model for moving fronts. The concentration profile of electrophoretically mobile BSA-FITC within the boundary layer at a polyacrylamde gel-buffer interface were visualized by epifluorescence microscopy. Measured boundary layer thickness exceeded that predicted for even a finite interface, indicating that the length scale associated with real boundaries is relevant to the modeling problem.     

Dissipative crystallization of aqueous mixtures of potassium salts of poly(riboadenylic acid) and poly(ribouridylic acid)

Dissipative drying patterns of aqueous mixtures of potassium salts of poly(riboadenylic acid) (KPolyA) and poly(ribouridylic acid) (KPolyU) were studied on a cover glass, a watch glass and a glass dish at room temperature. Accumulation of the polymers forming the broad rings near the outside edge and the inner area of the dried film was observed. The fine multiple ring structures formed when the affinity of the polymer with the substrate is strong. Microscopic drying patterns changed drastically depending on the location in the dried film. Microscopic drying patterns were mainly dendritic long rods and sword (halberd)-like rods. They are assigned to the crystals of double-stranded and triple-stranded helices of the A:U and A:2U complexes, respectively. Cross-like drying patterns are also observed originated from the salt-polymer interaction.     

Process of drying a polymeric paint by diffusion-evaporation and shrinkage. Determination of the concentration-dependent diffusivity

The process of drying a paint made of a dispersion of a polymer in a solvent is experimentally and theoretically studied at various constant temperatures. The diffusion of the solvent through the paint and evaporation from the surface is considered, as well as the subsequent shrinkage. From measurements made at the beginning and at the end of the drying operation on the kinetics of drying, the diffusivity is found to largely depend on the solvent concentration. An exponential relationship of the diffusivity versus the concentration is thus found and successfully tested for the whole process of drying, the diffusivity increasing with the solvent concentration.     

Dissipative crystallization of aqueous solution of sodium polymethacrylate

Drying dissipative structures of aqueous solution of sodium polymethacrylate (NaPMA) were studied on a cover glass, a watch glass, and a glass dish. Any convectional and sedimentation patterns did not appear during the course of dryness. Several important findings on the drying patterns are reported. Firstly, spherulite and hedrite dissipative crystals were observed when the polymer solutions were dried. The crystalline structures changed from hedrites to spherulites as polymer concentration increased. Secondary, the coupled structures of the spherulites and the broad rings were observed for NaPMA at the outside edge of the broad ring. However, the coupled crystalline structures of the lamellaes from the broad ring and the spherulites, which were observed for poly(ethylene glycol) (Okubo et al. 2009), were not observed clearly for NaPMA system. Thirdly, size of the broad ring at the outside edge of the dried film increased sharply as polymer concentration increased.     

Dissipative crystallization of aqueous mixtures of potassium salts of poly(riboguanylic acid) and poly(ribocytidylic acid)

Drying patterns of aqueous solutions of potassium salts of poly(riboguanylic acid) (KPolyG), poly(ribocytidylic acid) (KPolyC), and their mixtures KPolyG + KPolyC were studied on a cover glass, a watch glass, and a glass dish at room temperature. Accumulation of the polymers forming the broad rings near the outside edge and also in the inner area of the dried film was observed. The fine multiple ring structures formed, which supports the fact that the affinity of the polymer with the substrate is strong. Typical microscopic drying patterns of KPolyG, KPolyC, and KPolyG + KPolyC were spherulites, dendritic long rods, and sword (harberd)-like rods, respectively. The patterns changed depending on the location in the dried film. The dendritic long rods and sword-like rods were assigned to the crystals of double-stranded and/or triple-stranded helices of the G:C and 2G:C complexes. Cross-like drying patterns that originated from the salt-polymer interaction are also observed. The relationship between the polymer complexation of KPolyG + KPolyC systems and the drying patterns is similar to that of KPolyA (potassium salt of poly(adenylic acid)) + KPolyU (potassium salt of poly(uridylic acid)).     

Simulation of an ultrafiltration process of bovine serum albumin in hollow-fiber membranes

This work presents a numerical simulation of an ultrafiltration process of bovine serum albumin in solution, using hollow-fiber membranes. Such membranes are constituted of tiny polymer cylinders disposed in a tube-and-shell arrangement. The concentrate flows through the interior of the fibers and the pure solvent is recovered in the shell, assuming perfect solute rejection. In modeling the process, the flow of concentrate inside the fibers was considered to be laminar, with constant density, viscosity and solute diffusivity. Axial diffusion and angular effects were ignored. The model combines the effect of concentration polarization and adsorption, which are the two main limiting phenomena in ultrafiltration processes. The pressure on the shell side was considered constant and inside the fibers a linear pressure profile, dependent on the axial position, was adopted. The solution of the problem was achieved with the method of orthogonal collocation, with adequate choice of the weight function in the radial direction. In the axial direction, a finite-difference method was used. The numerical results were compared with experimental data available in the literature.     

Control of buckling in colloidal droplets during evaporation-induced assembly of nanoparticles

Micrometric grains of anisotropic morphology have been achieved by evaporation-induced self-assembly of silica nanoparticles. The roles of polymer concentration and its molecular weight in controlling the buckling behavior of drying droplets during assembly have been investigated. Buckled doughnut grains have been observed in the case of only silica colloid. Such buckling of the drying droplet could be arrested by attaching poly(ethylene glycol) on the silica surface. The nature of buckling in the case of only silica as well as modified silica colloids has been explained in terms of theory of homogeneous elastic shell under capillary pressure. However, it has been observed that colloids, modified by polymer with relatively large molecular weight, gives rise to buckyball-type grains at higher concentration and could not be explained by the above theory. It has been demonstrated that the shell formed during drying of colloidal droplet in the presence of polymer becomes inhomogeneous due to the presence of soft polymer rich zones on the shell that act as buckling centers, resulting in buckyball-type grains.     

The effect of a magnetic tape coating process on the structure and properties of a semi-crystalline polymer

A combination of structure identifying and bulk property experiments were combined with a two-phase analytical methodology to elucidate the influence of a magnetic tape coating process on the substrate polymer film. Employing a typical tape manufacturing process that utilizes coating, drying, and calendering stages, polyethylenete-rephthalate polymer film samples with and without the magnetic coating were prepared.Experiments and modeling studies performed on these samples demonstrate that the drying and calendering processes may increase the crystallinity and noncrystalline orientation of the substrate film. In addition, dynamic mechanical experiments identified a viscoelastic transition at 50 °C for the PET substrate film sample which is near the glass transition of the magnetic coating utilized. Overall, the results of this investigation provide a basis for evaluating structure property interrelations of polymer-based magnetic tapes.     

Preferential solvation dynamics in liquids: how geodesic pathways through the potential energy landscape reveal mechanistic details about solute relaxation in liquids

It is not obvious that many-body phenomena as collective as solute energy relaxation in liquid solution should ever have identifiable molecular mechanisms, at least not in the sense of the well-defined sequence of molecular events one often attributes to chemical reactions. What can define such mechanisms, though, are the most efficient relaxation paths that solutions take through their potential energy landscapes. When liquid dynamics is dominated by slow diffusive processes, there are mathematically precise and computationally accessible routes to searching for such paths. We apply this observation to the dynamics of preferential solvation, the relaxation around a newly excited solute by a solvent composed of different components with different solvating abilities. The slow solvation seen experimentally in these mixtures stems from the dual needs to compress the solvent and to do solvent-solvent exchanges near the solute. By studying the geodesic (most efficient) paths for this combined process in a simple atomic liquid mixture, we show that the mechanism for preferential solvation features a reasonably sharp onset for slow diffusion, and that this diffusion involves a sequential, rather than concerted, series of solvent exchanges.     

Direct measurement of the twist penetration length in a single smectic A layer of colloidal virus particles

In the 1970s, deGennes discussed the fundamental geometry of smectic liquid crystals and established an analogy between the smectic A phase and superconductors. It follows that smectic layers expel twist deformations in the same way that superconductors expel magnetic field. We make a direct observation of the penetration of twist at the edge of a single isolated smectic A layer composed of chiral fd virus particles subjected to a depletion interaction. Using the LC-PolScope, we make quantitative measurements of the spatial dependence of the birefringence due to molecular tilt near the layer edges. We match data to theory for the molecular tilt penetration profile and determine the twist penetration length for this system.