The multiphasic shear technique draws inspiration from two fabrication methods: phase inversion via nonsolvent-solvent (NS-S) exchange and interfacial polymerization or precipitation. These methods can produce particles9–10, fibers10–13, or membranes6,7 while allowing the controlled formation of nano- or microscopic features6,9,10. Our previous investigations reveled that such processes can be transcribed to multiphasic fluids and highlighted the role of hydrodynamic shear in developing diverse structures. Initial research revealed the formation of microrods and fibers within laminar flow1,14,15. Within turbulent flows, the shear-based technique led to the discovery of intricate structures such as dendritic soft matter (“dendricolloids”) and sheets1. Other researchers have since adopted derivative methods for producing polymer microrods16,17, other types of dendricolloids18, and sheets19. These studies highlight the technique’s capability for producing polymer structures tailored for specific applications.
We performed the initial experiments using polystyrene (PS) to ensure that the chemical composition does not contribute to structural variability. Initially, a polymer is dissolved in a good solvent. The polymer solution is injected into sheared nonsolvent medium, miscible with the polymer solvent, within two fabrication platforms. Laminar flow conditions were investigated using a Couette flow device (40-6,000 rpm) (Fig. 1a). Turbulent flow was produced through a high-shear colloidal mill, IKA Magic Lab® (3,000-26,000 rpm) (Fig. 1b, full details in the Methods section). The ultralow interfacial tension between the miscible solvent and nonsolvent allows for extreme liquid-liquid surface deformation upon shearing until the polymer is fully precipitated.
The outcome of this seemingly simple procedure is a result of the interplay of multiple underlying processes described briefly here and in more detail in the Supplementary Information. We constructed a simple conceptual framework that distinguishes three operational fabrication “stages”. The first physical mechanism determining the outcome is fluidic shear as the continuous-phase nonsolvent medium’s streamlines “template” themselves onto the dispersed polymer solution phase1,14. The second stage involves the competition between bulk shear and interfacial stress. It is associated with polymer molecular entanglement which affects the dynamic capillary stability of the polymer solution stream during precipitation20 or the mechanical integrity of the precipitated structure14,15. The final key mechanism is the precipitation rate, relating to the rate of interfacial NS-S exchange based on their mutual affinity or miscibility. It can be described by the interaction parameter, χ12, between the nonsolvent (1) and the solvent (2)6,7.
The types of polymer morphologies resulting from the process were investigated systematically by variations of two fluid regimes (laminar and turbulent), two states of polymer entanglement (low and high), and three precipitation rates (delayed, balanced, and rapid). These sub-stages resulted in twelve unique processing combinations and revealed a rich variety of polymeric structures. Simpler structures such as microparticles, rods, fibers, and ribbons are formed within laminar flows (Fig. 1c–h). Structures ranging from nanoparticles to porous nanosheets are formed within turbulent flows (Fig. 1i –n). We will focus on certain morphologies to investigate the critical control parameters. Other morphologies not discussed here in detail are described in the Supplementary Information.
We first elaborate on the effects of shear rate and polymer solution concentration on PS structures fabricated within a Couette flow device (Fig. 2a). This device produces flows ranging from strictly laminar Couette flow to modulated waves21. Several concentrations of PS and tetrahydrofuran (THF) solutions were injected into water. Cloud points were measured to approximate the binodal curve on the ternary diagram (Fig. 2b). The shapes and sizes of precipitated structures were found to depend on the polymer solution density (ρ) and viscosity (µ), interfacial tension (γ), fluid velocity (u), and precipitation rate (represented by χ12) (Fig. 2c). Most of these variables can be collectively described by the Reynolds (Re), Capillary (Ca), and Weber (We) numbers which express the relative importance of inertial, viscous, and cohesive forces. Furthermore, they can describe the fluid characteristics of the nonsolvent medium as well as injected solution stream dripping, jetting, and subsequent droplet breakup due to Rayleigh-Plateau instabilities20,22.
The simplest structures formed in the Couette cell device were microparticles. The formation of particles with low polymer chain entanglement is a “trivial” outcome of nonsolvent-induced precipitation, following supersaturation or nucleation and growth mechanisms23. Droplets can form through dripping mechanisms or break off after jetting due to capillary fragmentation before polymer precipitation can counteract the liquid-liquid phase breakup (Fig. 2d). Rods can be formed by two alternative pathways (Fig. 2e): additional shearing of polymer droplets formed as described above17 or through structural fragmentation due to limited mechanical integrity after the polymer has separated as a solid phase14.
Above a critical entanglement concentration, Ce, evaluated to be 2.5 – 3 wt. % PS in THF (Supplementary Fig. 1), fibers and ribbons are observed. A transition from fibers to ribbons can be observed with increased polymer concentration and shear rates. The concentration not only dictates molecular chain entanglement, but also the onset of phase separation as more concentrated solutions requires a lesser amount of nonsolvent influx to precipitate. The intensified interfacial mass transport at higher polymer concentrations is analogous to an earlier onset of phase separation, as suggested by the decreasing nonsolvent to solvent ratios (NS:S) (Fig. 2b). The subtle morphological difference between fibers and ribbons may be explained by different collapse modes of a denser “skin” layer due to variances in precipitation rate (Fig. 2a and Supplementary Fig. 2a).
On average, the characteristic dimensions (dc) of particles (diameter), fibers (diameter), and ribbons (principal axis of cross-section) decreased with increasing nonsolvent flow velocity and decreasing polymer concentration (Fig. 2a and Supplementary Fig. 2b). This was expected as the dc of the structures generally have an inverse relationship to the continuous-phase Rec, continuous-phase Cac, and dispersed-phase Wed (Supplementary Information). To illustrate, when the shear rate increased from 20 to 600 s−1 (approximately 60-fold increase in Rec and Cac), the diameters of fibers formed from 5 wt. % PS solution decreased 4-fold. Increasing shear stress by changing the viscosity of the nonsolvent medium was also found to decrease the characteristic size of structures (Supplementary Fig. 3).
The results deviated slightly from what was predicted through Re, Ca, and We analysis. Despite expecting of dripping characteristics at conditions with low Cac and Wed20, all particles were formed by droplet breakup after jetting, and even observed the production of fiber and ribbon-like structures in predicted dripping conditions (Supplementary Fig. 4 and Information). We also estimated that a transition from Couette flow to Taylor vortex flow around Rec of 125. Despite fabrication within strictly non-laminar flows, we still obtained “one-dimensional” fibers and ribbons within these regimes and do not observe branching morphologies until shear rates close to 600 s−1. These outcomes may point to contributions of more complex effects such as the elastocapillarity response of the injection polymer solution24,25.
In turbulent flow conditions, the resulting morphologies change profoundly. The disruptive nature of the turbulent flow leads to rapid droplet fragmentation, forming under low entanglement conditions particles of average size of several hundred nanometers in diameter (Fig. 1i). Slightly higher PS concentrations yielded the formation of “mesorods,” similar in shape to the microrods obtained in laminar flow but approximately one order of magnitude smaller in size (Fig. 1j). Ternary systems with faster precipitation rates and insufficient entanglement produced amorphous chunks (Fig. 1k). The results outlined in Fig. 3a show that above the critical entanglement concentration, we see three distinct classes of structures: dendricolloids (Fig. 1l), branched-ribbons (Fig. 1m), and nano-sheets (Fig. 1n).
Dendricolloids constitute a unique class of soft matter with high degree of branching that we reported earlier1. These particles are fabricated in turbulent flows which are characterized by multi-scale vortices that transport energy through a cascade until dissipated by viscous means at the Kolmogorov length scale, the smallest vortex scale26. We expected that an increased shear rate would increase the rate of energy dissipation and template finer features on the particles (Supplementary Information). The rheological properties of dendricolloid suspensions exhibited increasing gelation efficiency with increasing dissipation conditions despite lack of relation between lengths of the terminating branches of dendricolloid particles and the fabrication shear rate (Supplementary Fig. 5a,b). These results, alongside data from rheological experiments using dendricolloids fabricated with different rotor-stator gap dimensions (Supplementary Fig. 5c), indicate that larger shear stresses on the polymer solution droplets lead to the formation of dendricolloids with enhanced branching and gelation capabilities
Dendricolloids could transition into a previously unreported class of dendritic particles with ribbon-like branches, which have increased gelation propensity compared to dendricolloids (Supplementary Fig. 5d). The transition can be controlled by the polymer solution concentration (Fig. 3a) or by choice of the ternary system. PS solutions composed of two solvents, THF and 1,4-dioxane, were injected into sheared water (Fig. 3b,c). The THF ternary system has a lower NS-S affinity than the 1,4-dioxane system with χ12 values of 1.56 and 1.13, respectively, for 50 v.% water7,27. Thus, droplets of the THF system can undergo significant interfacial deformation compared to those of the 1,4-dioxane system, which has faster onset of precipitation as suggested by its smaller one-phase region, leading to the formation of thin sheets. The formation of branched ribbons was accomplished by injecting PS/1,4-dioxane solution into 20 v.% 1,4-Dioxane (aq.). These results confirmed that varying the precipitation rate is analogous to varying the polymer concentration and can achieve similar morphological transitions.
Porous nanosheets are structures that we have characterized in detail for the first time (Fig. 1k). We hypothesize that the morphology is a result of the rapid interfacial precipitation of a polymer “skin” which is subsequently peeled off by the vigorous flow. Slight differences in precipitation rate were observed to change the material’s nanoscale thickness and porosity (Fig. 3d). The 1,4-dioxane ternary system has relatively slower precipitation rates than other systems (Fig. 3e) and produced sheets with average thicknesses around 100 nm. In contrast, faster precipitation by the dimethyl formamide (DMF) system produced sheets with 4× larger thicknesses. This supports the theorized mechanism of sheet formation as the faster NS-S exchange will increase the precipitation depth into the solution droplet before the skin is sheared off. Superficially, faster precipitation rates also correlated with an increase in effective pore diameter. However, a closer look at the nanoscopic porous features rather suggest differences in phase separation mechanisms (Fig. 3d). The 1,4-dioxane system sheets have distinct individual pores while the DMF system produced bi-continuous sheets. The different pore characteristics are in line with previous observations of phase inversion mechanisms for membranes7,28.
One of the most surprising findings of this study is the diversity of materials morphologies that can be formed by this operationally simple method. It could be expected that the outcomes of such a complex process will be largely chaotic and difficult to control, especially for the case of turbulent flow. The nature of the fluid flows leads to some degree of size and mass variation in particles made in the same processing conditions. However, we observed that the resulting structures are distinctly different in their nano- or microscopic features, and their bulk properties are well reproducible. The scheme in Fig. 4 summarizes the mechanisms and outcomes leading to liquid-shear fabrication of twelve classes of polymer material morphologies. While this conceptual breakdown does not focus on the smaller intricate processing condition details, it highlights the delineation of the major physical effects involved. The key role of shear flow is established by the results which generally support the mechanism of fluid streamline templating of the precipitating polymer. The balance of interfacial and shear stresses heavily influenced the polymeric structures both during and after polymer precipitation. Finally, the rate of polymer precipitation was found to be a significant factor as the ratio of precipitation to shearing rates governs the extent to which fluidic characteristics are templated in the resulting morphologies.
The liquid shear-based fabrication technique investigated here combines complex multiphase flows with spontaneous interfacial mass-transport that induces phase separation and precipitation. We demonstrated the universal nature of these principles by transcribing them to the fabrication of three other common polymers of synthetic or biological origin. The polymers were selected for their diverse chemical composition and properties: copolyester, polyvinylidene fluoride (fluorinated polymer used in battery separators), and chitosan (common biopolymer). We found that the same morphologies that were produced with PS shown in Fig. 1 can be fabricated for these other polymers with judicious selection and adjustment of the processing parameters (Supplementary Fig. 6 with corresponding processing conditions listed in Supplementary Fig. 7). It is clear that the physical principles can also be adapted to a broad range of polymers in addition to systems that require different fabrication mechanisms such as cross-linking29. These results support our findings that liquid-liquid fabrication principles can be used to make a broad range of polymeric materials with diverse compositions and target areas of application. For example, microrods can be used as foam stabilizers15 or in liquid crystal systems16. Nanofibrillar or nanoporous materials can be used in water cleanup and remediation7,8,11. Other applications include adhesives and coatings1, homocomposite hydrogels29, and battery separators or electrodes18,19,30. The new structures reported here can find further use in applications ranging from colloidal systems to energy and biomedical materials.
In summary, the examination of the effects of hydrodynamic shear, molecular entanglement, and precipitation rate prove that liquid-liquid precipitation in sheared multiphasic fluid could be used in scalable fabrication of multiple classes of polymeric soft matter. It provides precise control over morphological transitions and nano- and micro-scale characteristics such as cross-section, thickness, or pore size. Polymer materials of all diverse morphologies presented here could be produced consistently, precisely, and potentially on unprecedented scale if the method is implemented in continuous flow devices. Most importantly, we believe that insights into the fundamental mechanisms underlying this shear-based process could help establish an intellectual framework guiding the development of future liquid-liquid nanomanufacturing technologies.