Microfluidics-Based Drying-Wetting Cycles to Investigate Phase Transitions of Small Molecules Solutions

Abstract

Drying-wetting cycles play a crucial role in the investigation of the origin of life as processes that both concentrate and induce the supramolecular assembly and polymerization of biomolecular building blocks, such as nucleotides and amino acids. Here, we test different microfluidic devices to study the dehydration-hydration cycles of the aqueous solutions of small molecules, and to observe, by optical microscopy, the insurgence of phase transitions driven by self-assembly, exploiting water pervaporation through polydimethylsiloxane (PDMS). As a testbed, we investigate solutions of the chromonic dye Sunset Yellow (SSY), which self-assembles into face-to-face columnar aggregates and produces nematic and columnar liquid crystal (LC) phases as a function of concentration. We show that the LC temperature-concentration phase diagram of SSY can be obtained with a fair agreement with previous reports, that droplet hydration-dehydration can be reversibly controlled and automated, and that the simultaneous incubation of samples with different final water contents, corresponding to different phases, can be implemented. These methods can be further extended to study the assembly of diverse prebiotically relevant small molecules and to characterize their phase transitions.

Keywords: chromonic liquid crystals; dry–wet cycles; microfluidics; self-assembly.

Figure A1

Volume of a droplet in a microfluidic channel. (a) Spere of D ≤ h. (b) Filled torus of D > h. (c) Straight border slice of D′ >> h.

Figure 1

Sketch of the microfluidic chip 1: (a) including a flow-focusing droplets generator and the trapping section, which is obtained by droplets anchoring to a hole in the channel roof due to surface energy minimization. A side view of the traps is reported in the inset (not in scale). (b) Scheme of the water evaporation process through the PDMS layer exploited to increase the c_SSY in the trapped droplets.

Figure 2

Measurement of the concentration phase diagram of SSY solutions during drying at T = 25 °C. (a) Bright-field (top) and polarized (bottom) microscopy images of an SSY droplet trapped in the microfluidic chip and undergoing phase transitions during evaporation. Numbered frames are the ones corresponding to the phase transitions I to I–N (1), I–N to N (2), N to N–C (3), and N–C to C (4). Pink and light blue arrows indicate the directions of the polarizer and analyzer, respectively. Scale bars are 100 μm. Graphs reporting the time evolution of the measured normalized droplet (b) diameter D/D₀, (c) volume V/V₀, (d) polarized intensity I_P, and (e) SSY yellow concentration c_SSY. Blue open circles and dashed lines indicate the phase transition points between the isotropic (I), isotropic–nematic coexistence (I–N), nematic (N), nematic–columnar coexistence (N–C), and columnar (C) phases.

Figure 3

Concentration–temperature phase diagram of SSY solutions obtained by drying droplets within the microfluidic chip (symbols) compared with the one reported in ref. [59] (solid lines). Data points and error bars are reported as the average and standard deviation of N = 5 measurements performed on different droplets.

Figure 4

Dry–wet cycles in the microfluidic device. (a) Sketches of the dehydration (i and iv) and rehydration (ii and iii) steps obtained by exchanging the flux in the reservoir channel (NaCl or water) and modifying the evaporation from the top PDMS layer. Red and blue arrows indicate the direction of the water transport. (b) Variation of c_SSY inside a droplet during time calculated from the measured droplet volume change. Vertical solid lines identify the different drying and wetting phases of the cycle. The colored background identifies the phase diagram of the SSY solution inside the droplet. (c) Partially de-crossed polarizers microscopy images of the SSY droplets during the dry–wet cycle showing droplet shrinkage and swelling and the transitions between the different phases.

Figure 5

Incubation of SSY solutions in different phases by controlled drying. (a) Sketch of the microfluidic chip 3 inspired from ref. [62] composed of an upper layer hosting wells for trapping SSY droplets (orange) and a bottom layer designed to repetitively mix two input salt solutions to obtain a linear gradient of salt concentrations in the reservoir channels placed in correspondence with the traps (black). Inset shows the detail of a well placed at the bifurcation of the main channel in two channels with different hydrodynamic resistance, which allows the trapping of SSY droplets in oil. (b) Normalized fluorescence intensity, I_f, measured in the reservoir channel during the mixing of 0.5 M and 2 M NaCl solutions, the latter doped with 1 mM FITC, as a function of the channel position. Data are reported as average values and error bars correspond to standard deviations of n = 5 measurements. The blue line represents the linear fit of data. (c) Fluorescence microscopy images of the initial part of the reservoir channels at different reservoir channel positions. (d) Bright-field microscopy images of traps loaded with SSY solution (initial concentration, c_SSY,0 = 0.1 M) at the beginning of the experiment (t₀). (e) Polarized microscopy images of the same traps t_f = 15 h, showing the reduction in droplet volume and the transition to different phases (I, N, C) and relative coexistence (I-N, N-C).

Figure 6

Plots of (a) the variation of normalized volume (V/V₀) and (b) the SSY concentration (C_SSY) over time of the droplets during incubation in chip 3. Volume was measured from a bright-field microscopy image sequence acquired at Δt = 10 min (Supplementary Materials). Curves correspond to droplets trapped in correspondence of channels 2, 5, 6, and 10 and thus exposed to a salt reservoir with increasing osmolarity. Droplets corresponding to higher osmolarity reservoir channels exhibit faster concentration and equilibration compared to those in lower osmolarity channels.