Correlating dynamic microstructure to observed color in electrophoretic displays via in situ small-angle x-ray scattering

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Correlating dynamic microstructure to observed color in electrophoretic displays via in situ small-angle x-ray scattering

Scott C. Bukosky, Joshua A. Hammons, Brian Giera, Elaine Lee, Jinkyu Han, Megan C. Freyman, Anna Ivanovskaya, Kerry G. Krauter, Joshua D. Kuntz, Marcus A. Worsley, T. Yong-Jin Han, William D. Ristenpart, and Andrew J. Pascall

Phys. Rev. Materials 4, 075802 – Published 27 July 2020

Abstract

Electrophoretic deposition (EPD) is an industrially relevant and scalable technique used to form particle deposits from colloidal suspensions. Highly concentrated particle suspensions generally prevent real-time in situ microscopy observations which limit the characterization of EPD films to ex situ, or postprocessed, laboratory techniques. For dynamic systems, such as tunable amorphous photonic crystals (APCs), only reversible deposits are formed during the EPD process. Since reversible deposits cannot be characterized with standard ex situ methods, the particle-particle and particle-field interactions that govern the displayed color and crystallinity of these systems are not well understood. Here, we present in situ small-angle x-ray scattering and UV-Vis techniques for measuring both the structural and optical response of an APC under applied electric fields. We also develop a computational model based on colloidal interactions to explain the observed change in the interparticle spacing of APCs due to the applied electric field which correlates to displayed color. Ultimately, this work provides a new in situ characterization method that could be expanded for other dynamic, tunable colloidal systems.

Received 9 March 2019
Revised 4 May 2020
Accepted 16 June 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.075802

Physics Subject Headings (PhySH)

Colloids

Small-angle x-ray scattering

Fluid Dynamics

Authors & Affiliations

Scott C. Bukosky ^1,2, Joshua A. Hammons ¹, Brian Giera ¹, Elaine Lee¹, Jinkyu Han¹, Megan C. Freyman¹, Anna Ivanovskaya¹, Kerry G. Krauter¹, Joshua D. Kuntz¹, Marcus A. Worsley ¹, T. Yong-Jin Han ¹, William D. Ristenpart², and Andrew J. Pascall ^1,*

¹Lawrence Livermore National Laboratory, Livermore, California 94551, USA
²Department of Chemical Engineering, University of California Davis, Davis, California 95616, USA

^*Corresponding author: pascall1@llnl.gov

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Issue

Vol. 4, Iss. 7 — July 2020

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Images

Figure 1
(a) Schematic of a sample device depicting particle packing near the surface of the working electrode in response to an applied DC field. (b) Diagram of the in situ experimental setup during x-ray scattering measurements (diagrams are not to scale). (c) Representative images of bulk ${Fe}_{2} O_{3} / {SiO}_{2}$ core/shell particles suspended in propylene carbonate at varying concentration.
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Figure 2
Representative images of the observed structural color response as a function of voltage for each concentration of ${Fe}_{2} O_{3} / {SiO}_{2}$ particles. After applying 5 V, the field was removed (OFF), and the color was observed to approach its initial state. The scale bar is 5 mm.
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Figure 3
Slit-smeared SAXS Intensity vs $q$ plots with associated fits for 42 $vol %$ samples. Data for each voltage are offset on the $y$ axis for better visualization. The vertical dashed line highlights the scattering peak shift near $q = 0.004$ $Å^{- 1}$ . (inset) Magnified view of intensity peak shift at $0.003 < q$ ( $Å^{- 1}) < 0.006$ . The final OFF scan was completed $\sim 30$ s after removal of the electric field.
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Figure 4
(a) Calculated hard sphere radius ( $R_{HS}$ ) vs voltage for 42 $vol %$ $(▪)$ and 54 $vol %$ $(▴)$ concentrations as well as the mesoscale simulation prediction $(\circ)$ . Above 3 V, the measured $R_{HS}$ approaches ${\bar{R}}_{p}$ (dashed line). Vertical error bars result from spatial variability within samples (see supplemental). (b) Diagram depicting the interparticle dimensions. (c) Cyclic voltammogram of 54 vol% ${Fe}_{2} O_{3} / {SiO}_{2}$ core/shell particles in propylene carbonate. At greater than 3.5 V, a sharp increase in the current is observed corresponding to the onset of propylene carbonate oxidation reactions. Arrows represent direction of voltage sweep.
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Figure 5
(a) Normalized visible reflectance spectra for a 42 $vol %$ concentration device. The inherent pigmentary color of iron oxide dominates at $λ > 550$ nm. Variations in structural color below $λ = 550$ nm show a blue-shift with increasing applied voltage. (b) Change in device reflectance ( $Δ {\hat{R}}_{i}$ ) from the initial 0 V state. $Δ {\hat{R}}_{i} > 0$ corresponds to wavelengths of increased scattering compared to 0 V.
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Figure 6
Simulation snapshots from the mesoscale model depicting particle correlation scattering of visible light at (a) 0 V (OFF) and (b) 5 V (ON). Particles are more tightly packed in the ON state of the device as compared to the initial OFF state which results in an observable color shift. (c) Measured $λ_{UV}$ peak values compared to the predicted $λ_{SAXS}$ values. The $45^{\circ}$ line denotes perfect agreement with theory. Vertical error bars result from error in Gaussian fitting of UV-Vis reflectance peaks (magnitude is smaller than the data points) while horizontal error bars result from spatial variability within samples [39].
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