Centrifugation-Mediated Crystal Growth of Attractive Colloids for Band Edge Lasing.

Attractive depletion interactions are utilized to organize colloidal particles into crystalline arrays with high crystallinity through spontaneous phase separation. However, uncontrolled nucleation frequently leads to the formation of crystalline grains with varied crystal orientations, which hampers the optical performance of photonic crystals. Here, colloidal crystals have been engineered with uniform orientation and high surface coverage by applying centrifugal force during the depletion-induced assembly of polystyrene particles. The centrifugal force encourages the particles to move toward the bottom surface, which fosters heterogeneous nucleation and supports rapid crystal growth, yielding densely-packed and uniformly-arranged crystal grains with high reflectivity. This study has observed that the nucleation and crystal growth behavior is significantly influenced by the salt concentration. Based on the pair potentials, the transition boundary has been quantitatively analyzed between fluid and crystal phases and identified the threshold for homogeneous nucleation. Utilizing the high-reflectivity colloidal crystals, band-edge lasing is achieved by dissolving the water-soluble dye into the aqueous suspensions. Upon optical excitation, a lasing emission characterized is observed by a narrow spectral width at the short-wavelength band edge. Notably, the laser wavelength can be adjusted by altering the salt concentration or particle diameter, offering a versatile approach to tuning the optical properties.

Polymer-induced adhesion of endothelial cells.

The effects of neutral dextran concentration and molecular mass on the adhesion of endothelial cells (EC) to siliclad-covered glass surfaces were studied using interference reflection microscopy (IRM). Results indicate that close contact of the EC to the glass slides is markedly enhanced in the presence of 500 kDa dextran, with this increase reflected by both the speed of forming close contact as well as the size of the contact area. This increased adhesion is attributed to the reduction in surface concentrations of large polymers and, therefore, to the attractive forces caused by depletion interaction. Our findings suggest that depletion could play an important role in cell-cell or cell-surface interactions via accelerating and enhancing close contacts. This interaction should thus be considered in vivo and in vitro for specific potential applications, such as cell culture and cell adhesion to biomimetic surfaces. It should therefore be of particular interest in a wide range of biomedical applications.

Viscoelasticity variation in a biofilm-mediated Bacillus subtilis suspension induced by adding polyethylene glycol.

Recent experiments show that synthetic polymers can influence the degree of microbial aggregation and the rheological properties of bacterial suspensions, the study of which can help us control biofilm formation. In this article, we add polyethylene glycol (PEG) with various molecular weights and concentrations into two types Bacillus subtilis cell cultures, Luria Broth (LB) and Minimal Salts glutamate glycerol (MSgg), respectively. We first observe cell clusters in cell suspensions with various concentrations of PEG, and measure cluster size in both static and dynamic fluid environments. We find that cells gather together into big clusters and most of the cells are arranged longitudinally; and the large cell clusters are divided into smaller aggregates under fluid shear. We then use a rheometer to measure the viscoelastic properties of various cell cultures, to represent the degree of aggregation of the bacterial suspensions. We find the storage modulus, the loss modulus and the viscosity of bacterial suspensions not only depend on the cell aggregation but also depend on the directionality of cellular motion.

Mechanically-driven phase separation in a growing bacterial colony.

Secretion of extracellular polymeric substances (EPSs) by growing bacteria is an integral part of forming biofilm-like structures. In such dense systems, mechanical interactions among the structural components can be expected to significantly contribute to morphological properties. Here, we use a particle-based modeling approach to study the self-organization of nonmotile rod-shaped bacterial cells growing on a solid substrate in the presence of self-produced EPSs. In our simulation, all of the components interact mechanically via repulsive forces, occurring as the bacterial cells grow and divide (via consuming diffusing nutrient) and produce EPSs. Based on our simulation, we show that mechanical interactions control the collective behavior of the system. In particular, we find that the presence of nonadsorbing EPSs can lead to spontaneous aggregation of bacterial cells by a depletion attraction and thereby generates phase separated patterns in the nonequilibrium growing colony. Both repulsive interactions between cell and EPSs and the overall concentration of EPSs are important factors in the self-organization in a nonequilibrium growing colony. Furthermore, we investigate the interplay of mechanics with the nutrient diffusion and consumption by bacterial cells and observe that suppression of branch formation occurs due to EPSs compared with the case where no EPS is produced.

Entropy-driven crystal formation on highly strained substrates.

In heteroepitaxy, lattice mismatch between the deposited material and the underlying surface strongly affects nucleation and growth processes. The effect of mismatch is well studied in atoms with growth kinetics typically dominated by bond formation with interaction lengths on the order of one lattice spacing. In contrast, less is understood about how mismatch affects crystallization of larger particles, such as globular proteins and nanoparticles, where interparticle interaction energies are often comparable to thermal fluctuations and are short ranged, extending only a fraction of the particle size. Here, using colloidal experiments and simulations, we find particles with short-range attractive interactions form crystals on isotropically strained lattices with spacings significantly larger than the interaction length scale. By measuring the free-energy cost of dimer formation on monolayers of increasing uniaxial strain, we show the underlying mismatched substrate mediates an entropy-driven attractive interaction extending well beyond the interaction length scale. Remarkably, because this interaction arises from thermal fluctuations, lowering temperature causes such substrate-mediated attractive crystals to dissolve. Such counterintuitive results underscore the crucial role of entropy in heteroepitaxy in this technologically important regime. Ultimately, this entropic component of lattice mismatched crystal growth could be used to develop unique methods for heterogeneous nucleation and growth of single crystals for applications ranging from protein crystallization to controlling the assembly of nanoparticles into ordered, functional superstructures. In particular, the construction of substrates with spatially modulated strain profiles would exploit this effect to direct self-assembly, whereby nucleation sites and resulting crystal morphology can be controlled directly through modifications of the substrate.

Non-adsorbing macromolecules promote endothelial adhesion of erythrocytes with reduced sialic acids.

BACKGROUND: Abnormal adhesion of red blood cells (RBCs) to vascular endothelium is often associated with reduced levels of sialic acids on RBC membranes and with elevated levels of pro-adhesive plasma proteins. However, the synergistic effects of these two factors on the adhesion are not clear. In this work, we tested the hypothesis that macromolecular depletion interaction originating from non-adsorbing macromolecules can promote the adhesion of RBCs with reduced sialic acid content to the endothelium. METHODS: RBCs are treated with neuraminidase to specifically remove sialic acids from their surface followed by the evaluation of their deformability, zeta potential and membrane proteins. The adhesion of these enzyme-treated RBCs to cultured human umbilical vein endothelial cells (ECs) is studied in the presence of 70 or 500kDa dextran with a flow chamber assay. RESULTS: Our results demonstrate that removal of sialic acids from RBC surface can induce erythrocyte adhesion to endothelial cells and that such adhesion is significantly enhanced in the presence of high-molecular weight dextran. The adhesion-promoting effect of dextran exhibits a strong dependence on dextran concentration and molecular mass, and it is concluded to originate from macromolecular depletion interaction. CONCLUSION: These results suggest that elevated levels of non-adsorbing macromolecules in plasma might play a significant role in promoting endothelial adhesion of erythrocytes with reduced sialic acids. GENERAL SIGNIFICANCE: Our findings should therefore be of great value in understanding abnormal RBC-EC interactions in pathophysiological conditions (e.g., sickle cell disease and diabetes) and after blood transfusions.

Regioselective Growth of Colloidal Crystals Induced by Depletion Attraction.

Colloidal crystals display photonic stopbands that generate reflective structural colors. While micropatterning offers significant value for various applications, the resolution is somewhat limited for conventional top-down approaches. In this work, a simple, single-step bottom-up approach is introduced to produce photonic micropatterns through depletion-mediated regioselective growth of colloidal crystals. Lithographically-featured micropatterns with planar surfaces and nano-needle arrays as substrates are employed. Heterogeneous nucleation is drastically suppressed on nano-needle arrays due to minimal particle-to-needles overlap of excluded volumes, while it is promoted on planar surfaces with large particle-to-plane volume overlap, enabling regioselective growth of colloidal crystals. This strategy allows high-resolution micropatterning of colloidal photonic crystals, with a minimum feature size as small as 10 µm. Stopband positions, or structural colors, are controllable through concentration and depletant and salt, as well as particle size. Notably, secondary colors can be created through structural color mixing by simultaneously crystallizing two different particle sizes into their own crystal grains, resulting in two distinct reflectance peaks at controlled wavelengths. The simple and highly reproducible method for regioselective colloidal crystallization provides a general route for designing elaborate photonic micropatterns suitable for various applications.

Polyethylene glycol regulates the pitch and liquid crystal behavior of cellulose nanocrystal-based photonic crystals.

Inspired by iridescent color in natural creations, cellulose nanocrystal (CNC) photonic crystals artificially created by nanotechnology have great application prospects due to their potential to control light propagation in the linear and nonlinear regimes. One of the most important development directions of photonic crystals is the diversification of colors, usually by adjusting the pitch. However, few researchers notice the effect of polymer molecular weight and content on pitch regulation and the interaction between polymer and CNC liquid crystals. Polyethylene glycol (PEG) were used as polymers to regulate the pitch of CNC photonic crystals and investigate the changes in microstructure, crystal structure, thermal properties, and liquid crystal texture of the composites by changing the PEG content and molecular weight. Different photonic crystal construction systems show that when the molecular weight of PEG is 0.4 k, it can be filled between CNCs to regulate the pitch of photonic crystals, while when the molecular weight of PEG is 20 k, it cannot always be filled between CNCs in evaporation-induced self-assembly (EISA) process due to the depletion interaction, which cannot effectively regulate the pitch. This study reveals the relationship between PEG and CNC liquid crystals, which supports the development of photonic crystals and the pitch regulation.

Preparation of liquid yogurt in the presence of pectin and its formation mechanism.

We had previously observed that adding pectin into milk before fermentation inhibited gelation of yogurt but did not affect the pH. Thus, this work aimed to prepare such liquid yogurt and clarify its formation mechanism. It was found that liquid yogurt was obtained in the presence of 0.10%-0.20% pectin. However, at lower or higher pectin concentrations, yogurt was gelled. Confocal laser scanning microscopy analysis demonstrated that 0.10%-0.20% pectin induced milk protein aggregating into separated particles rather than a continuous network, which explained why liquid yogurt was formed. Moreover, adding 0.10%-0.20% pectin into the casein micelle suspension induced aggregation of casein micelles at pH 6.8. After pH decreased to 4.3, casein micelles showed more aggregation but they were still separated particles, which was the same in the corresponding yogurt samples. These results suggested that pectin changed the aggregation mode of casein micelles and induced formation of liquid yogurt.

Freezing-mediated formation of supraproteins using depletion forces.

Hypothesis Long-acting formulations such as microparticles, injectable depots and implantable devices can realize spatiotemporally controlled delivery of protein drugs to extend their therapeutic in vivo half-lives. To efficiently encapsulate the protein drugs into such drug delivery systems, (sub)micron-sized protein particles are needed. The formation of micronized supraproteins can be induced through the synergistic combination of attractive depletion forces and freezing. The size of the supraproteins can be fine-tuned from submicron to several microns by adjusting the ice crystallization rate through the freeze-quench depth, which is set by the target temperature. Methods Supraprotein micron structures were prepared from protein solutions under various conditions in the presence and absence of nonadsorbing polyethylene glycol. Scanning electron microscopy and dynamic light scattering were employed to determine the sizes of the supraproteins and real-time total internal reflection fluorescent microscopy was used to follow the supraprotein formation during freezing. The protein secondary structure was measured before and after micronization by circular dichroism. A phase diagram of a protein-polyethylene glycol mixture was theoretically predicted to investigate whether the depletion interaction can elucidate the phase behavior. Findings Micronized protein supraparticles could be prepared in a controlled manner by rapid freeze-drying of aqueous mixtures of bovine serum albumin, horseradish peroxidase and lysozyme mixed with polyethylene glycol. Upon freezing, the temperature quench initiates a phase separation process which is reminiscent of spinodal decomposition. This demixing is subsequently arrested during droplet phase separation to form protein-rich microstructures. The final size of the generated protein microparticles is determined by a competition between phase separation and cooling rate, which can be controlled by target temperature. The experimental phase diagram of the aqueous protein-polyethylene glycol dispersion aligns with predictions from depletion theory for charged colloids and nonadsorbing polymers.

High salinity effects on the depletion attraction in colloid-polymer mixtures.

HYPOTHESIS: Depletion attraction induced by polymers can be employed across multiple disciplines. Previous studies implied that besides screening the electrostatic repulsion between colloids, adding salt may also affect the polymers even in a nonpolar solvent. Here, we study the depletion-induced gelation of a colloid-polymer model system, focusing on the salt effects on the depletion attraction. EXPERIMENTS: Confocal microscopy was used to quantitatively characterize the colloidal gels formed by the polymethylmethacrylate (PMMA)/polystyrene (PS) model system. The attraction experienced by colloids was estimated by correlating the colloidal dynamics with the local structure. Correspondingly, static light scattering was employed to systematically investigate the polymers. The resulting radius of gyration Rg and osmotic pressure were used to evaluate the depletion attraction offered by polymers. FINDINGS: Salt was discovered to lower the strength of inter-particle attraction, which can be attributed to the salt-induced decrease in Rg. The depletion attraction grew sublinearly with c, owing to the considerable decrease in Rg in the good solvent as c increased. We demonstrated how the close form equations in the framework of renormalization group theory can be employed to predict the depletion interaction using the properly determined zero-concentration radius of gyration.

Effects of polymer nonideality on depletion-induced phase behaviour of colloidal disks and rods.

Colloidal dispersions composed of either platelets or rods exhibit liquid crystalline phase behaviour that is strongly influenced by the addition of nonadsorbing polymers. In this work we examined how polymer segment-segment interactions affect this phase behaviour as compared to using either penetrable hard spheres (PHS) or ideal ('ghost') chains as depletants. We find that the simplified polymer description predicts the same phase diagram topologies as the more involved polymer descriptions. Therefore the PHS description is still adequate for qualitative predictions. For sufficiently large polymer sizes we find however that the precise polymer description significantly alters the locations of the phase coexistence regions. Especially the stability region of isotropic-isotropic coexistence is affected by the polymer interactions. To illustrate the quantitative effects some examples are presented.

Thermo-Statistical Effects of Inclusions on Vesicles: Division into Multispheres and Polyhedral Deformation.

The construction of simple cellular models has attracted much attention as a way to explore the origin of life or elucidate the mechanisms of cell division. In the absence of complex regulatory systems, some bacteria spontaneously divide through thermostatistically elucidated mechanisms, and incorporating these simple physical principles could help to construct primitive or artificial cells. Because thermodynamic interactions play an essential role in such mechanisms, this review discusses the thermodynamic aspects of spontaneous division models of vesicles that contain a high density of inclusions, with their membrane serving as a boundary. Vesicles with highly dense inclusions are deformed according to the volume-to-area ratio. The phase separation of beads at specific intermediate volume fractions and the associated polyhedral deformation of the membrane are considered in relation to the Alder transition. Current advances in the development of a membrane-growth vesicular model are summarized. The thermostatistical understanding of these mechanisms could become a cornerstone for the construction of vesicular models that display spontaneous cell division.

Depletion-Induced Chiral Chain Formation of Magnetic Spheres.

Experimental evidence is presented for the spontaneous formation of chiral configurations in bulk dispersions of magnetized colloids that interact by a combination of anisotropic dipolar interactions and isotropic depletion attractions. The colloids are superparamagnetic silica spheres, magnetized and aligned by a carefully tuned uniform external magnetic field; isotropic attractions are induced by using poly(ethylene oxide) polymers as depleting agents. At specific polymer concentrations, sphere chains wind around each other to form helical structures-of the type that previously have only been observed in simulations on small sets of unconfined dipolar spheres with additional isotropic interactions.

Modulating transparency and colour of cellulose nanocrystal composite films by varying polymer molecular weight.

HYPOTHESIS: Cellulose nanocrystals (CNC) can produce photonic composite films that selectively reflect light based on their periodic cholesteric structure. The hypothesis of this research is that by incorporating water-soluble polymer, photonic properties of CNC composite film can be designed by manipulating the polymer molecular weight. EXPERIMENTAL: Flexible free-standing composite films of five different poly (ethylene glycol) (PEG) molecular weights were prepared via air drying under a controlled environment, and characterised by reflectance UV-vis spectrometer, atomic force microscopy (AFM) and scanning electron microscopy (SEM). Films with each molecular weight were investigated over a concentration range. FINDINGS: The colour and transmission haze of the composite films was modified by varying both the PEG molecular weight and concentration. Depending on the molecular weight, the films were able to reflect light from the UV region (242 nm) across the visible spectrum to the near-infrared region (832 nm). Different trends in variation of the reflected light based on the molecular weight was found with increasing PEG concentration and was explained by weak depletion interactions occurring between CNC and PEG, which was reduced with increasing PEG molecular weight.

Non-monotonic phase behaviour of a mixture containing non-adsorbing particles and polymerising rod-like molecules.

HYPOTHESIS: Previous works have shown that many-body interactions induced by dispersants with increasing correlation length will generate a diminishing two-phase region [Soft Matter 14, 6921 (2018)]. We conjecture that the attenuation of the depletion attraction due to many-body interactions is a ubiquitous phenomenon in medium-induced interactions. We propose mixtures of colloidal particles and rod-like polymers as a feasible experimental system for verifying these predictions, since the intra-molecular correlations are not screened in a good solvent for rod-like polymers as they are in flexible polymers. The length of the rods can grow and become the dominant length scale that determines the range of the depletion interactions for the imbedded non-adsorbing particles. Simulations: We study many-body depletion forces induced by polymerizing rod-like polymers on spherical non-adsorbing colloids, using Metropolis Monte Carlo simulations. We also employ a simple mean-field theory to further justify our numerical predictions. FINDINGS: We demonstrate that the phase diagram displays the same qualitative features that have previously been predicted by many-body theory, for mixtures containing flexible polymers under theta solvent conditions. The contraction of the particle two-phase region that we observe, as the correlation length increases beyond some specific value, could be a signature of the weakening of the depletion caused by many-body effects.

Phase separation behavior of whey protein isolate particle dispersions in the presence of xanthan.

In this study, phase separation of colloidal whey protein isolate (WPI) particle dispersions was studied using a rod-like polysaccharide xanthan. Effects of different xanthan concentration, particle volume fraction, and temperature were analyzed by visual observations, turbidity measurements, and particle mobility tracking method. Particle mobility was determined using a diffusing wave spectroscopy (DWS) set up. Xanthan concentration was kept low in order not to increase the viscosity of dispersions, so that the phase separation could be observed easily. Visual observations showed that there was a minimum concentration of xanthan to induce phase separation at a constant particle volume fraction, and xanthan concentration was found to have an important effect on the degree of phase separation. The temperature was also found to have an effect on depletion mechanism. Phase separation was mainly a result of different sizes of WPI particles, and xanthan induced the depletion interaction between WPI particles, as supported by the data obtained from DWS. The results of this study explained both the mechanism and the stability range of particle dispersions in the presence of xanthan, which is important for the design of stable systems, including colloidal particles.

Red blood cell adhesion can be reduced by non-reactive macromolecules.

To date, the mechanisms behind red blood cell (RBC) adhesion remain unclear. However, polymer depletion at the red cell surface has been shown to play a significant role. Interestingly, most previous studies have focused on the adhesion-promoting effects of one type of large polymer or plasma protein. However, the situation in vivo is more complex in that one needs to consider a mixture of various bio-macromolecules. To explore this complexity, Interference Reflection Microscopy was used to investigate how mixtures of various polymers affect RBC adhesion. RBC adhesion to albumin-coated glass coverslips was studied in the presence of two pro-adhesion polymers [dextran70 kDa and 35 kDa poly(ethylene glycol) (PEG 35)] with and without three types of smaller polymers: dextran 10 kDa, PEG 10 kDa and Poloxamer 188. Our findings show that the presence of small polymers can inhibit the adhesion-promoting effects of dextran 70 and PEG 35, with a more pronounced reduction for heterogeneous mixtures. Interpretation of our results in terms of the depletion model appears appropriate, in that our findings are consistent with the assumption that this reduction occurs because of an increase of small molecules in the depletion region. This study thus suggests that depletion interaction can control cell-cell interactions in complex environments (e.g., in vivo), and indicates that considering the interplay of all plasma constituents is important in order to understand the pathophysiology of diseases associated with cell adhesion and vascular complications.

Molecular Crowding Tunes Material States of Ribonucleoprotein Condensates.

Ribonucleoprotein (RNP) granules are membraneless liquid condensates that dynamically form,dissolve, and mature into a gel-like state in response to a changing cellular environment. RNP condensation islargely governed by promiscuous attractive inter-chain interactions mediated by low-complexity domains(LCDs). Using an archetypal disordered RNP, fused in sarcoma (FUS), here we study how molecular crowdingimpacts the RNP liquid condensation. We observe that the liquid⁻liquid coexistence boundary of FUS islowered by polymer crowders, consistent with an excluded volume model. With increasing bulk crowderconcentration, the RNP partition increases and the diffusion rate decreases in the condensed phase.Furthermore, we show that RNP condensates undergo substantial hardening wherein protein-dense dropletstransition from viscous fluid to viscoelastic gel-like states in a crowder concentration-dependent manner.Utilizing two distinct LCDs that broadly represent commonly occurring sequence motifs driving RNP phasetransitions, we reveal that the impact of crowding is largely independent of LCD charge and sequence patterns.These results are consistent with a thermodynamic model of crowder-mediated depletion interaction, whichsuggests that inter-RNP attraction is enhanced by molecular crowding. The depletion force is likely to play akey role in tuning the physical properties of RNP condensates within the crowded cellular space.

Unravelling the Mechanism of Stabilization and Microstructure of Oil-in-Water Emulsions by Native Cellulose Microfibrils in Primary Plant Cells Dispersions.

It is long known that oil-in-water emulsions can be stable against coalescence in homogenized plant cell wall dispersions because of the presence of surface-active biopolymers. When plant cell wall material is homogenized to the extent of deagglomeration of the cellulose microfibrils (CMFs), a much more complex dispersed system is obtained. Here we show that in such complex systems both surface active soluble polymers and individual CMFs are at the origin of this stabilization against coalescence, as they form a shell around the oil droplets providing Pickering-like stabilization. Individual CMFs and bundles of them in the presence of soluble biopolymers form a hybrid network in the continuous phase linking the droplets, creating a viscoelastic network that prevents the droplets from coalescing. Depletion induced attraction caused by soluble biopolymers and dispersed CMFs induces the formation of oil droplet clusters at low CMF concentrations leading to a highly heterogeneous distribution of oil droplets. This effect diminishes at high CMF concentrations at which the strong viscoelastic network arrests the droplets. These findings are important steps toward controlling complex dispersed systems comprising CMF-polymers mixtures with a second liquid or solid dispersed phase.