Chemical composition from photos: Dried solution drops reveal a morphogenetic tree

Abstract

Under nonequilibrium conditions, inorganic systems can produce a wealth of life-like shapes and patterns which, compared to well-formed crystalline materials, remain widely unexplored. A seemingly simple example is the formation of salt deposits during the evaporation of sessile droplets. These evaporites show great variations in their specific patterns including single rings, creep, small crystals, fractals, and featureless disks. We have explored the patterns of 42 different salts at otherwise constant conditions. Based on 7,500 images, we show that distinct pattern families can be identified and that some salts (e.g., Na₂SO₄ and NH₄NO₃) are bifurcated creating two distinct motifs. Family affiliations cannot be predicted a priori from composition alone but rather emerge from the complex interplay of evaporation, crystallization, thermodynamics, capillarity, and fluid flow. Nonetheless, chemical composition can be predicted from the deposit pattern with surprisingly high accuracy even if the set of reference images is small. These findings suggest possible applications including smartphone-based analyses and lightweight tools for space missions.

Keywords: chemical analysis; evaporites; pattern formation; self-organization.

Fig. 1.

Deposit patterns of 10 μL drops of different aqueous salt solutions formed under ambient conditions on glass. (A) Time sequence showing the evaporation-driven formation of NH₄Cl deposit. Time between snapshots: 190 s. Time-lapse movies are provided in SI Appendix. (B−M) Representative photos of key deposit patterns. Both scale bars: 1 cm (the Lower Right scale bar applies to all panels in B−M).

Fig. 2.

Image analysis and a family tree of salt deposits. (A) Schematic drawing of a deposit pattern (gray) with deposit-free holes (blue). Red curves denote deposit borders; the green curve is the best-fit ellipse yielding eccentricity data. (B) Radial distribution of deposit distances from the pattern centroid. Blue, red, and black lines indicate the corresponding mode, mean, and median values. (C) Analysis applied to the KNO₃ example in Fig. 1H. Red and green curves are the same as in (A) while the additional colors highlight different deposit-free regions. (D) Heatmap of the image metrics for different salts. Colors represent the deviation from the global average in units of SDs. The data yield a dendrogram of similar deposit patterns. (E) Deposit patterns projected into the principal component plane for 500 replicate patterns of 12 key salts. Different salts are represented by different colors; different markers of the same color distinguish pattern types of the “bifurcated” salts [e.g., Na₂SO₄ (1; dominant) and (2; subtype)]. (F) Multidimensional scaling map of the centroids of the data groups in (E). Black and dashed blue lines connect the closest and second-closest neighbors, respectively.

Fig. 3.

Identifying composition from deposit patterns. (A) Confusion matrix with each square showing the number of predictions for each salt based on 25 deposit patterns that were not included in the training dataset of 6,000 images. (B) Similar heat map where each set of 25 reference images is collectively analyzed for its normalized distance to the centroids of salt patterns in the main dataset. White x-markers and black o-markers denote the closest and second-closest matches, respectively. (C) Deposit patterns of eight additional salts. All scale bars are 500 μm. (D) Confusion matrix of 42 different salts (see SI Appendix for compound indices) as obtained for a very small set of 21 images per salt. (E) Number of correct assignments based on one (blue) and five attempts (orange). Brown colors indicate overlap.

Fig. 4.

Deposit patterns of binary salt mixtures. (A) Typical deposit patterns observed for drops containing two dissolved salts. The numbers above each column denote the percentage volume of the saturated salt solution specified on the left as mixed with the saturated salt solution on the right. The scale bar is 1 cm and applies to all panels. (B) EDS maps of the magnified deposit area produced by a KNO₃/NaCl drop (60:40 v/v). The panels show the spatial distribution of the specified elements (K, O, Na, and Cl). (Scale bar: 500 μm.) (C) Projection of the salt mixture patterns into the PCA plane of Fig. 2E. Each open marker represents the average coordinates obtained from 30 images. These markers trace the deposits’ changes as the mixing ratio is varied. Squares, triangles, and stars indicate KNO₃/NaCl, NH₄Cl/NaCl, and KNO₃/NH₄Cl mixtures, respectively. Circles indicate pure deposit coordinates for reference.