Research ArticleChemistry

Long-range, selective, on-demand suspension interactions: Combining and triggering soluto-inertial beacons

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Science Advances  16 Aug 2019:
Vol. 5, no. 8, eaax1893
DOI: 10.1126/sciadv.aax1893
  • Fig. 1 Using SI beacons of one or more types to drive controlled colloidal migration.

    (A) A single SI beacon source that maintains solute outflux, driving DP migration (here down gradient) of particles, giving SI “repulsion.” (B-1) Triggered release: A thermosensitive beacon keeps solute trapped at high-enough temperature (T > TLCST); (B-2) upon cooling below LCST, the beacon releases the solute, initiating SI migration. (C) SI dipole: SI beacon source emits solute, as in (A), whereas the SI beacon sink absorbs the solute, establishing an SI dipolar flux that directs particle migration from one beacon to the other. (D) Combining SI beacon sources releasing distinct solutes may attract one particle species (yellow) to one source, a second species (green) to a second source, and repelling a third species of particle (black) from both. (E) SI beacon sources releasing solutes that react with one another (e.g., acid-base) establish long-lasting fluxes from each beacon to the reaction zone, driving particles accordingly.

  • Fig. 2 Triggered solute release and SI migration using pNIPAm beacons.

    (A) A pNIPAm gel above the LCST can be used to trap solute molecules. The dashed red line shows the gel boundary. (B) The gel swells when it cools below the LCST, and the trapped solute diffuses out, establishing a concentration gradient that causes neighboring particles to migrate via DP. Control experiment: (C) The gel above the LCST closes its pores and occupies a smaller volume. (D) As the gel cools down, the pores open up and it expands, pushing the neighboring particles out of the way. (E) Points represent the change in the particle interface radius (Ri) as the gels undergo transition at the LCST.

  • Fig. 3 SI source-sink dipole.

    Streak lines showing the trajectories of negatively charged PS particles migrating due to (A) an SDS source, (B) both SDS source and sink, and (C) an SDS source and an inert obstacle that does not absorb SDS. (D) Time-averaged azimuthal particle velocities as a function of angular position with respect to the source center under the scenarios depicted in (A) to (C). Error bars represent SD. Points show experimental data. Lines are predictions from COMSOL computations. Details of the COMSOL model are provided in the Supplementary Materials. The shaded region shows angle subtended by sink at source center, φsink = 12.5o. The inset shows the solute concentration field and local particle velocities as computed by COMSOL. (E) Flux of particles, Jt, entering a region with radius r = 1.5*Rbeacon around the sink. (F) Cumulative particles captured in the region as function of time (as a fraction of number of particles in that region at t = 0, N0). Dashed lines are for visual guidance.

  • Fig. 4 SI migration driven by two sources releasing distinct solutes.

    (A) Streak lines show negatively charged PS particles repelled by SDS source and attracted to ionic liquid (IL), [C6mim][I] source. (B) SI particle velocities, averaged in the interbeacon region, are larger when migrating under the composite gradient (black squares), compared with under either source in isolation, although slower than simple superposition of the two sources (purple dash). Error bars show SD. (C) An SDS source repels negative PS particles (blue streaks) but simultaneously attracts decane drops (white streaks). (D) Distinct beacon sources, one releasing SDS (left) and the other releasing IL, [C6mim][I] (right). Decane drops (white) migrate toward the SDS source beacon, whereas negative PS beads migrate toward the [C6mim][I] source beacon. Inset shows accumulation of decane (black) and PS (white) particles around the two beacons at the end of the experiment.

  • Fig. 5 SI migration driven by reactive solutes.

    Streak lines showing migration under the flux generated by the reaction between Ca(OH)2 and benzoic acid. (A) Aminated (positive) PS particles migrate toward the acidic beacon, while (B) sulfonated (negative) PS particles migrate toward the basic Ca(OH)2 source. (C) Beads migrate the fastest and for a longer duration when there is a reaction between the acid and the base (black diamonds) as opposed to migrating under the flux of acid (blue squares) or base (red circles) alone. Error bars show SD. Inset shows Ca(OH)2 and C7H6O2 crystals stored in the PEG-DA beacons in their solid form.

  • Fig. 6 Focusing and defocusing of particles migrating under an acid-base flux.

    (A) Positive PS particles focus at the reaction zone. (B) Negative PS particles defocus at the reaction zone, which is defined as the region where a maximum change in velocity is observed. The reaction zone (xR) gradually shifts to the left (toward the acid source) over the course of the experiment. Error bars represent SD in average velocities over the time intervals. Positions are normalized by the distance between the two beacons.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/8/eaax1893/DC1

    Fig. S1. Colloidal migration under α-ketoglutaric acid gradients.

    Fig. S2. Negatively charged PS particles move down α-ketoglutaric acid gradients.

    Fig. S3. DP focusing/defocusing under acid-base gradients.

    Table S1. Parameters used for numerically calculating the concentration profile of PS particles migrating under an SDS flux, generated by a source-sink pair of SI beacons.

    Table S2. Diffusivity and solubility of benzoic acid and calcium hydroxide in water.

    Movie S1. A pNIPAm beacon with residual photoinitiator (α-ketoglutaric acid) undergoing phase transition at the LCST (33°C).

    Movie S2. A pNIPAm beacon from which all the solute has been flushed out, undergoing phase transition at the LCST.

    Movie S3. SI dipole interaction between a beacon source (left) that releases SDS and a beacon sink (right) that absorbs the solute.

    Movie S4. COMSOL computations revealing the behavior of particles under an SI dipolar flux, reproducing the experimental observations of movie S3.

    Movie S5. SI interaction between two sources releasing distinct solutes.

    Movie S6. A beacon source releasing SDS attracts decane drops (black) and simultaneously repels negative PS particles (white).

    Movie S7. Two distinct beacon sources, one releasing SDS (left) while the other releasing [C6mim][I] (right), attract decane drops (black) and negative PS beads (white), respectively, separating them from the mixture.

    Movie S8. SI migration of positively charged particles driven by beacon sources that release solutes that react with each other.

    Movie S9. SI migration of negatively charged particles driven by beacon sources that release solutes that react with each other.

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. Colloidal migration under α-ketoglutaric acid gradients.
    • Fig. S2. Negatively charged PS particles move down α-ketoglutaric acid gradients.
    • Fig. S3. DP focusing/defocusing under acid-base gradients.
    • Table S1. Parameters used for numerically calculating the concentration profile of PS particles migrating under an SDS flux, generated by a source-sink pair of SI beacons.
    • Table S2. Diffusivity and solubility of benzoic acid and calcium hydroxide in water.
    • Legends for movies S1 to S9

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). A pNIPAm beacon with residual photoinitiator (α-ketoglutaric acid) undergoing phase transition at the LCST (33°C).
    • Movie S2 (.mp4 format). A pNIPAm beacon from which all the solute has been flushed out, undergoing phase transition at the LCST.
    • Movie S3 (.mp4 format). SI dipole interaction between a beacon source (left) that releases SDS and a beacon sink (right) that absorbs the solute.
    • Movie S4 (.avi format). COMSOL computations revealing the behavior of particles under an SI dipolar flux, reproducing the experimental observations of movie S3.
    • Movie S5 (.mp4 format). SI interaction between two sources releasing distinct solutes.
    • Movie S6 (.mp4 format). A beacon source releasing SDS attracts decane drops (black) and simultaneously repels negative PS particles (white).
    • Movie S7 (.mp4 format). Two distinct beacon sources, one releasing SDS (left) while the other releasing C6mimI (right), attract decane drops (black) and negative PS beads (white), respectively, separating them from the mixture.
    • Movie S8 (.mp4 format). SI migration of positively charged particles driven by beacon sources that release solutes that react with each other.
    • Movie S9 (.mp4 format). SI migration of negatively charged particles driven by beacon sources that release solutes that react with each other.

    Files in this Data Supplement:

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