Published papers

© 2018 The Royal Society of Chemistry. We have studied the self-assembled morphologies of tripod nanoparticle solutions confined in nanoslits using dissipative particle dynamics (DPD) simulations. In the present study, composite-shaped nanoparticles, like a planar tripod model with an amphiphilic surface pattern, were also synthesized experimentally. Using the DPD results at various values of a hydrophobic ratio (HR) and arm length (AL), we create phase diagrams of the self-assembled morphologies of tripod nanoparticles. With each of the two parameters, we observe three morphologies in the tripod nanoparticle solution. Further, potential energy vs. temperature for various tripod nanoparticle solutions is compared for different self-assembled structures. Our simulations offer a guide to control the morphologies of self-assembled tripod nanoparticles, which constitute novel systems that may find applications in nanofluidic devices.

© The Royal Society of Chemistry. Peptide amphiphiles are one of the most promising materials in the biomedical field, so much effort has been devoted to characterizing the mechanism of their self-assembly and thermosensitive gelation. In this work, vapor pressure osmometry measurements were carried out to parameterize the thermosensitivity of interactions between peptide amphiphiles in an aqueous solution. The osmometry measurement verified that the peptides became more hydrophobic as temperature increased, which was quantitatively described with the Flory-Huggins χ parameter. Thereafter, a coarse-grained molecular model was used to simulate peptide amphiphiles dissolved in an aqueous solution. The temperature sensitive coarse-grained parameter aHW, which is the repulsive force between the hydrophilic head of the peptide amphiphile and water was estimated from the aforementioned experimentally obtained χ. Furthermore, the effects of concentration and temperature on the self-assembly behavior of peptide amphiphiles were quantitatively studied by dissipative particle dynamics. The simulation results revealed that aHW plays an important role in self-assembly characteristics and in the resulting microstructure of the peptide amphiphiles, which coincides with previous experimental and computational findings. The methodology in quantitatively linking the coarse-grained parameter from experiment and theory provides a sensible foundation for bridging future simulation studies with experimental work on macromolecules.

Janus nanoparticles(JNPs) have received considerable attention because of their characteristic physical properties that are due to more than two distinct chemical or physical surfaces. We investigated the rheological properties of a JNP solution in the nanotubes using a computer simulation. Prediction and control of the self-assembly of colloidal nanoparticles is of critical importance in materials chemistry and engineering. Herein, we show computer simulation evidence of a new type of velocity profile and a hallmark shear-thinning behavior by confining a JNP solution to a nanotube with hydrophobic and hydrophilic wall surfaces. We derived curves of the shear rate versus the viscosity for two quasi-one-dimensional nanotube systems including diluted and concentrated volume fractions of JNP solutions. For the diluted system, under relatively low shear rates, shear-thinning behavior with a moderate slope or behavior similar to a Newtonian fluid is observed because of the clustering of JNPs. Under relatively high shear rates, the slope of shear thinning changes markedly because the self-assembled structures are rearranged. Moreover, for concentrated systems, when the nanotube wall is hydrophobic, new characteristic velocity profiles that have not been reported before are observed. Our simulation offers a guide to control the rheological properties of JNP solutions by the chemical patterns on the surfaces of nanochannels, the effect of confinement, and the self-assembled structure.

Researchers have studied the origin of life and the process of evolution on early Earth for decades. However, we lack a comprehensive understanding of biogenesis, because there are many stages in the formation and growth of the first cell. We investigate the self-replication processes of coacervate protocells using computer simulations of single-chain lipid and phospholipid aqueous mixtures. Based on a morphological phase diagram, we develop a model of prebiotic self-replication driven by only environmental factors (i.e. temperature and lipid concentrations) without any external force. Moreover, we investigate high concentration structures during the process of self-replication. These structures have an advantage in fusion and repair of cell membranes. Therefore, lipid hot spots may have existed in primordial soup.

© 2016 The Royal Society of Chemistry. We performed molecular simulations to investigate the morphologies and phase diagrams of self-assembled diblock Janus nanoparticles (JNPs) confined in nanotubes. A JNP is a unique anisotropic nanoparticle, which typically has more than two distinct surfaces, each with different properties. We derived qualitative phase diagrams of axial pressure versus the ratio of the diameter of the nanoparticle and the nanotube. Three distinct types of nanotube walls were considered: hydrophobic, hydrophilic, and hydroneutral. We observed diverse morphologies in JNP solutions, many of which have not been observed in bulk solutions. We also compared the self-assembled structures of diblock and triblock patchy particles. Under weak confinement, significant differences were observed between the different JNP designs. The orientation ordering and self-assembly behaviours of the JNPs can be qualitatively predicted based on the chemical nature of the nanotube wall and the JNP design.

© 2016 American Chemical Society. A vesicle in a cell is an enclosed structure in which the interior fluid is encompassed by a lipid bilayer. Synthetic vesicles are known as the liposomes. Liposomes with a single phospholipid bilayer are called unilamellar liposomes; otherwise, they are called multilamellar liposomes or onion-like liposomes (vesicles). One prototype synthetic onion-like vesicle, namely, onion-like dendrimersomes, have been recently produced via the self-assembly of amphiphilic Janus dendrimers (Proc. Natl. Acad. Sci. U.S.A. 2016, 113, 1162). Herein, we show computer simulation evidence of another type of onion-like vesicle, namely, onion-like oligomersomes, via the self-assembly of amphiphilic Janus oligomers in water. Specifically, we investigate the minimum-sized oligomers (or minimalist model) that can give rise to the onion-like oligomersomes as well as the composition-dependent phase diagrams. Insights into the formation condition and formation process of the onion-like oligomersomes are obtained. We demonstrate that the discharge of the in-vesicle water is through the remarkable "peeling-one-onion-layer-at-a-time" fashion, a feature that can be utilized for a clinical dosing regimen. The ability to control the formation of onion-like oligomersomes by design can be exploited for applications in drug and gene delivery.

© 2019 IOP Publishing Ltd. The self-assembly of surfactant molecules can spontaneously result in a variety of micelle morphologies, such as spherical micelles, threadlike micelles, and vesicles, and it is therefore crucial to predict and control the self-assembly to achieve a helpful process in the fields of materials chemistry and engineering. A dissipative particle dynamics (DPD) method used in a coarse-grained molecular simulation is applied to simulate various self-assembling soft matter systems because it can handle greater length and time scales than a typical molecular dynamics simulation (MD). It should be noted that the thorough sampling of a system is not assured at low temperatures because of large complex systems with coarse-grained representations. In this article, we demonstrate that the replica exchange method (REM) is very effective for even a DPD in which the energy barrier is comparatively lower than that of a MD. A replica exchange on DPD (REDPD) simulation for threadlike micellar aqueous solutions was conducted, and the values of the potential energy and the mean aggregation number were compared. As a result, the correct values and a self-assembled structure within a low-temperature range can only be obtained through the REDPD.

In recent years, self-assembly of complex fluids has received considerable attention in the fields of nanoscience and nanotechnology. Self-assembly is involved in various functional materials that are widely used in industry and there is a strong relationship between self-assembled structures and specific functions. Hence, the understanding and controlling of the self-assembly controlling of complex fluids is important and a useful process in materials chemistry and engineering. In this paper, the recent developments on studies of the structure formation and flow properties of amphiphilic molecules and Janus colloids in solution, using coarse-grained simulation that takes into account hydrodynamic interactions are reviewed. We first briefly introduce the dissipative particle dynamics method and discuss in detail the self-assembly behavior and viscosity properties of surfactant aqueous solution in nanotubes. Following this, we describe another simulation method in which solvent particles are modeled explicitly as point particles, which is the so-called "the multi-particle collision dynamics (MPCD)". The relation between the structural and rheological properties of Janus colloids in solution, using molecular dynamics simulations coupled to MPCD is reported. In general, the shear viscosity of the dispersion correlates strongly with the size of the aggregates not aggregates shape. These results allow us to determine the dynamic properties solely based on structural information.

In this study, a coarse-grained molecular simulation was performed to investigate the morphologies and phase diagrams of self-assembled polymer-tethered nanoparticles (NPs) confined in nanotubes (NTs). Unlike ordinary NPs, polymer-tethered NPs have two distinct characteristic lengths, which are key factors that determine their self-assembly. Herein, two distinct types of NT walls and three types of polymer-tethered NPs were considered: hydrophilic and hydrophobic walls, and hydrophilic, hydrophobic, and Janus surfaces. First, the qualitative phase diagrams of the axial pressure,Pz,versusthe ratio of the NT radius to the NP radius,L, were derived. The results revealed that diverse self-assembled morphologies, which are not formed in non-tethered NPs, were observed in the polymer-tethered NPs. For example, three types of ordered structures with different structural characteristic lengths, depending onPz, were obtained. In addition, the effect of the chemical nature of the polymer-tethered NP surface on the self-assembled morphology confined in NTs was investigated. Clusters of water molecules were formed, particularly in the hydrophobic polymer-tethered NPs, and these clusters caused the structural distortion of the NP. Moreover, in the polymer-tethered NPs with the Janus amphiphilic surface, the hydrophobic and hydrophilic polymer tethered NPs assembled in the axial direction to form an ordered structure, and a double-helix structure was formed atL= 3.0 in the hydrophobic NT. The results of these simulations indicate that the self-assembly behaviours of polymer-tethered NPs can be qualitatively predicted based on the chemical nature of the NT walls and the surface design of the polymer-tethered NP.

Self-assembly of multi-component systems can provide a large variety of functions, which is impossible for a single-component system, and those multi-component derived functional materials attract attention in various fields such as biomedical, engineering, and materials science. This gives multi-component systems importance, but molecular elucidation of such systems is a major challenge, since this type of complex system has large degrees of freedom. In this study, we focused on cleansing agents as multi-component systems, which are cosmetics products mainly used to remove oily compounds such as makeup, and are multi-component aqueous solutions consisting of many surfactants, moisturizers, and pH regulators. Our aim is to link the macroscopic properties of multi-component systems with the mesoscale structures to elucidate the molecular evidence of functions. We have studied the self-assembled morphologies of cleansing agents using coarse-grained molecular simulations as mesoscale structures and compared them to cleansing performance as a macroscopic property. From the simulation results of morphologies and the experimental results of performance, the key features which explain their functionalities were examined. As a result, it was quantitatively shown that the distribution of specific particles or molecules such as hydrophobic particles both in the bulk state and when they are applied on dirty surfaces is one of the clues to explain the performance. Through molecular designing based on this result, it will be much easier to reach the desired property or performance in the field of multi-component functional materials. Thus, this molecular simulation method will push the field of materials science and materials informatics forward.

Self-assembly is currently one of the most studied branches of materials chemistry, and has attracted a lot of attention due to its diverse potential applications in electronics, engineering, biomedical, and optical fields. Thus far, many previous studies have reported on the self-assembly of surfactants for developing various functional materials. In this study, we focus on the chemical nature of the wall surface as a critical parameter which can drastically change the self-assembled structure. We have studied the rheological properties of surfactant solutions in chemically distinct nanotubes using a computer simulation. Graphs of the Weissenberg number (Wi) versus the viscosity (17) in hydrophilic, hydroneutral, and hydrophobic nanotubes at different concentrations of surfactant were derived. We found that differences in the viscosity can be obtained depending on the chemical nature of the wall surface, even in systems without surfactant molecules. With the surfactant molecules, characteristic viscosity behaviors were observed with rich steadystate morphologies. For example, although the concentration of the surfactant is the same (c = 30%), completely different viscosity behaviors were observed in hydrophilic (shear-thinning) and hydrophobic (shear-thickening) nanotubes. Our simulations offer a guide to controlling the Theological properties of surfactant aqueous solutions by altering the chemical nature of the wall surface and elucidating the effects of confinement, the concentration of the surfactant aqueous solution, and the self-assembled structure. (C) 2018 Elsevier B.V. All rights reserved.

© 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. Recently, various techniques have been developed using photonic crystals. Liquid crystals (LC) confined in a nanodroplet mimicked photonic crystals, such as those of opal. Therefore, investigating the phase behaviour of LC molecules in nanodroplets is very important in the next-generation optical field. In this study, the chemical interaction between surfactants and LCs in nanodroplets is reproduced using a dissipative particle dynamics method. We identify the phase behaviour of LCs and investigate how the chemical interaction affect on the orientation of LCs. In particular, by adding surfactant molecules, various morphological behaviours were observed in the LC nanodroplet. The phase transition temperature varied depending on RND (amount of surfactant molecules). Furthermore, difference of the self-assembly structure also appeared inside the droplet depending on RND. Our simulation offers a theoretical guide to control morphologies of self-assembled LCs inside a nanodroplet, a novel system that may find applications in nanofluidic devices or in photonic crystal technology.

© 2018 the Owner Societies. The lipid bilayer membrane facilitates various biological reactions and is thus an essential structure that sustains all higher forms of life. The unique local environment of the lipid bilayer plays critical roles for the diffusion of biomolecules as well as water molecules in biological reactions. Although fluctuation of the cell membrane is expected to allow for the transport of some water molecules, the flip-flop of lipid molecules corresponds to lipid transport between membrane leaflets, and is considered to be an important process to regulate the lipid composition of biological membranes. However, the relationship between these flip-flop phenomena and surrounding water molecules remains poorly understood. We hypothesized that the flip-flop is caused by water molecules permeating through the cell membrane. To test this hypothesis, we used millisecond-order coarse-grained molecular simulations (dissipative particle dynamics) to investigate the distance between water molecules and lipid molecules depending on the position of the lipid molecule. The results clearly showed that water molecules affect the flip-flop motion in the early stage, but have minimal contribution to the subsequent behavior. Moreover, based on the results of dissipative particle dynamics simulation, we computed several first-passage-time (FPT) quantities to describe the detailed dynamics of water permeation. We modeled arrangements in the middle of the flip-flop process, which were compared with the arrangement without lipid molecules. Overall, our results indicate that lipid molecules located both in perpendicular and parallel arrangements largely affect water permeation. These findings provide new insight into the detailed relationship between water permeation and the flip-flop motion.

On basis of molecular simulation of confined surfactant solutions, we show that by adding chemical patterns on the inner surface of nanochannels dynamical properties the confined surfactant solutions could be modified from shear thinning to shear thickening. To this end, we select uniformly hydrophobic and hydrophilic surfaces as well as a stripe-patterned Janus surface as three prototype confining surfaces of nanochannels. In all three nanochannels, when the surfactant solution is under relatively low shear rates, it shears thin. Under moderate shear rates, a sharp decrease in the shear viscosity could occur due to surfactant morphology transition. Under relatively high shear rates, a shear-thinning-to-thickening transition can emerge due to the tendency of stratification normal to the confining surface. Our simulation study offers a guide to steering dynamic properties of surfactant fluids in nanofluidic devices through engineering surfaces of nanochannels by design.

© 2020 by the authors. Combining atomistic and coarse-grained (CG) models is a promising approach for quantitative prediction of polymer properties. However, the gaps between the length and time scales of atomistic and CG models still need to be bridged. Here, the scale gaps of the atomistic model of polyethylene melts, the bead-spring Kremer-Grest model, and dissipative particle dynamics with the slip-spring model were investigated. A single set of spatial and temporal scaling factors was determined between the atomistic model and each CG model. The results of the CG models were rescaled using the set of scaling factors and compared with those of the atomistic model. For each polymer property, a threshold value indicating the onset of static or dynamic universality of polymers was obtained. The scaling factors also revealed the computational efficiency of each CG model with respect to the atomistic model. The performance of the CG models of polymers was systematically evaluated in terms of both the accuracy and computational efficiency.

We have studied equilibrium morphologies of dimyristoylphosphatidylcholine lipid solution and cholesterol solution confined to nanotubes using dissipative particle dynamics (DPD) simulations. Phase diagrams regarding monomer concentration c versus radius of nanotube r for both solutions are attained. Three types of the inner surface of nanotubes, namely hydrophobic, hydrophilic, and hydroneutral are considered in the DPD simulations. A number of phases and molecular assemblies for the confined solutions are revealed, among others, such as the spiral wetting and bilayer helix. Several phases and assemblies have not been reported in the literature, and some are non-existence in bulk solutions. The ability to control the morphologies and self-assemblies within nanoscale confinement can be exploited for patterning interior surface of nanochannels for application in nanofluidics and nanomedical devices.

How do lipid molecules in membranes perform a flip-flop? The flip-flops of lipid molecules play a crucial role in the formation and flexibility of membranes. However, little has been determined about the behavior of flip-flops, either experimentally, or in molecular dynamics simulations. Here, we provide numerical results of the flip-flops of model lipid molecules in a model membrane and investigate the statistical properties, using millisecond-order coarse-grained molecular simulations (dissipative particle dynamics). We find that there are three different ways of flip-flops, which can be clearly characterized by their paths on the free energy surface. Furthermore, we found that the probability of the number of the flip-flops is well fitted by the Poisson distribution, and the probability density function for the inter-occurrence times of flip-flops coincides with that of the forward recurrence times. These results indicate that the occurrence of flip-flops is a Poisson process, which will play an important role in the flexibilities of membranes. (C) 2014 AIP Publishing LLC.

© 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group. Machine learning is a category of AI technology that is utilized in the product development cycle in numerous industries. It was reported that machine learning can be applied to predict the chemical properties of functional materials, for example, the self-assembly of surfactant solutions (Nanoscale, 2018, 10, 16013) and the chemical reactions of copper nanoparticles (ACS Energy Lett., 2018, 3, 2983). A liquid crystal (LC) molecule is a representative functional material. Monodisperse systems have been targeted in many theoretical studies; however, the resulting products usually have a polydisperse distribution. It has been reported in several studies that the properties exhibited by polydisperse systems are different from those observed in monodisperse systems. In this study, we focus on the physical properties of polydisperse systems i.e. the ratio of binary LC molecules on the phase transition temperature and the possibility of predicting the physical properties of the system using machine learning. A relatively high prediction accuracy was obtained using machine learning. This study demonstrates that machine learning can be utilized to predict the phase transition temperature of polydispersed LC systems. The results of this study will contribute to the further development of material science and molecular design via the application of machine learning.

© The Author(s) 2019. Despite several research efforts, the detailed mechanism that dominates the thermal conductivity of nanofluids has not been explained. We investigated the effect of the chemical surface design of NPs (hydrophobic–hydrophilic balance of the NPs containing Janus surface) and verified the influence of their self–assembled structures on the thermal conductivity of a nanofluid using molecular simulation. When hydrophobic (HO) NPs are added to a nanofluid, they only form one large cluster because they do not prefer to be in contact with water molecules. Hence, an increment in the thermal conductivity of the nanofluid added with HO NPs was observed. On the contrary, the mean cluster size of Janus NPs was limited because of their limited HO superficial areas (attractive domains). Hence, an increment in the enhancement ratio of thermal conductivity was not observed even when the volume fraction of the NPs (φNP) was increased. Thus, our results demonstrate that the mean cluster size of NPs is the key factor for thermal conductivity enhancement. In addition, this study reveals that it is possible to control the thermal conductivity of nanofluids (or the mean cluster size of Janus NPs) using Janus NPs with surfaces designed using anisotropic chemical interactions.

Site-selective surface modification on the shape-controlled nanocrystals is a key approach in the programmed self-assembly of inorganic colloidal materials. This study demonstrates a simple methodology to gain self assemblies of semiconductor nanocrystals with branched shapes through tip-to-tip attachment. Short-chained watersoluble cationic thiols are employed as a surface ligand for CdSe tetrapods and CdSe/CdS core/shell octapods. Because of the less affinity of arm-tip to the surface ligands compared to the arm-side wall, the tip-surface becomes uncapped to give a hydrophobic nature, affording an amphiphilic surface pattern. The amphiphilic tetrapods aggregated into porous agglomerates through tip-to-tip connection in water, while they afforded a hexagonally arranged Kagome-like two-dimensional (2D) assembly by the simple casting of aqueous dispersion with the aid of a convective self-assembly mechanism. A 2D net-like assembly was similarly obtained from amphiphilic octapods. A dissipative particle dynamics simulation using a planar tripod model with an amphiphilic surface pattern reproduced the formation of the Kagome-like assembly in a 2D confined space, demonstrating that the lateral diffusion of nanoparticles and the firm contacts between the hydrophobic tips play crucial roles in the self-assembly.