Unconventional multiple ring structure formation from evaporation-induced self-assembly of polymers

A Facile Approach to Improve Interchain Packing Order and Charge Mobilities by Self-Assembly of Conjugated Polymers on Water

Development of facile and economic approaches for assembling organic semiconductors into more ordered structures toward high charge mobilities is highly demanding for the fabrication of organic circuits. Here a simple and facile approach is reported to prepare conjugated polymer thin films with improved crystallinities and charge mobilities by self-assembling semiconducting polymers on water. The formation of polymer thin films with more ordered structures is attributed to coffee ring effect induced by solvent-evaporation on water, and the hydrophobic nature of conjugated polymers that forces the polymer chains to pack densely and orderly on water surface. This approach is applicable to typical semiconducting polymers, and charge mobilities of their thin films are boosted remarkably. Finally, this new method can be utilized to easily fabricate the array of field-effect transistors with high charge mobilities in an economic way.

Multi-ring Deposition Pattern of Drying Droplets

We propose a theory for the multi-ring pattern of the deposits that are formed when droplets of the suspension are dried on a substrate. Assuming a standard model for the stick-slip motion of the contact line, we show that as droplets evaporate many concentric rings of deposits are formed but are taken over by a solid-circle pattern in the final stage of drying. An analytical expression is given to indicate when the ring pattern changes to a solid-circle pattern during the evaporation process. The results are in qualitative agreement with existing experiments, and the other predictions on how the evaporation rate, droplet radius, and receding contact angle affect the pattern are all subject to an experimental test.

Deposition Patterns of Two Neighboring Droplets: Onsager Variational Principle Studies

When two droplets containing nonvolatile components are sitting close to each other, asymmetrical ring-like deposition patterns are formed on the substrate. We propose a simple theory based on the Onsager variational principle to predict the deposition patterns of two neighboring droplets. The contact line motion and the interference effect of two droplets are considered simultaneously. We demonstrate that the gradients of evaporation rate along two droplets is the main reason for forming asymmetrical deposition patterns. By tracing the relative motion between the contact line and the solute particles, we found that the velocities of solute particles have no cylindrical symmetry anymore because of the asymmetrical evaporation rate, giving the underlying mechanism of forming asymmetrical patterns. Moreover, controlling the evaporation rate combined with varying the contact line friction, fan-like and eclipse-like deposition patterns are obtained. The theoretical results of pinned contact line cases are qualitatively consistent with the pervious experimental results.

Dynamics of the evaporative dewetting of a volatile liquid film confined within a circular ring

The dewetting dynamics of a toluene film confined within a copper ring on a deformable PMMA film is studied. The toluene film experiences evaporation and dewetting, which leads to the formation of a circular contact line around the center of the copper ring. The contact line recedes smoothly toward the copper ring at a constant velocity until reaching a dynamic "stick" state to form the first circular polymer ridge. The average receding velocity is found to be dependent on the dimensions of the copper ring (the copper ring diameter and the cross-sectional diameter of the copper wire) and the thickness of the PMMA films. A model is presented to qualitatively explain the evaporative dewetting phenomenon.

Evaporation-induced formation of self-organized gradient concentric rings on sub-micron pre-cast PMMA films

A "particle-on-film" template is developed to fabricate self-organized surface patterns through solvent evaporation on initially featureless, sub-micron PMMA films. The small particle placed on the pre-cast PMMA film is able to confine a toluene droplet and influence the evaporative process. Well-ordered gradient concentric rings are formed around the particle due to the unconventional "advancing-receding" motion of the contact line in the "stick" state on the surface of the PMMA films. Both the center-to-center distance between adjacent rings (wavelength) and the height of the rings (amplitude) are strongly dependent on the particle size and the film thickness, and decrease with the decrease of the distance to the center of the particle. A linear dependence of the amplitude of the rings on the wavelength is observed under experimental conditions. The results demonstrate that the "particle-on-film" template has the potential to fabricate highly-ordered surface patterns economically and efficiently.

Formation of self-organized gradient stripes on precast poly(methyl methacrylate) films

A "wire-on-film" template has been developed to construct surface gradient patterns on precast poly(methyl methacrylate) films. Solvent evaporation around a Cu wire leads to the formation of self-organized gradient stripes, with the longitudinal direction of the stripes being parallel to the axis of the Cu wire. Both the spatial wavelength and the amplitude of the stripes near the Cu wire decrease with a decrease of the distance to the center of the Cu wire. The amplitude of the stripes varies linearly with the spatial wavelength for the experimental conditions used, which is a function of the diameter of the Cu wires and the film thickness. Using two parallel copper wires, we demonstrate the possibility of controlling the characteristics of the gradient stripes. A featureless zone is formed between the copper wires, the size of which is dependent on the distance between two wires and the diameter of the copper wires. The results of this study provide a simple method to fabricate gradient gratings on polymer films in an economical and efficient way.

Pattern formation and mass transfer under stationary solutal Marangoni instability

According to the seminal theory by Sternling and Scriven, solutal Marangoni convection during mass transfer of surface-active solutes may occur as either oscillatory or stationary instability. With strong support of Manuel G. Velarde, a combined initiative of experimental works, in particular to mention those of Linde, Wierschem and coworkers, and theory has enabled a classification of dominant wave types of the oscillatory mode and their interactions. In this way a rather comprehensive understanding of the nonlinear evolution of the oscillatory instability could be achieved. A comparably advanced state-of-the-art with respect to the stationary counterpart seemed to be out of reach a short time ago. Recent developments on both the numerical and experimental side, in combination with assessing an extensive number of older experiments, now allow one to draw a more unified picture. By reviewing these works, we show that three main building blocks exist during the nonlinear evolution: roll cells, relaxation oscillations and relaxation oscillations waves. What is frequently called interfacial turbulence results from the interaction between these partly coexisting basic patterns which may additionally occur in different hierarchy levels. The second focus of this review lies on the practical importance of such convection patterns concerning their influence on mass transfer characteristics. Particular attention is paid here to the interaction between Marangoni and buoyancy effects which frequently complicates the pattern formation even more. To shed more light on these dependencies, new simulations regarding the limiting case of stabilizing density stratification and vanishing buoyancy are incorporated.

Formation of well-ordered finger-like structures on pre-cast thin films

In this work, we developed a simple method to construct well-ordered finger-like structures on pre-cast polymer films for the first time.