Regular Article
Interfacial self-assembly of polysaccharide rods and platelets bridging over capillary lengths

https://doi.org/10.1016/j.jcis.2021.02.017Get rights and content

Abstract

Hypothesis

Generation of long-range ordering of colloidal particles through anisotropic interactions is of growing interest in material designing. At submicron-scale, routine works use synthetic spheres or rods but the knowledge pertaining to assembly of binary combination of particles is severely restricted. Improved understanding of the fundamental aspects that drive self-assembly, can lead to robust strategies for fabrication of topographically oriented films.

Experiment

The fluidical geometry of a liquid crystalline (LC) solution of polysaccharide consisting of micron-sized rod and platelet units was explored. The solutions, characterized for their rheological behavior, were evaporated from a rectangular cavity. The assembly and orientation of the units was monitored by polarizing microscopy and the interparticle capillary forces approximated mathematically.

Findings

The units deposited into an uninterrupted membrane upon interfacial evaporation, forming a bridge along the 8 mm gap, linking the substrates. The membrane, composed of a lamellar structure, was uniaxially oriented along the direction of the gap. The rheological estimations corroborated an extremely high value of viscosity with the presence of crosslinking junctions in this solution when compared to a solution with only rod units, capable of bridging a maximum of 1 mm. It has been demonstrated for the very first time that the presence of platelet-units contributes lateral capillary interactions and assist rod-units towards a wider, self-assembled structure.

Introduction

Self-assembly of colloids through tunable interactions facilitate material fabrication for optics, catalysis, sensing and cleansing applications [1], [2], [3], [4], [5]. It has been reported that capillary forces exhibited by particles and even bacteria, are responsible for inversing the prevalent coffee-ring effect during evaporation [6], [7], [8]. Recent advances in directional interactions between particles gravitates around the efforts to unravel long-range order and geometrical control in natural biopolymers [9], [10], [11], [12]. However, research works featuring a binary combination of particles and their extended applicability over larger scales are limited and thus offer attractive avenues for further investigation. A typical convective self-assembly is characterized by deposition of colloidal particles along the pinning-depinning contact line of the evaporating solution. The method leads to patterned deposition on the substrate ensuing tedious optimizations and control parameters in order to achieve high orientation.

We have developed a method to fabricate uniaxially oriented membranous structures, by evaporating polysaccharide solutions, in their liquid crystalline (LC) state, from a limited space of 1 mm. The polysaccharides, xanthan gum and sacran, having a high molecular weight > 106 gmol−1 and lyotropic LC properties, deposit by accumulating as a nucleus for a thin membrane that subsequently bridges the two substrates and grows with the descending air-LC interface [13], [14], [15]. Additionally, the deposited membrane behaves as an anisotropic hydrogel when crosslinked by thermal annealing at 80 – 140 °C [16]. However, factors enumerating the geometrical limits of this bridging-deposition ability of the polysaccharides, are still unclear.

By employing a solution comprising of rod- and platelet-shaped LC units in this work, the maximum extent of the bridging deposition is substantiated, and the interplay of solution rheology explained. An aqueous solution of cyanobacterial exopolysaccharide, sacran (a), consisting of rod-shaped LC units, deposits by bridging a 1 mm gap between the substrates of the drying cell. In contrast, the solution of sacran obtained by bulk extraction, sacran (b), containing rod- and platelet-shaped LC units, could bridge an extraordinary thicker gap of 8 mm. As conceptualized in Fig. 1A, when sacran (b) solution, consisting of platelet- and rod-shaped LC units is evaporated, the units align along the air–liquid-solid three phase contact line. Strong interactions at the drying air-LC interface help the platelets to unequivocally align with the rods. Together, they support the bridging of a gap, that is larger than twice the standard capillary length, Lc ~ 2 mm right in the center [14], considering both sides of the glass substrates (Fig. 1B). Capillary forces prevail over the gravitational forces, near the substrate at least within the Lc value [17] and help bridge Δy < Lc. The self-assembly of sacran (b) wide membranous structure from the evaporative air-LC interface is a direct consequence of strong lateral capillary forces acting along the contact lines, in a gap Δy > 2Lc. Such a deposition, generating a partition in a wide space, conforms to be a highly synergistic approach in an accessibly simple process of drying.

Section snippets

Materials

Sacran (a) was extracted in lab from Aphanothece sacrum as per previous reports [18]. Sacran (b) is extracted commercially and kindly provided by OGIC Technologies Co. Ltd., Japan. Sacran (a) consist of rod-shaped liquid crystalline units whereas sacran (b) is composed of rod and platelet-shaped units. Topside-open cells for the drying experiments were prepared with two non-modified glass slides (Matsunami Co. Ltd. S1111) and silicone spacer of the desired width. The glass slides were

Results & discussion

Sacran (a) solution with only rod units and sacran (b) solution with rod plus platelet units (Fig. S2), were analyzed for their rheological characteristics to differentiate between their solution properties. The apparent shear viscosity for sacran (b) solution was found to be considerably higher than that of sacran (a) solution as represented by the time profiles in Fig. 2A. Sacran solution shows rheopectic behavior wherein its shear viscosity increases with time at low shear rates [19]. Upon

Conclusion

The evaporation-induced self-assembling characteristic of an aqueous polysaccharide solution, comprising of rod- and platelet-shaped LC units was investigated as a significant development over the previous reports [13], [15], [35], [36]. With the applied combination of LC units, it was demonstrated for the very first time, that the deposition of a polysaccharide membrane bridging a gap greater than twice the capillary length was feasible. Due to the exceptionally high storage modulus of the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

G. J. is grateful for the Research Fellowship from the Japan Society for the Promotion of Sciences (JSPS) and the JSPS Kakenhi Grant number JP18J11881. Authors also acknowledge the financial support provided by Grant-in-Aid from A-step (AS2915173U) of JST, Grant-in-Aid from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (JP20H05213), The Asahi Glass Foundation, Innovation inspired by Nature Research Support Program, Sekisui Chemical Co. Ltd. and Shibuya Science

Author Contributions

G.J. performed the drying experiments, analyzed and conceptualized the data and wrote the manuscript. F. A. A. Y. and T. M. evaluated the viscosity. M. K. O. extracted sacran (a). K. O. and T. K. supervised the project. All the authors discussed the results and commented on the manuscript.

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    Present address: B CUBE—Center for Molecular Bioengineering, Technische Universität Dresden 01307, Germany

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