Research ArticleCHEMICAL PHYSICS

Salt creeping as a self-amplifying crystallization process

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Science Advances  20 Dec 2019:
Vol. 5, no. 12, eaax1853
DOI: 10.1126/sciadv.aax1853
  • Fig. 1 Creep left by different salts.

    Photographs of the salt creep at the end of the creeping process, when all the solution has evaporated. Solutions (from left to right): NaCl, KCl, Na2SO4-A, and Na2SO4-B. For all experiments: RH = 6% and T = 21°C.

  • Fig. 2 Contact angle evolution of the salt solutions.

    (A) Time evolution of the contact angle θ of the liquid meniscus for the four salt solutions under study as they evaporate (RH = 6% and T = 21°C). The dashed lines indicate the time at which precipitation starts in the solution. The creeping on the rod starts right after the last data point (highlighted in black). (B) Typical photograph of the meniscus, as used to determine the contact angles. (C) Schematic of the meniscus evolution as the solution evaporates.

  • Fig. 3 Crystal precipitation during evaporation.

    (A) At high contact angles θa > θc, crystals forming on the meniscus fall into the bulk. (B) At lower contact angles θb ≤ θc, the crystals are confined within the liquid/air and solid/liquid interfaces at the contact line. The inset shows the parameters used to calculate the typical crystal size at this onset of creeping. (C) Illustration of the creeping front and the self-amplified growth process: a single entrapped crystal in contact with the bulk solution (blue) gets covered by a thin film of solution from which new crystals grow ahead, and so forth.

  • Fig. 4 Self-amplified growth.

    Mass of the salt creep on the glass rod as function of time for the four salts studied. Dashed curves are fits of the experimental data with Eq. 1. Fitting parameters are listed in the inset.

  • Fig. 5 Crystal shapes in salt creep.

    SEM images of creep formed from the four salt solutions. Note the different scales. The arrows indicate locations slightly ahead of the creep edge where high-resolution SEM revealed the presence of nanocrystals (see Fig. 6).

  • Fig. 6 Effect of surfactants on salt creeping.

    (A) Comparison of the final salt creep of NaCl solutions with and without the addition of CTAB or Tween 80 surfactants (RH = 6% and T = 21°C). (B) Time evolution of the contact angle (top) and creeping mass (bottom) for the same solutions as they evaporate. (C) SEM pictures of creep found for NaCl (bottom) and NaCl + CTAB (top).

  • Fig. 7 Crystallization at the front of the creep zone.

    Top: Image of NaCl creep formed on a glass slide and zoom around the creep boundary with a high-resolution SEM (zone A). Bottom: High-resolution SEM images of nanocrystals spreading over the glass substrate from the precursor film (zone B) in regions beyond the creeping line (zone A) (also indicated by arrows in Fig. 5). Note the different scales. NaCl nanocrystal [panel NaCl (1)] initially without a well-defined structure, which could correspond to the amorphous state evolving toward whisker growth [panel NaCl (2)] from a thin film of solution, while KCl crystals are seen to grow by means of multiple nucleation even at an early stage of growth at the nanoscale [panels KCl (1) and (2)]. The precipitation of the whisker-shaped thenardite (anhydrous crystals) (some laying down and some pointing up) can also be seen for Na2SO4-B. Na2SO4-A forms a porous structure made of anhydrous nanocrystals upon removal of the bound water during the drying of the hydrated crystals (zone A).

  • Fig. 8 Location of crystal precipitation at the onset of nucleation and growth in evaporating droplets for the four solutions used in this study.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/12/eaax1853/DC1

    Supplementary Materials and Methods

    Table S1. Properties of the salt solutions studied.

    Fig. S1. Phase diagram of Na2SO4 in water as a function of temperature (9).

    Fig. S2. Influence of surfactants, CTAB and Tween 80, on the nucleation of NaCl.

    Fig. S3. Influence of the surfactants on the wetting properties.

    Fig. S4. Experimental setup.

    Fig. S5. Initiation and growth of salt creeping during the evaporation.

    Fig. S6. Evolution of the contact line.

    Fig. S7. Evolution of the contact angle up to creeping.

    Fig. S8. Effect of the curvature of the glass rod.

    Fig. S9. Mass of the salt creep (normalized).

    Fig. S10. Experiment of creeping of NaCl and Na2SO4 on the glass slide.

    Fig. S11. Salts creeping on glass slide.

    Movie S1. NaCl solution—Zoom at the contact line.

    Movie S2. NaCl + CTAB solution—General view of salt creeping.

    Movie S3. Na2SO4-B—General view of salt creeping of the anhydrous sodium sulfate (thenardite).

    Reference (25)

  • Supplementary Materials

    The PDFset includes:

    • Supplementary Materials and Methods
    • Table S1. Properties of the salt solutions studied.
    • Fig. S1. Phase diagram of Na2SO4 in water as a function of temperature (9).
    • Fig. S2. Influence of surfactants, CTAB and Tween 80, on the nucleation of NaCl.
    • Fig. S3. Influence of the surfactants on the wetting properties.
    • Fig. S4. Experimental setup.
    • Fig. S5. Initiation and growth of salt creeping during the evaporation.
    • Fig. S6. Evolution of the contact line.
    • Fig. S7. Evolution of the contact angle up to creeping.
    • Fig. S8. Effect of the curvature of the glass rod.
    • Fig. S9. Mass of the salt creep (normalized).
    • Fig. S10. Experiment of creeping of NaCl and Na2SO4 on the glass slide.
    • Fig. S11. Salts creeping on glass slide.
    • Legends for movies S1 to S3
    • Reference (25)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.avi format). NaCl solution—Zoom at the contact line.
    • Movie S2 (.avi format). NaCl + CTAB solution—General view of salt creeping.
    • Movie S3 (.avi format). Na2SO4-B—General view of salt creeping of the anhydrous sodium sulfate (thenardite).

    Files in this Data Supplement:

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