Research ArticleMATERIALS SCIENCE

Free-standing liquid membranes as unusual particle separators

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Science Advances  24 Aug 2018:
Vol. 4, no. 8, eaat3276
DOI: 10.1126/sciadv.aat3276
  • Fig. 1 Concept of liquid membrane.

    (A) Schematics showing the materials architectural differences between solid membranes, liquid-infused porous materials [for example, slippery liquid-infused porous surfaces (SLIPS) (5)], and the liquid membrane presented in the current work. (B) Conventional solid membranes use porous geometries to allow small particles to pass through while mechanically inhibiting the passage of large particles. (C) Liquid membranes rely on entirely different mechanisms for particle separation and allow reversed separation behavior: Small particles can be retained, while large ones pass through the membrane.

  • Fig. 2 Particle separation demonstration and membrane design.

    (A) This image is an overlay of four time-lapse images extracted from a video capturing two PTFE beads (one small and one large) falling into a liquid membrane at the same time from the same drop height. Tweezers used to hold the beads here were only shown for the image before the beads were released for clarity. (B) Data from 718 independent bead drop experiments used to determine the criteria for retention versus pass-through (each marker represents an individual bead drop event, and the error bars indicate the possible errors resulting from the measurements of physical parameters in Eq. 3). Scale bar, 1 cm.

  • Fig. 3 Liquid membranes as selective microorganism and particulate barrier.

    (A) This plot shows the type of organisms that theoretically can pass through (or be retained by) a specified liquid membrane (with a radius of 1.5 cm and surface tension of ~35 mN/m) according to the value of E* based on the characteristic size, locomotion speed, and mass or density of the organism of interests. (B) Demonstrations of retention of fruit flies (Drosophila hydei), houseflies (Musca domestica), and mosquitoes [Culicidae (Diptera)] by the liquid membranes at impact speeds of ~0.5, ~1.1, and ~0.9 m/s, respectively (see movie S3). Here, dead insects are used in an effort to control the impact velocity of the insect. Each panel represents an overlay of multiple images from one video to show insect position over time. (C) Time series of a live fruit fly flying into a liquid membrane (see movie S4). In the left panel, the short arrows point to fruit flies that have already been trapped in the membrane, and the long arrow points to the fruit fly of interest. In the next two panels, we see the fruit fly of interest flying up into the membrane, where it is retained in the last panel. Scale bars, 1 cm.

  • Fig. 4 Potential use of liquid membranes in nonfouling particle separation and surgery.

    (A) Dynamic reconfigurability: Unlike those in solid membranes, objects embedded in the liquid membranes can move freely in the plane of the membrane due to the mobility of the liquid molecules (see movie S5). (B) Particle transport: Particles retained in the film can also move within the plane of the liquid membrane, allowing them to be transported away if needed (see movie S6). Note also that these particular liquid membranes are transparent, allowing them to be used in applications requiring through-film visibility. (C) Self-cleaning of liquid membranes: Here, a tilted liquid membrane passively removes contaminates (that is, small sand particles) from the separation region by gravity, allowing the large particle to be collected in the left petri dish. The small particles are collected downstream, forming a growing aggregate that will later fall from the membrane into the petri dish on the right when the weight of the aggregates exceeds the capillary force exerted by the liquid membrane (“aggregate removal”; see movie S7). (D) Simulated surgery: We demonstrated that the liquid membrane can block contaminants during simulated surgical procedures without inhibiting visibility or in-film maneuverability and can passively and continuously collect and remove contaminants (see movie S8). (E) Plot showing the retention of various amounts of contaminants on a liquid membrane. Note that, in all the trials, no measurable amount of contaminant leaked through the membranes and, therefore, no data are shown for the red data bar representing “mass passed through.” The contaminant was different quantities of fluorescent powder [the same as that used in (D)] sprinkled from a drop height of ~1 cm. Scale bars, 1 cm.

  • Fig. 5 Potential use of liquid membranes as selective solid/gas filters.

    (A) A liquid film can serve as a gas diffusion barrier while allowing macroscopic objects to pass through (for example, odor management; see movie S9). (B) Schematic of the experimental setup in assessing the ability of a liquid membrane (barrier) to prevent the passage of fog. (C) Quantitative measurements showing that a liquid membrane can be as effective as a solid glass barrier in blocking the fog passage. (D) Gas sequestration experimental setup. Here, a hexane-filled test tube was covered with either parafilm or a liquid membrane. (E) The measured sensor output (signal voltage Vs normalized by the maximum voltage in ambient conditions, Vs,max) over time when exposed to ambient (black circles) and hexane vapor (red circles) and when covered by either a parafilm sheet (green circles) or a liquid membrane (blue circles). The liquid membrane consists of a 7:3 ratio of deionized water to glycerol by volume and 8.5 mM SDS.

  • Fig. 6 Longevity and mechanical perturbation of a liquid membrane.

    (A) Time sequence of images showing the mechanical perturbation of a liquid membrane by a smooth glass rod for >3 hours. Probing took place at a period of 3.55 s over 3042 cycles (see movie S10), and liquid was replenished at a rate of ~1 to 2 ml/min. We note that the glass rod is hydrophilic, so some liquid from the membrane wets its surface as it passes into the membrane repeatedly. Scale bar, 1 cm. (B) Plot showing the mechanical perturbation cycle of the liquid membrane. The experiment was set up to acquire images at various points in the cycles (shown in red open circles). Images were acquired at a rate of 1 frame/s. Note that A is the vertical location of the probe tip, and A0 is the amplitude of the probing cycle.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/8/eaat3276/DC1

    Section S1. Descriptions of movies S1 to S10

    Section S2. Sources of energy dissipation

    Fig. S1. A schematic diagram showing a bead passing through a liquid membrane.

    Fig. S2. Comparison of the relative magnitude of different energy terms.

    Fig. S3. Theoretical dependence of E* on relevant parameters.

    Fig. S4. Effect of mechanical perturbation frequency on the longevity of liquid membranes.

    Fig. S5. Residual liquid on both hydrophilic and hydrophobic particles.

    Table S1. Surface tension and density of different liquid membrane solutions.

    Table S2. Liquid membrane mass and thickness characterization.

    Table S3. Surface roughness measurements of various bead materials.

    Table S4. Advancing angles of liquid membrane solution droplets on a flat surface.

    Table S5. Reported inertial parameters for various organisms and particles.

    Movie S1. Large and small bead separation.

    Movie S2. Particle filtration.

    Movie S3. Insect retention.

    Movie S4. Live insect retention.

    Movie S5. In-film probe movement.

    Movie S6. Particle transport.

    Movie S7. Self-cleaning of liquid membranes.

    Movie S8. Simulated surgery.

    Movie S9. Liquid membranes as selective gas/solid barriers.

    Movie S10. Liquid membrane longevity.

    References (2740)

  • Supplementary Materials

    The PDF file includes:

    • Section S1. Descriptions of movies S1 to S10
    • Section S2. Sources of energy dissipation
    • Legend for Movies S1 to S10
    • Fig. S1. A schematic diagram showing a bead passing through a liquid membrane.
    • Fig. S2. Comparison of the relative magnitude of different energy terms.
    • Fig. S3. Theoretical dependence of E* on relevant parameters.
    • Fig. S4. Effect of mechanical perturbation frequency on the longevity of liquid membranes.
    • Fig. S5. Residual liquid on both hydrophilic and hydrophobic particles.
    • Table S1. Surface tension and density of different liquid membrane solutions.
    • Table S2. Liquid membrane mass and thickness characterization.
    • Table S3. Surface roughness measurements of various bead materials.
    • Table S4. Advancing angles of liquid membrane solution droplets on a flat surface.
    • Table S5. Reported inertial parameters for various organisms and particles.
    • References (2740)

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mov format). Large and small bead separation.
    • Movie S2 (.mov format). Particle filtration.
    • Movie S3 (.mov format). Insect retention.
    • Movie S4 (.mov format). Live insect retention.
    • Movie S5 (.mov format). In-film probe movement.
    • Movie S6 (.mov format). Particle transport.
    • Movie S7 (.mov format). Self-cleaning of liquid membranes.
    • Movie S8 (.mov format). Simulated surgery.
    • Movie S9 (.mov format). Liquid membranes as selective gas/solid barriers.
    • Movie S10 (.mov format). Liquid membrane longevity.

    Files in this Data Supplement:

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