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

Solvent-induced deflection of polydimethylsiloxane plates: Effects of dimensions and solvent volume

The development of flexible electronics and robots has stimulated interest in the deformation behavior of polymers under liquid environments. In this work, we introduce a "local" method to study the toluene-induced deflection of polydimethylsiloxane (PDMS) plates. Placing a graphite rod on the surface of a PDMS plate, we are able to confine the spreading and evaporation of toluene and the uptake of toluene by the PDMS plate. The local uptake of toluene by a PDMS plate causes the deflection of the PDMS plate, which is dependent on the geometric dimensions of the plate and the toluene volume. An empirical relationship is proposed to correlate the largest deflection of a PDMS plate induced by local uptake of toluene to geometric dimensions of the plate and toluene volume. The approach used in this work provides a simple approach to study the kinetics of the solvent-induced deformation of a polymer, which can contribute to the design and applications of a stimuli-sensitive self-shaping polymer.

Growth of Polystyrene Pillars in Electric Field

Surface patterning on polymer films, which is a self-assembly process under the action of external and/or internal impetus, has a variety of applications, including drug delivery and flexible electronics. In this work, we study the growth of polystyrene pillars in the electric field for different combinations of annealing temperature, film thickness, and electrode separation (electric field intensity). There are five stages for the growth of the polystyrene pillars for all the configurations used in this work, including a nucleation stage, a linear growth stage, an acceleration stage in the pillar length prior to the contact between the top surface of a pillar and the upper electrode, a radial growth stage after the contact, and a stationary stage without further growth of the pillar. In the linear growth stage, there exist linear relationships between the pillar length and the annealing time and between the square of the pillar diameter and the annealing time. The activation energies for the rate processes controlling the radial growth and the length growth in the linear growth stage are 30.2 and 25.3 kJ/mol, respectively. There are two rate processes controlling the radial growth of the pillars: one is the field-induced flow of polymer through the polymer film to the roots of pillars and the other is the coalescence of pillars. The activation energy for the coalescence is 16.5 kJ/mol. The results obtained in this work offer a practical route to control the geometrical dimensions of polymer pillars through the processing parameters.

Coexistence and Sudden Entrapment between Two Dissimilar, Miscible Oil Lenses

The property of substrates is one of the important factors determining the interaction between two lenses (droplets). There likely exist different interactions between two dissimilar oil lenses (droplets) floating on the surface of a liquid phase from the interaction between two dissimilar oil droplets on a rigid substrate, for example, coalescence or coexistence. The interaction between two dissimilar oil lenses (droplets) is dependent on the intrinsic properties of both oil lenses (droplets) and external environmental factors. In this work, we investigate the contact interaction between two dissimilar, miscible oil lenses (toluene and silicone oil) on the surface of deionized water (DI water). The morphological evolution of two dissimilar, miscible oil lenses during the interaction under different experimental conditions is recorded and analyzed. The effects of the volume ratio of two dissimilar, miscible oil lenses, temperature of DI water, and viscosity of silicone oil on characteristic parameters are systematically studied. A sudden "entrapment" of a toluene lens into a silicone oil lens occurs after a period of the "mass exchange" (coexistence) between these two oil lenses. Several characteristic parameters, including the duration of the "mass exchange" and critical sizes of the toluene lens at the onset of the entrapment and after the entrapment, are found to be dependent on experimental conditions.

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.

A review on suppression and utilization of the coffee-ring effect

Evaporation of sessile droplets containing non-volatile solutes dispersed in a volatile solvent leaves behind ring-like solid stains. As the volatile species evaporates, pinning of the contact line gives rise to capillary flows that transport non-volatile solutes to the contact line. This phenomenon, called the coffee-ring effect, compromises the overall performance of industrially relevant manufacturing processes involving evaporation such as printing, biochemical analysis, manufacturing of nano-structured materials through colloidal and macromolecular patterning. Various approaches have been developed to suppress this phenomenon, which is otherwise difficult to avoid. The coffee-ring effect has also been leveraged to prepare new materials through convection induced assembly. This review underlines not only the strategies developed to suppress the coffee-ring effect but also sheds light on approaches to arrive at novel processes and materials. Working principles and applicability of these strategies are discussed together with a critical comparison.

Dynamics of Dissolutive Wetting: A Molecular Dynamics Study

Dissolutive wetting, i.e., dynamic wetting of a liquid droplet on dissolvable substrates, has been studied by molecular dynamics simulations. In dissolutive wetting, the geometry and properties of the solid-liquid interface evolve with the solid dissolving into the droplet; meanwhile, the droplet spreads on the receding solid surfaces. The droplets advance on the dissolvable substrate following different dynamic laws, compared with spreading on nondissolutive substrate. On the basis of molecular kinetic theory, we develop a theoretical model to reveal physical mechanisms behind the dissolutive wetting phenomena. We also find that solid particles are pulled by their hydration shells to dissolve into liquid, changing the flow field, the atomic structure, and the hydrogen bond network in the droplet. Our findings may help to comprehend the dynamics of dissolutive wetting and assist future design in practical applications.

Evaporation-induced self-assembly of quantum dots-based concentric rings on polymer-based nanocomposite films

The "ball-on-film" template is used to construct concentric rings on the surface of PMMA-QDs (polymethyl methacrylate - quantum dots) nanocomposite films via the evaporation of pure chloroform droplets, which are confined by a steel ball. The concentric rings consist of QDs, as revealed by the fluorescence images of the concentric rings. The photoluminescence intensity of the concentric rings increases with the increase of the distance to the ball center, suggesting that the amount of QDs accumulated around the contact line at individual stick state increases with the increase of the distance to the ball center. Both the wavelength and cross-sectional area (width) of the concentric rings increase approximately linearly with increasing distance to the ball center, independent of the ball size, the film thickness and the QDs concentration. For the PMMA-QDs nanocomposite films prepared from the same QDs concentration in chloroform, the thicker the PMMA-QDs nanocomposite film, the larger the wavelength for the same distance to the ball center. The effect of confinement of two steel balls on the surface patterns over the PMMA-QDs nanocomposite films is studied via a template of "two spheres on film". Symmetric surface patterns are formed. There exist two types of featureless zone between the two balls, depending on the distance between the two balls: one is the inner featureless zone and the other is the outer featureless zone. The size of both featureless zones increases with the increase of the ball distance.

Evaporation of a Volatile Liquid Lens on the Surface of an Immiscible Liquid

The evaporation behavior of toluene and hexane lenses on the surface of deionized (DI) water is studied. The toluene and hexane lenses during evaporation experience an advancing stage and a receding stage. There exists a significant difference of the evaporation behavior between the toluene lenses and the hexane lenses. The lifetime and largest diameter of both the toluene and hexane lenses increase with increasing the initial volume of the lenses. For the evaporation of the toluene lenses, the lifetime and largest diameter of the lenses decrease with increasing the temperature of DI water. The effect of the residual of the oil molecules on the evaporation of toluene lenses at a temperature of 21 °C is investigated via the evaporation of a series of consecutive toluene lenses being placed on the same position of the surface of DI water. The temporal evolution of the toluene lenses placed after the first toluene lens deviates significantly from that of the first toluene lens. Significant increase of the receding speed occurs at the dimensionless time in a range 0.7-0.8.

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.

Fabrication of asymmetric-gradient-concentric ring patterns via evaporation of droplets of PMMA solution at different substrate temperatures

Asymmetric-gradient-concentric ring patterns are fabricated via evaporating a PMMA solution droplet with a circular copper ring as template. Various micro-patterns are formed in the trench between the polymer rings.

Cooling-induced formation of honeycomb patterns on pre-cast PMMA films at low temperatures

Surface patterns formed on pre-cast PMMA films. A typical AFM image shows the hexagon network.

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.

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.