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Abstract
The crystallization of metastable liquid “phase change materials” releases stored energy as latent heat upon nucleation and may therefore provide a triggerable means of activating downstream processes that respond to changes in temperature. In this work, we describe a strategy for controlling the fast, exothermic crystallization of sodium acetate from a metastable aqueous solution into trihydrate crystals within a polyacrylamide hydrogel whose polymerization state has been patterned using photomasks. A comprehensive experimental study of crystal shapes, crystal growth front velocities and evolving thermal profiles showed that rapid growth of long needle-like crystals through unpolymerized solutions produced peak temperatures of up to 45˚C, while slower-crystallizing polymerized solutions produced polycrystalline composites and peaked at 30˚C due to lower rates of heat release relative to dissipation in these regions. This temperature difference and propagating heat waves, which we describe using a proposed analytical model, enable the use of this strategy to selectively activate thermoresponsive processes in predefined areas.
Supplementary materials
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Supplementary Information
Description
Supplementary Information Sections S1-S6, Figures S1-S8, and Tables S1-S6. 16 pages.
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Video S1. Crystallization of a metastable solution polymerized through a photomask.
Description
Sample was exposed to UV light in all areas outside the letter H, which was covered by a mask. Precursor solution composition: 2.8 M acrylamide, 26 mM N,N’-methylenebisacrylamide, 7.0 M sodium acetate, 2 mM alpha-ketoglutaric acid. This video also demonstrates the procedure for crystallizing samples suspended in air; a paper grid supports the sample. Ruler shown for scale; numbered increments are centimeters. Video is shown in real time.
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Video S2. Video microscopy (10x) through crossed linear polarizers showing progression of crystal growth along interface between masked and unmasked regions
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Video shown at 10x magnification and 0.2x speed with automatic exposure. First frame shows faint phase boundary between masked (left) and unmasked (right) regions before growth front reaches field of view. Crystals grow rapidly through masked region with large, needle-like domains, then slowly outward through unmasked region where additives have been photopolymerized, forming small crystallites. Higher magnification view of unmasked region is shown in Figure S1; electron micrographs of similar samples are shown in Figure S2. After some time has passed, a secondary cascade of small crystallites engulfs the large needles in the masked region; this corresponds to the growth of bright white areas visible in Figure 4A and Video S4. Secondary crystallite growth appears to be restricted to masked areas. Precursor solution composition: 2.8 M acrylamide, 26 mM N,N’-methylenebisacrylamide, 7.0 M sodium acetate, 2 mM alpha-ketoglutaric acid.
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Video S3. Video microscopy (5x) through crossed linear polarizers showing progression of crystal growth along interface between masked and unmasked regions
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Video shown at 5x magnification and 0.2x speed with automatic exposure. See caption from Video S2. This video also shows converging fronts and bubble formation, which is common in regions where fronts converge and hypothesized to be a result of gel contraction during crystallization due to the higher density of the solid phase.
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Video S4. Visible, infrared, and threshold temperature videos of crystal growth in hydrogel polymerized through “hello” photomask
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10x speed; sample was crystallized on a Peltier cold plate held at 18˚C. Precursor solution composition: 2.8 M acrylamide, 13 mM N,N’-methylenebisacrylamide, 7.0 M sodium acetate, 2 mM alpha-ketoglutaric acid. Threshold temperature shown: 34.5 ˚C.
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Video S5. Visible, infrared, and threshold temperature videos of crystal growth in hydrogel polymerized through hexagonal grid photomask.
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10x speed; sample was crystallized while suspended in air on a paper grid, slowing heat dissipation in areas with converging fronts. Precursor solution composition: 2.8 M acrylamide, 13 mM N,N’-methylenebisacrylamide, 7.0 M sodium acetate, 2 mM alpha-ketoglutaric acid. Threshold temperature shown: 42.5 ˚C.
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Video S6. Confocal Z-stack showing crystallization-induced wave of thermoresponsive gel contraction
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30x speed. Top-left is XZ plane (1.28 x 0.24 mm), bottom-left is XY plane (1.28 x 1.28 mm), bottom-right is YZ plane (0.24 x 1.28 mm). Film composition: N-isopropylacrylamide and Nile blue acrylamide cross-linked with polyethylene glycol diacrylate in water, frozen and thawed twice to induce microporosity and improve swelling kinetics. Fluorescence emission appears in red (excitation wavelength was 488 nm); transmitted light appears in white and vanishes once crystals pass under sample. Metastable hydrogel precursor solution composition: 2.8 M acrylamide, 19.5 mM N,N’-methylenebisacrylamide, 7.0 M sodium acetate, 2 mM alpha-ketoglutaric acid; 2 mm spacer. Channel below area on screen was unmasked during polymerization.
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Video S7. Confocal Z-stack showing crystallization-induced wave of thermoresponsive gel contraction
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35x speed. Top-left is XZ plane (2.56 x 0.26 mm), bottom-left is XY plane (2.56 x 2.56 mm), bottom-right is YZ plane (0.26 x 2.56 mm). Film composition: N-isopropylacrylamide and Nile blue acrylamide cross-linked with polyethylene glycol diacrylate in water, frozen and thawed twice to induce microporosity and improve swelling kinetics. Fluorescence emission appears in red (excitation wavelength was 488 nm). Metastable hydrogel precursor solution composition: 2.8 M acrylamide, 19.5 mM N,N’-methylenebisacrylamide, 7.0 M sodium acetate, 2 mM alpha-ketoglutaric acid; 2 mm spacer. Channel below area on screen was unmasked during polymerization.
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Video S8. Confocal Z-stack showing selective gel contraction over masked region of crystallizing sample
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13x speed. Top-left is XZ plane (2.56 x 0.30 mm), bottom-left is XY plane (2.56 x 2.56 mm), bottom-right is YZ plane (0.30 x 2.56 mm). Film composition: N-isopropylacrylamide and Nile blue acrylamide cross-linked with polyethylene glycol diacrylate in water, frozen and thawed twice to induce microporosity and improve swelling kinetics. Fluorescence emission appears in red (excitation wavelength was 488 nm). Metastable hydrogel precursor solution composition: 2.8 M acrylamide, 19.5 mM N,N’-methylenebisacrylamide, 7.0 M sodium acetate, 2 mM alpha-ketoglutaric acid; 2 mm spacer. Channel below area on screen was masked on left side during polymerization.
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Video S9. Wax melting and wetting processes patterned using heat from crystal growth in hydrogel polymerized through “HOT” photomask
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10x speed. A layer of wax had been deposited onto the underside of the paper, which was attached using thermally conductive glue to the top of a channel bearing a patterned metastable hydrogel. Sample was crystallized on a Peltier cold plate held at 20˚C. Wax composition: 1% (w/v) Oil Blue N in icosane. Hydrogel precursor solution composition: 2.8 M acrylamide, 19.5 mM N,N’-methylenebisacrylamide, 7.0 M sodium acetate, 2 mM alpha-ketoglutaric acid.
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