Polymer dewetting in solvent-non-solvent environment- new insights on dynamics and lithography-free patterning.

HYPOTHESIS: We show that one may employ polymer dewetting in solvent-non-solvent environment to obtain lithography-free fabrication of well-defined nano- to micro- scale polymer droplets arrays from pre-patterned polymer films. The polymer droplet pattern may be converted to a series of hybrid organic-inorganic and inorganic well-defined nano-patterns by using sequential infiltration synthesis (SIS). In particular, we scrutinize the physical parameters which govern the dewetting of flat and striped polymer thin films, which is the key to obtaining our objective of lithography-free ordered nano-patterns. EXPERIMENTS: We immerse polystyrene (PS) and polymethyl methacrylate (PMMA) thin films in water in the presence of chloroform vapors. We study the ensuing polymer dewetting dynamics and the pattern formation of nanospheres by employing in-situ light microscopy and scanning electron microscopy. We then investigate pattern formation by dewetting of polymer stripes, fabricated by directed solvent evaporation, and SIS of AlOx from vapor phase precursors, trimethyl aluminum (TMA) and H2O, within the nanosphere patterns. FINDINGS: We find that solvent- non-solvent environments render film dewetting rates, which are an order of magnitude faster than solvent vapor dewetting, and supports the formation of small solid polymer droplets, down to sub-100 nm droplet size, of large contact angles with the solid substrate. Pre-patterned polymer film stripes support the formation of highly ordered structures of polymer droplets, which are easily transformed to hybrid polymer-AlOx nanosphere patterns and templated AlOx nanosphere via SIS.

Coupling between wetting dynamics, Marangoni vortices, and localized hot cells in drops of volatile binary solutions.

HYPOTHESIS: A sessile drop comprising a mixture of volatile solvents supports spatial variations in interfacial energy, which gives rise to solutal Marangoni flow, alongside evaporative loss of drop mass. Both the Marangoni flow and evaporation bring about a dance of concurrent and inter-connected phenomena: internal Marangoni vortices, localized hot cells, and complex wetting dynamics. EXPERIMENT: We employ Particle Image Velocimetry and Infra-Red Microscopy to visualize Marangoni vortices, temperature variations, and the wetting dynamics of drops of toluene and ethanol mixtures. FINDINGS: The intensity of the measured phenomena vary concurrently in time and in like manner according with the initial composition of drops. In particular, we observe maximum intensity levels when the initial toluene proportion in the drops is 60%, and none of these phenomena in the case of pure toluene. Moreover, the drops initially expand on the solid in response to Marangoni flow, then contract due to evaporation; between these dynamic wetting regimes, we further observe a regime of one or periodic wetting/de-wetting cycles at low toluene concentrations. Our findings indicate that both the solutal Marangoni flow and evaporation drive the different phenomena we observe and confirm the connection between Marangoni vortices and the formation of localized hot cells.

Analysis of the oscillatory wetting-dewetting motion of a volatile drop during the deposition of polymer on a solid substrate.

A recent experimental work revealed an oscillatory wetting-dewetting motion of the three phase contact line during the deposition of polymer from a volatile solution. Here we employ a theoretical model to explain the wetting-dewetting motion of the contact line by incorporating opposing evaporation and Marangoni induced flows in the deposition process. We take into account the contribution of polymer concentration to the surface tension of the volatile drop and show that by changing the different parameters of the system we are able to traverse the dynamics of the three phase contact line from a simple dewetting regime to the wetting-dewetting regime, observed in experiment. We further show that deposition patterns, which were previously attributed to stick and stick-slip modes of the contact line motion may be generated by the wetting-dewetting mode. We summarize our theoretical findings in phase diagrams, which show the expected regimes of contact line motion and the resulting types of patterned deposits which are to be obtained under different physical conditions.

Transitions between different motion regimes of the three-phase contact line during the pattern deposition of polymer from a volatile solution.

HYPOTHESIS: The interplay between different transport mechanisms during polymer deposition from a volatile solution determines the motion regime of the three-phase contact line, e.g. monotonous slip, stick-slip, and oscillatory wetting-dewetting regimes of motion, which define the morphology of the deposit. EXPERIMENT: To investigate the transitions between the motion regimes of the contact line, we evaporate solutions of Poly-methyl-methacrylate (PMMA) and Poly-dimethyl-siloxane (PDMS) in toluene. The solutions are confined in a well-defined (micro-chamber) geometry, where we adjust the system temperature, initial polymer concentration and molecular mass, and precisely determine the rate of evaporation. FINDINGS: We show that transitions between particular motion regimes of the contact line are connected to two types of competition between physical mechanisms. A transport competition between polymer diffusion and convection determines the distribution of polymer in the volatile meniscus and hence of spatial variations in the surface energy of the solution. A competition between evaporative and surface energy stresses in the liquid meniscus determines the motion of the contact line. We report the temporal variations of the contact line position during each motion regime and give a phase diagram to quantify the physical parameters that are responsible to transitions between the different regimes.

Connecting Monotonic and Oscillatory Motions of the Meniscus of a Volatile Polymer Solution to the Transport of Polymer Coils and Deposit Morphology.

We study the deposition mechanisms of polymer from a confined  meniscus of volatile liquid. In particular, we investigate the physical processes that are responsible for qualitative changes in the pattern deposition of polymer and the underlying interplay of the state of pattern deposition, motion of the meniscus, and the transport of polymer within the meniscus. As a model system we evaporate a solution of poly(methyl methacrylate) (PMMA) in toluene. Different deposition patterns are observed when varying the molecular mass,  the initial concentration of the solute, and temperature; these  are systematically presented in the form of morphological phase diagrams. The modi of deposition and meniscus motion are correlated. They vary with the ratio between the evaporation-driven convective flux and the diffusive flux of the polymer coils in the solution. In the case of a diffusion-dominated solute transport, the solution monotonically dewets the solid substrate by evaporation, supporting continuous contact line motion and continuous polymer deposition. However, a convection-dominated transport results in an oscillatory ratcheting dewetting-wetting motion of the contact line with more pronounced dewetting phases. The deposition process is then periodic and produces a stripe pattern. The oscillatory motion of the meniscus differs from the well documented stick-slip motion of the meniscus, observed as well, and is attributed to the opposing influences of evaporation and Marangoni stresses, which alternately dominate the deposition process.