The observed magnetic tunability of light transmission through a ferrofluid can be effectively understood in terms of the inter-particle interaction that can be estimated from the magnetic and rheological properties of these fluids. The present study reports complementary magnetic and rheological measurements of aqueous dispersions of ferrite nanoparticles and a commercial ferrofluid. The room temperature magnetization measured in a SQUID magnetometer up to fields of 1 to 2 Tesla showed superparamagnetic behaviour of the particles and the dispersion with the background signal of the liquid showing a diamagnetic behaviour. The room temperature rheological behaviour in zero magnetic field of the fluids was investigated by measuring the viscosity as a function of shear rate from 1-100 s-1. The particle size and the nature of the carrier liquid determine the viscosity and is expected to have an effect on the inter-particle interaction.
Thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) particles of different sizes are synthesized by varying the concentration of sodium dodecyl sulphate (SDS) in a one-pot method. The sizes, size polydispersities and the thermoresponsivity of the PNIPAM particles are characterized by using dynamic light scattering and scanning electron microscopy. It is observed that the sizes of these particles decrease with increase in SDS concentration. Swelling ratios of PNIPAM particles measured from the thermoresponsive curves are observed to increase with decrease in particle size. This observation is…
Hypothesis: The structural details, viscosity trends and dynamic phenomena in t-butanol/water solutions are closely related on the molecular scales across the entire composition range. Utilizing the experimental small- and wide-angle x-ray scattering (SWAXS) method, molecular dynamics (MD) simulations and the complemented-system approach method developed in our group it is possible to comprehensively describe the structure-viscosity-dynamics relationship in such structurally versatile hydrogen-bonded molecular liquids, as well as in similar, self-assembling systems with pronounced molecular an…
Chitosan, a natural, cationic polysaccharide, may be a hydrocolloid strategic to formulate acidic food products, as it can act as both bio-functional and technofunctional constituent. Typically, acetic acid is used to disperse chitosan in aqueous media, but the use of this acid is limited in food formulations due to its flavor. In this study, chitosan was firstly dispersed (0.1% m/V) in lactic acid aqueous solutions (pH 3.0, 3.5 or 4.0), and then evaluated regarding its thickener and emulsion stabilizer properties. O/W emulsions were prepared and characterized in terms of rheological propertie…
Investigating microstructure of suspensions with particles having anisotropic shape that share complex interactions is a challenging task leading to competing claims. This work investigates phase behavior of one such system: aqueous Laponite suspension, which is highly contested in the literature, using rheological and microscopic tools. Remarkably, we observe that over a broad range of Laponite (1.4 to 4 weight %) and salt concentrations (0 to 7 mM), the system overwhelmingly demonstrates all the rheological characteristics of the sol-gel transition leading to a percolated network. Analysis o…
We investigated the formation of cocontinuous structures in polymer blends. These polymeric bijels (bicontinuous interfacially jammed emulsion gels) were composed of polystyrene oligomer, polybutene and fluorescent hydrophobic silica nanoparticles. A micron-sized cocontinuous morphology was stabilized by a monolayer of silica nanoparticles at the interface. Real-time observation of coalescence dynamics in co-continuous polymer blends stabilized by interfacial particles was for the first time achieved via laser scanning confocal microscopy. We demonstrated that suppression of coalescence arises…
Field dependent Anisotropic Micro Rheological and Microstructrual properties of Ferrofluids
采用被动显微流变学方法，在自制的倒置显微镜上测量了由Mn $_{0.75}$ Zn $_{0.25}$ Fe $_2$ O $_4$（MZF）纳米颗粒组成的超顺磁性磁流体的微流变学和微结构特性。测量了探针微球在不同外加磁场下的热运动并进行了分析。分析表明，磁粘效应的各向异性。另外的微观流变性能，如储能模量和损耗模量和它们的过渡。根据\cite{Oliver Muller, Stefan Mahle}中给出的分析，我们获得了微观结构特性，如拉伸流动系数$λ_2$、弛豫时间常数$τ$、耗散磁化系数$α$等。上述参数的计算值与早期的理论计算和宏观流变实验测量值相一致。
We present a neutron spin echo study (NSE) of the nanosecond dynamics of polyethylene glycol (PEG) functionalised nanosized gold particles dissolved in D$_2$O at two temperatures and two different PEG molecular weights. The analysis of the NSE data was performed by applying a Bayesian approach to the description of time correlation function decays in terms of exponential terms, recently proved to be theoretically rigorous. This approach, which addresses in a direct way the fundamental issue of model choice in any dynamical analysis, provides here a guide to the most statistically supported way…
Synthetic hectorite clay Laponite RD/XLG is composed of disk-shaped nanoparticles that acquire dissimilar charges when suspended in an aqueous media. Owing to their property to spontaneously self-assemble, Laponite is used as a rheology modifier in a variety of commercial water-based products. Particularly, aqueous dispersion of Laponite undergoes liquid - to - solid transition at about 1 volume % concentration. The evolution of the physical properties as dispersion transforms to solid state is reminiscent of physical aging in molecular as well as colloidal glasses. The corresponding soft glas…
Results of broadband dielectric spectroscopy and rheological studies of poly(propylene glycol) + SiO${_2}$ nanocomposites are presented. They show that the dynamics in high-concentrated composite is determined by confinement and adsorption effects, resulting from interactions of confined polymers correspond with the host system at the interface between PPG and solid nanoparticles. The evolution of relaxation times follows the clear Vogel-Fulcher-Tammann pattern, what is proved by the supplementary activation energy temperature index analysis. The strong influence of nanoparticles on the transi…
The linear and nonlinear rheological behavior of two rod-like particle suspensions as a function of concentration is studied using small amplitude oscillatory shear, steady shear and capillary breakup extensional rheometry. The rod-like suspensions are composed of fd virus and its mutant fdY21M, which are perfectly monodisperse, with a length on the order of 900 nm. The particles are semi flexible yet differ in their persistence length. The effect of stiffness on the rheological behavior in both, shear and extensional flow, is investigated experimentally. The linear viscoelastic shear data is …
We study the structural changes occurring in the isotropic phase of the C$_{12}$=EO$_6$/H$_2$O lyotropic mixture (up to 35% surfactant weight concentration) upon increasing the concentration and temperature, from small individual micelles to an entangled network which subsequently becomes connected. High-frequency (up to $ω= 6 \times 10^4$ rad/s) rheological measurements give us access to the viscoelastic relaxation spectrum, which can be well described by the sum of two Maxwell models with very different temperature behaviors: the slower one ($τ_1 \simeq 10^{-4}$ s) is probably due to reptati…
We investigate physical origin of ergodicity breaking in an aqueous colloidal dispersion of synthetic hectorite clay, LAPONITE(R), by performing dissolution and rheological experiments with monovalent salt and tetrasodium pyrophosphate solution. We also study the effect of interface, nitrogen and paraffin oil on the same. Dissolution experiments carried out for dispersions with both the interfaces show similar results. However, for samples with nitrogen interface, all the effects are observed to get expedited in time compared to paraffin oil interface. When kept in contact with water, 1.5 wt. …
Lanthanum phosphate (LaPO4) nanorods dispersed in the non-aqueous solvent of ethylene glycol form a system exhibiting large intrinsic birefringence, high colloidal stability and the ability to self-organize into liquid crystalline phases. In order to probe the electro-optical response of these rod dispersions we study here the electric-field-induced birefringence, also called Kerr effect, for a concentrated isotropic liquid state with an in-plane a.c. sinusoidal electric field, in conditions of directly applied (electrodes in contact with the sample) or externally applied (electrodes outside t…
Double emulsions, due to their compartmental structures, are essential in food, agricultural, and pharmaceutical applications. Traditionally, double emulsifications rely on the presence of both oil-soluble and water-soluble surfactants or external stimuli responsive materials and require sequential droplet formation settings or unique fluidic designs. We report on unusual phenomenon where double emulsions are spontaneously formed as soon an aqueous nanoparticle dispersion is placed in contact with an oleic micellar solution. Nanoscale water droplets nucleate in oil in the form of swollen micel…
Inhaled airborne particles smaller than 100 nm entering the airways have been shown to deposit in significant amount in the alveolar region of the lungs. The interior of the alveoli is covered with a ~ 1 micron thick lining fluid, called pulmonary surfactant. Inhaled nanoparticles are susceptible to interact with the lung fluid and modify pulmonary functions. Here we evaluate the structural and rheological properties of the pulmonary surfactant substitute Curosurf which is administered to premature babies for the treatment of respiratory distress syndrome. Curosurf is considered a reliable mod…
Thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) particles of a nearly constant swelling ratio and with polydispersity indices (PDIs) varying over a wide range (7.4% - 48.9%) are synthesized to study the effects of polydispersity on the dynamics of suspensions of soft PNIPAM colloidal particles. The PNIPAM particles are characterized using dynamic light scattering (DLS) and scanning electron microscopy (SEM). The zero shear viscosity ($η_{0}$) data of these colloidal suspensions, estimated from rheometric experiments as a function of the effective volume fraction $ϕ_{eff}$ of the suspensi…
Nanotribology of biopolymer brushes in aqueous solution using dissipative particle dynamics simulations: an application to PEG covered liposomes in theta solvent
Using non-equilibrium molecular dynamics simulations, we study the rheology of a model hybrid mixture of liquid crystals (LCs) and dipolar soft spheres (DSS) representing magnetic nanoparticles. The bulk isotropic LC-DSS mixture is sheared with different shear rates using Lees-Edwards periodic boundary conditions. The steady-state rheological properties and the effect of the shear on the microstructure of the mixture are studied for different strengths of the dipolar coupling, $λ$, among the DSS. We find that at large shear rates, the mixture shows a shear-thinning behavior for all considered …
Dispersion of tubules made of diacetylenic phospholipids (DC 8,9 PC), in aqueous colloidal dispersions of magnetic nanoparticles is studied as a function of the sign of particles surface charges. In every case the tubule-vesicle transition temperature is decreased by the presence of the magnetic nanoparticles. Electrophoresis experiments on the tubules in pure water permits to conclude on a negative apparent surface charge. We study the magnetic response of the system to a static or stationary rotating field and to a magnetic field gradient. These experiments reveal an excess or a lack of magn…
It is shown that porous spherical particles can be obtained via the freezing of silica nanoparticle aqueous suspensions emulsified in a continuous oil phase. After two freeze-thaw cycles, nanoparticles turn aggregated into flocculated objects with microstructure that depends upon emulsion volume fraction and droplet size. For low volume fractions, regular microspheres are produced while for large ones, irregular beads with several tens of micrometer radius are formed. Electronic microscopy, mercury porosimetry and nitrogen adsorption are used to get insights into these porous particles typical…
Porous magnetic silica beads are promising materials for biological and environmental applications due to their enhanced adsorption and ease of recovery. This work aims to develop a new, inexpensive and environmentally friendly approach based on agglomeration of nanoparticles in aqueous droplets. The use of an emulsion as a geometrical constraint is expected to result in the formation of spherical beads with tunable composition depending on the aqueous phase content. Magnetic silica beads are produced at room temperature by colloidal destabilization induced by addition of calcium chloride to a…
Ferrofluids are strongly magnetic fluids consisting of magnetic nanoparticles dispersed in a carrier fluid. Besides their technological applications, they have a tendency to form beautiful and intriguing patterns when subjected to external static and dynamic magnetic fields. Most of the patterns occur in systems consisting of two fluids: one ferrofluidic and one non-magnetic (oil, air, etc.), wherein the fluid-fluid interface deforms as a response to magnetic fields. Usually, the fluids are completely immiscible and so the interfacial energy in this systems is very large. Here we show that it …
The aggregation of clay particles in aqueous solution is a ubiquitous everyday process of broad environmental and technological importance. However, it is poorly understood at the all-important atomistic level since it depends on a complex and dynamic interplay of solvent-mediated electrostatic, hydrogen-bonding, and dispersion interactions. With this in mind we have performed an extensive set of classical molecular dynamics simulations (included enhanced sampling simulations) on the interactions between model kaolinite nanoparticles in pure and salty water. Our simulations reveal highly aniso…
Preparation of nanoemulsions of triglyceride oils in water usually requires high mechanical energy and sophisticated equipment. Recently, we showed that alpha-to-beta (viz. gel-to-crystal) phase transition, observed with most lipid substances (triglycerides, diglycerides, phospholipids, alkanes, etc.), may cause spontaneous disintegration of micro-particles of these lipids, dispersed in aqueous solutions of appropriate surfactants, into nanometer particles/drops using a simple cooling/heating cycle of the lipid dispersion (Cholakova et al. ACS Nano 14 (2020) 8594). In the current study we show…
In Specific Power Absorption (SPA) models for Magnetic Fluid Hyperthermia (MFH) experiments, the magnetic relaxation time of the nanoparticles (NPs) is known to be a fundamental descriptor of the heating mechanisms. The relaxation time is mainly determined by the interplay between the magnetic properties of the NPs and the rheological properties of NPs environment. Although the role of magnetism in MFH has been extensively studied, the thermal properties of the NPs medium and their changes during of MFH experiments have been so far underrated. Here, we show that ZnxFe3-xO4 NPs dispersed throug…
Aqueous dispersion of Laponite, when exposed to carbon dioxide environment leads to in situ inducement of magnesium and lithium ions, which is, however absent when dispersion is exposed to air. Consequently, in the rheological experiments, Laponite dispersion preserved under carbon dioxide shows more spectacular enhancement in the elastic and viscous moduli as a function of time compared to that exposed to air. By measuring concentration of all the ions present in a dispersion as well as change in pH, the evolving inter-particle interactions among the Laponite particles is estimated. DLVO anal…
A systematically designed study has been conducted to understand and clearly demarcate the degree of contribution by the constituting elements to the surface tension of nanocolloids. The effects of elements such as surfactants, particles and the combined effects of these on the interfacial tension of these complex fluids are studied employing pendant drop shape analysis method by fitting Young Laplace equation. Only particle has shown considerable increase in surface tension with particle concentration in a polar medium like DI water whereas only marginal effect particles on surface tension in…
