Mesoscale Patterns Formed by Evaporation of a Polymer Solution in the Proximity of a Sphere on a Smooth Substrate:  Molecular Weight and Curvature Effects

Abstract

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.

Directed Self‐Assembly of Gradient Concentric Carbon Nanotube Rings

“…The axially symmetric sphere-on-flat geometry provides a unique environment (i.e., a bound solution) for controlling the flow within an evaporating droplet, which in turn regulates the structure formation. [35][36][37][38][39][40][41] Thus, in sharp contrast to the irregular concentric rings formed due to stochastic ''stick-slip'' cycles of the contact line when a droplet evaporates from a single surface (i.e., an unbound solution), [44,45,47,48] highly ordered, gradient concentric rings of MEH-PPV are produced using the sphere-on-flat geometry.…”

“…[35][36][37][38][39][40][41][42][43] Rather than allowing the solvent to evaporate over the entire droplet area as in copious past work, in which droplets evaporated from a single surface, [44][45][46] the evaporation was restricted at the droplet edges. [35,36] Concentric rings were formed by controlled, repetitive pinning and depinning of the contact line (i.e., ''stick-slip'' motion).…”

Robust Self‐Assembly of Highly Ordered Complex Structures by Controlled Evaporation of Confined Microfluids

“…Branching at the transition region (location 2) can be attributed mainly to the difference in height between the MEH-PPV capillary bridge under the arm and the body of the triangular-slice sphere, where H body is smaller than H arm (Figure 3 a), which led to a difference in the evaporation rate of the solvent. Because a larger capillary bridge has a higher evaporation rate, [27] the MEH-PPV meniscus beneath the arm retracted faster than that beneath the body of the triangular-slice sphere. The imbalance between the fast "stick-slip" cycles of the contact line caused by the larger capillary bridge (H arm ) and the slower "stick-slip" cycles at the smaller capillary entrance (H body ) was responsible for a locally branched pinning of the contact lines at location 2.…”

Synthesis of carbon microrings using polymer blends as templates

“…First, an amylose/PDP mixture was dissolved in DMSO. [33][34][35][36][37][38] Due to its higher surface tension, PDP was aggregated at the lm surface of the pores and created a ring at the rim of the pore as reported previously. Upon drying of the sample under a continuous ow of nitrogen, phase separation of the ternary amylose/PDP/DMSO solution had occurred.…”

“…Upon drying of the sample under a continuous ow of nitrogen, phase separation of the ternary amylose/PDP/DMSO solution had occurred. 18,20,21,37,39,40 The obtained ring morphology was cured into a highly crosslinked phenolic resin in the presence of formaldehyde vapor at 110 C for 5 h. 24,41,42 The cross-linked resin was carbonized under a nitrogen atmosphere at 700 C for 3 h to obtain the carbon microrings. 30 In the present study, a ring morphology was observed and the process of ring formation is believed to be caused by the two-step phase separation.…”