Microstructures by Solvent Drop Evaporation on Polymer Surfaces: Dependence on Molar Mass (original) (raw)
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Polymer Patterns in Evaporating Droplets on Dissolving Substrates
Langmuir, 2004
Self-organized polymer patterns resulting from the evaporation of an organic solvent drop on a soluble layer of polymer are investigated. The patterns can be modulated by changing the rate of evaporation and also the rate of substrate dissolution controlled by its solubility. Both of these affect the contact zone motion and its instabilities, leading to spatially variable rates of substrate etching and redeposition that result from a complex interplay of several factors such as Rayleigh-Benard cells, thermocapillary flow, solutal Marangoni flow, flow due to differential evaporation, osmotic-pressure-induced flow, and contact-line pinning-depinning events. The most complex novel pattern, observed at relatively low rates of evaporation, medium solubility, and without macroscopic contact-line stick-slip, consists of a regularly undulating ring made up of a bundle of parallel spaghetti-like threads or striations and radially oriented fingerlike ridges. Increased rate of evaporation obliterates the polymer threads, producing more densely packed fingers and widely separated multiple rings due to a frequent macroscopic pinning-depinning of the contact line. Near-equilibrium conditions such as slow evaporation or increased solubility of the substrate engender a wider and less undulating single ring.
“Coffee-ring” patterns of polymer droplets
2013
A theoretical study on the inverted phase formation in diblock copolymer solutions J. Chem. Phys. 137, 224902 (2012) Scattering function of semiflexible polymer chains under good solvent conditions J. Chem. Phys. 137, 174902 (2012) Effect of solvent quality on the dispersibility of polymer-grafted spherical nanoparticles in polymer solutions J. Chem. Phys. 137, 094901 Component dynamics in polyvinylpyrrolidone concentrated aqueous solutions J. Chem. Phys. 137, 084902 (2012) Additional information on AIP Conf. Proc.
Journal of Colloid and Interface Science, 2006
Patterning in the intensively evaporated polymer solutions based on polystyrene and poor solvent (acetone) was investigated. SEM and AFM studies demonstrated that annular elements of the surface topography are formed in this case, in contrast to the honeycomb patterns obtained under the evaporation of the good solvent (chloroform). The authors suggest that the theory of viscous dewetting developed by de Gennes explains the phenomenon satisfactorily.
Macromolecules, 2007
A drop of polymer solution was constrained in a sphere-on-flat geometry, resulting in a liquid capillary bridge. As solvent evaporated, intriguing surface patterns of polymer formed, which were strongly dependent on the molecular weight (MW) of polymer. Dotted arrays were formed at low MW; concentric rings were produced at intermediate MW; concentric rings, rings with fingers, and punch-hole-like structures, however, were yielded at high MW. Rings with fingers as well as punch-hole-like structures were manifestations of simultaneous occurrence of the "stick-slip" motion of the contact line and the fingering instabilities of rings. In addition, the curvature of the sphere in the sphere-on-flat geometry was found to affect the pattern formation. A decrease in the curvature of the sphere led to an earlier onset of the formation of punch-hole-like structures when high-MW polymer was employed as the nonvolatile solute.
Compound redistribution due to droplet evaporation on a thin polymeric film: Theory
Journal of Applied Physics, 2019
A thin polymeric film in contact with a fluid body may leach low-molecular-weight compounds into the fluid. If this fluid is a small droplet, the compound concentration within the liquid increases due to continuous leaching in addition to the evaporation of the droplet. This may eventually lead to an inversion of the transport process and a redistribution of the compounds within the thin film. In order to gain an understanding of the compound redistribution, we apply a macroscopic model for the evaporation of a droplet and combine that with a diffusion model for the compound transport. In the model, material deposition and the resulting contact line pinning are associated with the precipitation of a fraction of the dissolved material. We find three power law regimes for the size of the deposit area as a function of the initial droplet size, dictated by the competition between evaporation, diffusion, and the initial compound concentrations in the droplet and the thin film. The strength of the contact line pinning determines the deposition profile of the precipitate, characterized by a pronounced edge and a linearly decaying profile toward the center of the stain. Our predictions for the concentration profile within the solid substrate resemble patterns found experimentally.
Morphology of Evaporating Sessile Microdroplets on Lyophilic Elliptical Patches
Langmuir, 2019
The evaporation of droplets occurs in a large variety of natural and technological processes such as medical diagnostics, agriculture, food industry, printing, and catalytic reactions. We study the different droplet morphologies adopted by an evaporating droplet on a surface with an elliptical patch with a different contact angle. We perform experiments to observe these morphologies and use numerical calculations to predict the effects of the patched surfaces. We observe that tuning the geometry of the patches offers control over the shape of the droplet. In the experiments, the drops of various volumes are placed on elliptical chemical patches of different aspect ratios and imaged in 3D using laser scanning confocal microscopy, extracting the droplet's shape. In the corresponding numerical simulations, we minimize the interfacial free energy of the droplet, by employing Surface Evolver. The numerical results are in good qualitative agreement with our experimental data and can be used for the design of micropatterned structures, potentially suggesting or excluding certain morphologies for particular applications. However, the experimental results show the effects of pinning and contact angle hysteresis, which are obviously absent in the numerical energy minimization. The work culminates with a morphology diagram in the aspect ratio vs relative volume parameter space, comparing the predictions with the measurements.
Self-Assembly in Evaporated Polymer Solutions: Patterning on Two Scales
Israel Journal of Chemistry, 2007
Self-assembly processes in intensively evaporated polymer solutions have been investigated under low and high humidity. Patterning on two scales has been revealed: mesoscopic and microscopic. The dependence of the characteristic size of a mesoscopic pattern on the polymer concentration is revealed and studied. Various possible mechanisms of patterning on the two scales are discussed. It has been shown that the widespread hypothesis of micro-patterning induced with water droplets needs further justification. The influence of substrate defects on the mesoscopic patterning is also investigated.
Controlling and characterising the deposits from polymer droplets containing microparticles and salt
The European Physical Journal E, 2016
A coffee ring-stain is left behind when droplets containing a wide range of different suspended particles evaporate, caused by a pinned contact line generating a strong outwards capillary flow. Conversely, in the very peculiar case of evaporating droplets of poly(ethylene oxide) solutions, tall pillars are deposited in the centre of the droplet following a boot-strapping process in which the contact line recedes quickly, driven by a constricting collar of polymer crystallisation: no other polymer has been reported to produce these central pillars. Here we map out the phase behaviour seen when the specific pillar-forming polymer is combined with spherical microparticles, illustrating a range of final deposit shapes, including the standard particle ring-stain, polymer pillars and also flat deposits. The topologies of the deposits are measured using profile images and stylus profilometery, and characterised using the skewness of the profile as a simple analytic method for quantifying the shapes: pillars produce positive skew, flat deposits have zero skew and ring-stains have a negative value. We also demonstrate that pillar formation is even more effectively disrupted using potassium sulphate salt solutions, which change the water from a good solvent to a thetapoint solvent, consequently reducing the size and configuration of the polymer coils. This inhibits polymer crystallisation, interfering with the bootstrap process and ultimately prevents pillars from forming. Again, the deposit shapes are quantified using the skew parameter.
Influence of experimental conditions on the incorporation of water droplets in polystyrene
Polímeros, 2009
concentration was high near the surface of the particles and decreased near their center resulting in a non-uniform expanded structure. Crevecoeur et al. (1999a) [2] proposed a process using only water as blowing agent, totally avoiding the use of volatile hydrocarbons. The process was divided in two stages: the first one was a water dispersion stage in which water was added in the reactor with the monomer, the initiator and a suitable surfactant under strong stirring at the polymerization temperature. The second one was a suspension stage, when the continuous suspension medium (water and a suspension stabilizer) was added and the system was allowed to react for more 4 hours. Up to 11 wt% of water was homogeneously incorporated in the particles by this method. In another work [3] , the authors demonstrated that the internal structure of the water expanded polystyrene was very similar to that of the pentane expanded one. One can observe in the works described above that the water incorporation step is of mayor importance because it determines the final morphology of the incorporated water droplets. In turn, the water droplets morphology determines the expansion behavior of the material as well the cell formation since each cell is originated from one single droplet. Furthermore, water must be homogeneously incorporated along the polystyrene particle to assure that a homogeneous expanded structure will be obtained. It can also be noted that a reduced number of works reported the production of expandable polystyrene using only water as blowing agent. However, the influence of several