Research ArticleAPPLIED SCIENCES AND ENGINEERING

Photothermal trap utilizing solar illumination for ice mitigation

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Science Advances  31 Aug 2018:
Vol. 4, no. 8, eaat0127
DOI: 10.1126/sciadv.aat0127
  • Fig. 1 Concept of photothermal trap for melting of ice.

    (A) Schematic of the photothermal trap applied on the base substrate as a laminate and the associated heat transfer mechanisms. This laminate consists of (top to bottom) highly absorbing cermet, thermal spreader, and insulating layer. The thickness of the absorber layer is exaggerated. (B) Heating scenarios for the laminate compared to the reference cases: insulating and conductive layers. The white-to-red color scale indicates the obtained surface heating after a few seconds. (C) Sketch of the experimental setup showing separation of the inner experimental chamber (orange) from the environment and the flow of cooling air (blue arrows).

  • Fig. 2 Single-drop experiments.

    Freezing (A) and melting (B and C) under an illumination of 1.8 kW/m2. (A) Change of surface temperature T (red) upon drop freezing at an ambient temperature of approximately −20°C. The ambient temperature increases slightly due to the passive equilibration process (no active cooling). Release of latent heat upon recalescence (t = 0 s) causes an increase in surface temperature and formation of a condensation ring. Snapshots show the progression of freezing (see also movie S1). (B) Change of surface temperature T upon illumination of various surfaces at an ambient temperature of −25°C (h =17.3 W m−2 K−1): Only the photothermal trap (red) induces melting, while the droplet placed on a thin aluminum surface (blue), thick aluminum surface (cyan), or insulating carbon foam (gray) remains frozen. The dashed lines show the corresponding model predictions (not accounting for phase change). Insets show the initial frozen droplet (left inset) and final state (melted or frozen; right insets). Snapshots in the lower row show intermediate steps in the progression of melting on the photothermal trap. (C) Representative snapshots of melting on the photothermal trap when illuminated immediately upon recalescence. Left snapshot: The melting front (lower dashed line) catches up with the freezing front (upper dashed line). Here, Tamb = −15°C (h =16.7 W m−2 K−1). For visualization purposes, and to keep wetting properties constant across all tested surfaces, we coated the surfaces with a ~100-nm Teflon layer, yielding a large contact angle of ~120°; this does not compromise light absorption (see the Supplementary Materials). Initial drop volume is 40 μl, corresponding to a base diameter of 4.6 ± 0.2 mm (s = 0.03) [see scale bar in (A)].

  • Fig. 3 Performance of photothermal trap versus reference surfaces.

    (A) Performance diagram, showing the melting delay t0 upon illumination as a function of Tamb. Solid colored lines show the numerical results, Eq. 1, for the various substrates (red, blue, and cyan) at an illumination of 1.8 kW/m2 and an ice coverage fraction s = 0.03. Red dashed lines indicate varying illumination intensity: 1.0 and 0.5 kW/m2 for the photothermal trap. For each surface, the curves divide the state of the drop into fully frozen (left of the curve) versus, at least partially, melted (right of the curve). The markers show the experimental data. Colored arrows indicate (left) an increase in ΔTeq (shift in asymptote Tamb,min) with increased absorbed energy and (down) a decrease in time scale τ with decreased thermal mass. (B) Nondimensional phase diagram obtained by plotting t0/τ (black line) as a function of |ΔTeq/Tamb| shows a collapse of data. Left of the asymptote (black dashed line), the droplet remains frozen. Representative snapshots of a 40-μl droplet highlight frozen and melted regions. (C) Influence of heat transfer coefficient on photothermal trap performance: minimum ambient temperature Tamb,min for which the photothermal trap induces melting, as a function of heat transfer coefficient. Numerical results shown correspond to illumination of the laboratory-scale photothermal trap with 1 kW/m2 [the cross denotes the asymptote of the red short dashed lines in (A)].

  • Fig. 4 Application tests.

    (A) Sliding of a frozen drop on a 30° tilted substrate. Sliding starts promptly when a thin liquid film is present (here, 19.8 s after illumination). Insets show the liquid layer 0.5 and 0.7 s after the start of melting (highlighted by yellow dashed boxes). Illumination is at the lower side of the incline, explaining the increase of brightness when the drop starts moving. Drop volume is 40 μl. (B) Melting of a frost layer (s = 1). The substrate is illuminated, through the frost layer, at its right side outside the video frame. Melting proceeds from right to left in two stages: First, the surface layer melts (white dashed lines indicate the melting front), followed by bulk melting that allows collection of most of the liquid by dewetting (red dashed lines), leaving only a few residual microdroplets on the surface. Conditions in both panels: an illumination of 1.8 kW/m2 at an ambient temperature of −15°C.

  • Fig. 5 Outdoor performance.

    (A) The experimental setup is taken outside in Cambridge, MA. It has a thin aluminum surface layer (left) and the photothermal trap (right). Both have areas of 0.004 m2. Photo credit: J. de Ruiter, Massachusetts Institute of Technology, Cambridge. (B) Temperature increase upon illumination, and resulting sliding-off of frozen 0.5-ml puddles (snapshot). The ambient temperature is below zero, and slightly fluctuating, −3.5 ± 1.3°C (black curve), due to variations in wind velocity and sun intensity. The solar intensity is ~0.6 kW/m2. (C) Melting of snow on the photothermal trap, shown at 2 and 5 min after initial exposure to sunlight (t = 0).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/8/eaat0127/DC1

    Section S1. Absorptivity of materials

    Section S2. Estimation of temporal temperature rise of the substrate due to illumination

    Section S3. Freezing and melting movies

    Section S4. Droplet sliding upon illumination

    Section S5. Demonstration experiment with forced convection

    Section S6. Melting of frost layer

    Fig. S1. Sample absorptivity.

    Fig. S2. Control volume approach to determine the transient temperature evolution of the metal spreader upon illumination.

    Fig. S3. Substrate under direct shear flow.

    Movie S1. Droplet freezing.

    Movie S2. Droplet melting.

    Movie S3. Droplet sliding on melt layer.

    Movie S4. Melting of frost layer.

  • Supplementary Materials

    The PDF file includes:

    • Section S1. Absorptivity of materials
    • Section S2. Estimation of temporal temperature rise of the substrate due to illumination
    • Section S3. Freezing and melting movies
    • Section S4. Droplet sliding upon illumination
    • Section S5. Demonstration experiment with forced convection
    • Section S6. Melting of frost layer
    • Fig. S1. Sample absorptivity.
    • Fig. S2. Control volume approach to determine the transient temperature evolution of the metal spreader upon illumination.
    • Fig. S3. Substrate under direct shear flow.
    • Legends for movies S1 to S4

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.avi format). Droplet freezing.
    • Movie S2 (.avi format). Droplet melting.
    • Movie S3 (.avi format). Droplet sliding on melt layer.
    • Movie S4 (.avi format). Melting of frost layer.

    Files in this Data Supplement:

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