Breath Figure Assembly on Evaporating Polymer Solution Droplets in Levitation

Open Access

Breath Figure Assembly on Evaporating Polymer Solution Droplets in Levitation

Róisín A. O’Connell, William N. Sharratt, and João T. Cabral

Phys. Rev. Lett. 131, 218101 – Published 22 November 2023

Abstract

We investigate the drying of isolated polymer solution droplets, employing acoustic levitation, and demonstrate the spontaneous generation of breath figures (BF) on the resulting polymer particles and capsules ( $\sim 5 - 1000 μ m$ ) with controlled surface pore arrays ( $< 1 - 20 μ m$ ). By contrast with supported polymer thin films, the evaporative cooling experienced by suspended droplets suffices to yield ubiquitous BF formation, owing to their thermal insulation and the synchronous condensation and self-assembly of water microdroplets, accompanied by capsule skin formation and kinetic arrest. A simple model describes simultaneously the radius and temperature evolution along the droplet-to-particle transformation, and the scaling of surface pore dimensions, with environmental parameters. The generality of the approach is demonstrated with a range of model polymers, and the coupled roles of solution thermodynamics and droplet environment are shown to permit the facile design of capsules with tunable transport and dissolution kinetics.

Received 11 January 2023
Accepted 25 October 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.218101

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Smart materials Surface & interfacial phenomena Surface instabilities

Polymers & Soft Matter

Authors & Affiliations

Róisín A. O’Connell , William N. Sharratt , and João T. Cabral ^*

Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom

^*j.cabral@imperial.ac.uk

Article Text

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Supplemental Material

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References

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Issue

Vol. 131, Iss. 21 — 24 November 2023

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Images

Figure 1
(a) Polymer solution droplet drying in acoustic levitation, depicting: solvent evaporation, skin and cavity formation, and surface buckling; under prescribed conditions, evaporative cooling leads to water condensation, BF, and regular pore array formation. (b) Drying of $PS / DCM$ droplets (0.5, 4 and $8 w / w %$ PS) at 40% RH, $21 ° C$ . Inset: water microdroplet condensation on the droplet surface. (c) Normalized equivalent radii as a function of time for $0.5 - 8 w / w %$ PS droplets; solid lines are model fits (outlined in Supplemental Material [26]), and $τ_{1}$ and $τ_{2}$ indicate skin formation and kinetic arrest timescales. (d) Surface temperature profile for DCM droplets at 10%–100% RH (detailed in Supplemental Material [26], Sec. 7), and $τ_{1}$ values obtained in (c).
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Figure 2
(a) SEM images of polymer particles generated from acoustically levitated droplets of $0.5 - 8 w / w %$ PS/DCM solutions at $21 ° C$ , 40% RH, showing the final particle morphology and porous surface structure. (b) Voronoi nearest-neighbor tessellations. (c) Predominant 5 (pink), 6 (green), and 7 (yellow) neighboring pore arrangements. (d) Normalized frequency of nearest pore neighbors. (e) Dependence of half width half maximum (HWHM) of distributions in (d) on initial polymer concentration. (f) Area-weighted average pore radius distributions and (g) average pore radii concentration dependence.
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Figure 3
(a) SEM images of particle surfaces produced from $4 w / w %$ $PS / DCM$ droplets at $21 ° C$ and varying RH. (b) Corresponding Voronoi nearest-neighbor tessellations. (c) Film surfaces cast onto glass under identical conditions (with Voronoi insets). (d),(e) Surface morphology maps obtained for levitated particles and films: ordered hexagonal BF (green), nonuniform BF (yellow), no pores (red). (f) Normalized frequency of pore nearest neighbor number as a function of RH for $4 w / w %$ PS particles (HWHM in inset). (g) Average pore size and maximum variation. (h) Pore surface area coverage with varying RH.
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Figure 4
(a) Dissolution profiles estimated by the change in (projected) particle area normalized by initial area ( $A_{0}$ ) for $4 w / w %$ $PS / DCM$ particles, produced at varying RH, immersed in $θ$ -solvent cyclohexane; inset shows the dissolution time. (b) SEM images of particle cross sections formed at 10% and 100% RH. (c) Optical images depicting time-resolved dissolution of the PS particles formed at 10% and 100% RH, $21 ° C$ in cyclohexane.
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