3D printed sessile platform and construction of confined flexible droplet crystallizer (CFDC). In this study, the CFDC with different volume and morphology were subtly created on the self-designed 3D printed sessile platform. Hydrophilic polyacrylic acid (PAA) was selected as main material to precisely manufacture the sessile platform (radius of 600 μm and height of 800 μm) by 3D printing as exhibited in Supplementary Fig. 1, which can effectively restrain the spread and shrinkage of droplet. Thus, a series of CFDCs were facilely constructed on sessile platform by tuning the droplet volume from 0.1 μL, 0.2 μL, to 2.0 μL via precise droplet dripping system (Fig. 1a-c). More detailed demonstrations on the CFDC were provided in Supplementary Fig. 2. Correspondingly, the contact angle of CFDC was increased from 51°, 60°, to 140° with a stable location on the sessile platform, respectively. Notably, to avoid the electrostatic interaction between platform and solute, the sessile platform was immersed in ammonium hydroxide solution for 48 h to neutralize the residual carboxyl groups before the CFDC construction, which was proved by the reduced negative zeta potential from -26.1 mV to -2.9 mV (Fig. 1e). The neutralized property was also verified by the increased N element and reduced O element, which were detected by energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) as demonstrated in Supplementary Figs. 3, 4. Meanwhile, the hydrophilicity and flat surface of sessile platform was maintained after the neutralization treatment, which was testified by the inherent contact angle of 77° and low roughness of Ra = 30 nm in scanning electron microscopy (SEM) and atomic force microscopy (AFM) images (Fig. 1a, d). All these results manifested the fabrication of a flat, hydrophilic, and neutral sessile platform, facilitating the exploration of droplet crystallization process.
Synergized micro-flows mechanism inside CFDC. Benefiting from the implementation of various CFDCs on the sessile platform, we unfolded the synergized micro-flows mechanism inside CFDC. CFDCs with contact angle of 90°, 120°, and 140° were selected to explore the different synergized micro-flows mechanism induced by the shape of CFDC. Firstly, CuSO4 and Na2SO4 aqueous solutions were used to evaluate the crystal growth and deposition due to the virtue of dendritic crystal on illustrating the initial nucleation sites and growth pathway. The classic dome deposition was formed at the center of sessile platform when the initial contact angle of CFDC was 120° as depicted in Fig. 2b. Unfortunately, small CFDC (contact angle 90°) endowed deposition with a classic coffee-ring deposition along the contact line while the large CFDC (140°) left a random aggregation (Fig. 2a, c). In particular, the nucleation sites of dome deposition and coffee-ring deposition were separately focused on the interior and contact line of sessile platform. And then both crystals grew towards the center of sessile platform, hinting a radially inward vortex. While, the nucleation sites and growth pathways would become random when the contact angle was 140°. These phenomena implied that the morphology of CFDC can effectively and simply regulate the crystal deposition morphology via the potential synergized micro-flows inside CFDC.
To explore the evolution of the deposition morphology, neutral nanoparticle SiO2, with radius of 200 nm, was introduced to simulate the small crystal particle transfer during the crystallization process via eliminating the impact of the solution concentration gradient. In this scenario, the compensation of solution was replenished from everywhere inside droplet driven by the surface tension, rather than from the droplet center (lowest concentration region) due to the concentration difference. Under this condition, Marangoni flow can dominate the formation of synergized micro-flows by impairing the effect of capillary flow, which was clearly verified by the radially inward orientation of micro-flows in all CFDCs as shown in Supplementary Video 1. Furthermore, the trajectory of one nanoparticle was recorded via on-line monitoring camera to calculate the translational speed of nanoparticle, which can illustrate the Marangoni flow intensities in different CFDCs (Fig. 2d and Supplementary Fig. 5). These results demonstrated that the motion of suspended nanoparticles was accelerated with the increase of initial CFDC contact angle from 90° to 140°, indicating the enhanced intensity of Marangoni flow. After drying the solvent, a uniform deposition of nanoparticles was observed on the sessile platform after evaporating a small CFDC (90°) as displayed in Fig. 2a. It should be attributed to the weak Marangoni flow inside the small CFDC (90°), which is incapable of transporting the particles towards sessile platform center. And then the increased initial contact angle (120°) endowed the Marangoni flow with stronger driven force for the nanoparticle transfer, realizing an aggregation of particles towards the platform center (Fig. 2b). Identical tendency was further intensified when the initial contact angle of CFDC increased from 120° to 140°, implying much stronger Marangoni flow on conveying nanoparticles towards the center of the sessile platform. Therefore, this aggregation was further intensified, and a dome deposition was shown in Fig. 2c. These findings distinctly confirmed that the deposition morphology can be controlled by modifying CFDC morphology via the regulation of Marangoni flow intensity.
Next, to further investigate the undergoing mechanism of the synergized Marangoni flow and capillary flow in CFDC, the computational fluid dynamics (CFD) simulations were implemented to analyze the temperature, concentration, and velocity distributions inside the CFDCs (20% NaCl aqueous solution) as depicted in Fig. 3. The corresponding contours and independence verifications were provided in Supplementary Fig. 6. Firstly, these distributions along the axes of CFDCs was supplied to exploited the synergized micro-flows mechanism in Fig. 3a-c. Noting that a corner of temperature distribution and a peak of concentration distribution along the axis were observed in Fig. 3a, b, which indicated a special region (center shaft region) with the lowest temperature and highest concentration was formed inside the CFDCs as marked in Supplementary 7. Driven by the temperature and concentration gradients, a radially inward vortex was created at the contact line region while another radially outward vortex was appeared at the dome region, which transported the solution towards the center shaft region, realizing the highest velocity at center shaft region as manifested in Fig. 3c. With the increased contact angle, a connection was constructed between the vortex at dome region and another enhanced vortex at contact line region, which inverted the transfer of solutes and or micro-crystals towards contact line. Concretely, when contact angle was increased from 90° to 140°, the reduced corner temperature (from 296.5 to 293.7 K) and the increased concentration peak value (from 21.4 % to 24.3 %) declared the enhanced temperature and concentration gradients, certifying the strengthened Marangoni flow intensity. Together with the up-shifts of the temperature corner and concentration peak position from 0.34 to 1.06 mm, the radially inward vortex would be constantly enhanced and enlarged, which was evidenced by the similar variations for the peak value and position of velocity distribution. More detailed distributions for other CFDCs were validated in Supplementary Fig. 8, which were consistent with the aforementioned experimental results.
In addition, the temperature, concentration, and velocity distributions along the gas-liquid interface of CFDC were also obtained from CFD simulations to explain the synergized micro-flows. Specifically, the dome of all CFDCs possessed the lowest temperature, in which the solution was transported towards the dome region along the gas-liquid interface following the temperature gradient (Fig. 3d and Supplementary Fig. 9). This was similar to the detection of infrared thermal imager as displayed in Supplementary Fig. 10. Combined with the concentration gradient (Supplementary Fig. 11), we found that the solution would be transported along the two vortexes at the dome and contact line regions, which was kept in agreement with the effect of the temperature and concentration gradients along the axis. Besides, the temperature and concentration gradients were also boosted with the increase of the CFDC initial contact angle. These conjectures were intuitively proved by the fully developed velocity vectors projection on the cross-section plane of CFDC in Fig. 3e. With the increase of CFDC contact angle, induced by the raised center shaft region, the vortex at the contact line region was constantly enhanced until the CFDC was thoroughly occupied (140°) by invading the vortex at dome region (Fig. 3e). Conversely, the vortex at dome region would be reduced until it was completely devoured.
In a word, the Marangoni flow intensity was constantly enhanced by increasing the CFDC contact angle, thereby synergized micro-flows were subtly regulated by controlling the integration of capillary flow and Marangoni flow. As illustrated in Fig. 3f, for the 30° CFDC, the synergized micro-flows were dominated by the radially outward capillary flow due to the weak Marangoni flow intensity (Ma, 154.1). Thus, the solution was transported from center to contact line. Noting that the Ma was calculated by the CFD simulation as listed in Supplementary Table 1. By contrast, when the contact angle was increased to 60°, a small radially inward vortex at contact line region was induced by the synergy of capillary flow and intensive Marangoni flow (Ma, 452.4), realizing a solution transfer away from the contact line of CFDC. Then, the intensity (i.e., size and velocity) of such radially inward vortex was further strengthened along with the increased contact angle from 60° to 140° ascribed to the enhanced Marangoni flow intensity (Ma, from 452.4 to 2493.1). Notably, when it comes to 140°, the synergized micro-flows were dominated by the strong Marangoni flow (Ma, 2493.1), exhibiting a big radially inward vortex as illustrated in Fig. 3f, where the sharp changes from 120° to 140° could be exposed by the velocity vector of 130° in Supplementary Fig. 12. These interesting findings hinted that a magic shape and featured angle can be obtained by regulating the morphology of CFDC, which achieved the ideal synergized micro-flows by integrating the Marangoni flow and capillary flow, presenting great potential on the precise modulation of crystal preparation. Meanwhile, the flexibility of the gas-liquid interface and the confinement of spatial structure inside CFDC opened an avenue for the regulation of complex combined flow.
Precise regulation of targeted crystal morphology. Following above guidance about the synergized micro-flows, NaCl aqueous solution was introduced to further evaluate the crystal morphology due to the insensitive solubility of NaCl to temperature and humidity. First of all, various nanoliter CFDCs were constructed on sessile platform with controllable contact angle (from 90°, 120° to 140°). Subsequently, the crystallization process of CFDC was captured by on-line camera and the crystal morphology was distinctly analyzed as shown in Fig. 4. Particularly, a coffee-ring crystal along the sessile platform edge was acquired after drying a small CFDC (90°) as demonstrated in Fig. 4a, b. This should be attributed to the capillary flow dominated micro-flows due to the synergy of capillary flow and weak Marangoni flow (Ma, 828.1<1000), which conveyed the solutes and/or micro-crystals towards the contact line region and generated the coffee-ring crystal as illustrated in Fig. 4g. And then the Marangoni flow (Ma, 1373.7) was enhanced in the moderate CFDC (120°), which realized a perfectly synergized micro-flows by controlling the match of capillary flow and Marangoni flow. Thus, the synergized micro-flows conveyed the solutes and/or micro-crystals towards the center shaft region, and ultimately a standard cubic crystal was assembled at the center of the sessile platform as displayed in Fig. 4c, d. Unfortunately, for the large CFDC (140°), the match of capillary flow and Marangoni flow was destroyed by the excessive Marangoni flow intensity (Ma, 2493.1>2000). The synergized micro-flows were dominated by the Marangoni flow, where the overhigh solutes and/or micro-crystals transfer velocity caused drastic collision between micro-crystals and sessile platform, resulting in a formation of random crystal as shown in Fig 4e-g. These phenomena confirmed that the shape of CFDC possessed pronounced impact on the modulation of the synergized micro-flows by regulating the Marangoni flow intensity, further controlling the crystal morphology and deposition.
By virtue of this perfect synergized micro-flows inside moderate CFDC (120°), we furtherly exploited the massive production of such cubic crystal in spatial (scale-up, enlarged platform size) and temporal (stage-up, supplementary feeding droplet) detail by expanding the CFDC size and replenishing another 1.0 μL crystallization solution onto the crystal as exhibited in Fig. 5. More detailed crystal morphologies and the corresponding CFDCs were displayed in Supplementary Fig. 13. Concretely, the size of CFDC was enlarged by injecting more crystallization solution on much larger sessile platform (also constructed via 3D printing), whose size was correspondingly enlarged to match the enlarged CFDC (120°). The side views of resultant CFDCs and the corresponding crystal morphologies after being completely dried were probed by contact angle goniometry and SEM, respectively (Fig. 5a, b). To evaluate such scale-up and the effect of gravity on the flexible gas-liquid interface, an error of CFDC shape was defined as the ratio of aspect ratio for actual and ideal CFDC, as exhibited in Supplementary Fig. 14. It was worth noting that the aspect ratio of the actual CFDC (1 μL) was 0.766 while the ideal CFDC (120°) was 0.750, which exhibited an error of 2.16 % as displayed in Supplementary Table 2. When the volume of the enlarged CFDC was below 16.0 μL (16 times in volume), the error of aspect ratio was lesser than 8.0 % although suffering the gravity. This should be attributed to the effect of surface tension, which overcame the gravitational effect onto the flexible gas-liquid interface of CFDC (Fig. 5a). Thus, the desired synergized micro-flows were maintained inside CFDC, which conveyed the solutes and micro-crystals towards sessile platform center through the center shaft region. And then the standard cubic crystal was scaled up at the center of sessile platform (Fig. 5a, b). While, when the volume of CFDC was above 20.0 μL, this scale-up was failed with the excessive error of aspect ratio above 10.0 % due to the intensified gravitational effect. Thereby, the perfect synergized micro-flows were destroyed, resulting in the disappearance of cubic crystal at the sessile platform center (Fig. 5b).
In addition, benefiting from the flexibility of droplet, new CFDC can be consecutively constructed on the initial CFDC. Thus, the stage-up (secondary to forth feeding) of cubic crystal was realized for diverse CFDCs by replenishing another 1.0 μL crystallization solution onto the original crystal (Fig. 5c, d). This not only caused the dissolution of original coffee-ring or cubic crystals, but also accomplished the regrowth of new crystal during the droplet crystallization (as manifested in Supplementary Videos 2, 3). Compared with the original crystals, the crystal morphology was normalized to a regular cubic crystal with much larger crystal size. As shown in Fig. 5c, the classical coffee-ring crystal was obtained by evaporating 0.4 μL NaCl solution, which constructed a small CFDC (90°) on sessile platform. In this case, the capillary flow dominated the formation of synergized micro-flows inside CFDC and thus conveyed the solutes and/or micro-crystals towards contact line region, acquiring a coffee-ring crystal along the contact line. After replenishing another 1.0 μL solution, a new CFDC (120°) was constructed on the sessile platform, where the coffee-ring crystal was dissolved and reassembled a new large cubic crystal (from 1.4 μL solution) at sessile platform center by optimizing the match of capillary flow and Marangoni flow (Fig. 5c). Besides, when the new CFDC (about 120°) was established by replenishing another 1.0 μL solution onto the residual droplet with nascent cubic crystal, the original cubic crystal was dissolved, and then a much larger cubic crystal was attained at the center of sessile platform from 2.0 μL solution (Fig. 5d). These results collectively indicated that the perfect synergy of capillary flow and Marangoni flow can be fully maintained during the scale-up and stage-up operation, until a certain degree of scale-up (16 times) and stage-up (2 to 4 times). Thus, the CFDC constructed on the developed sessile platform had great potential, feasibility, and adjustability in engineering the crystallization process.
Considering the effect of gravity on the droplet, hanging droplet method was regarded as a common strategy for micro-droplet crystallization (especially for biomacromolecule crystal preparation)49–52. In this study, based on the virtues of sessile platform and CFDC, we inverted the sitting CFDC (120°) to construct a hanging CFDC on the reversed sessile platform (Fig. 6). And then a potential sheet crystal was appeared and developed along the gas-liquid interface. Ultimately, the sheet crystal was fully developed and restricted on the sessile platform center by surface tension as manifested in Fig. 6a and Supplementary Video 4. By contrast, a cubic crystal could be obtained after the growth of initial crystal surrounded by crystallization solution. To analyze the crystallization mechanism of sitting and hanging CFDCs (120°), CFD simulations were applied to probe the temperature, concentration, and velocity distributions, thereby evaluating the synergized micro-flows mechanism during crystallization process (Supplementary Fig. 15). Influenced by the reversed gravity direction, the temperature and concentration distributions inside the hanging CFDC were dramatically changed when it was compared with that of sitting CFDC (Fig. 6b, c). Particularly, the orientation of the center shaft region was reversed towards the gas-liquid interface. Besides, a higher value of corner (296.2 K) in temperature distribution and lower peak (22.0 %) in concentration distribution along the axis of hanging CFDC were observed while those of sitting CFDC were 295.6 K and 22.6 %, respectively (Fig. 6e, f), which signified the declined driving force in the hanging CFDC. This was verified by the reduced peak value of velocity distribution from 0.072 to 0.054 mm/s compared with the sitting CFDC (Fig. 6g). Meanwhile, the shifts of corner and peak positions in temperature and concentration distributions from 0.66 to 0.76 mm induced the similar shift of peak position (velocity distribution) towards gas-liquid interface of CFDCs. These results further testified the migration of center shaft region towards the gas-liquid interface, which was evidenced by the velocity vector as exhibited in Fig. 6d. This shift conveyed solutes and/or micro-crystals towards the gas-liquid interface (hanging CFDC), rather than the sessile platform center (sitting CFDC). Therefore, for the crystal with higher density than the solution, the sitting CFDC may possess much larger potential on crystal nucleation and growth in the investigated system, paving the way for the preparation of cubic crystal at the sessile platform center. In addition, the developed 3D-printed matrix-type and regular platform had illustrated multiple functions of revelation the undergoing crystallization mechanism and parallel preparation the target crystal in diverse CFDCs, etc.