Dilute Colloidal Dispersion Research Articles

Probing nonlinear velocity profiles of shear-thinning, nematic platelet dispersions in Couette flow using x-ray photon correlation spectroscopy

We report experiments employing x-ray photon correlation spectroscopy (XPCS) to characterize the velocity profiles of complex fluids in Couette flow. The approach involves modeling the XPCS correlation functions obtained with the incident x-ray beam passing tangentially through the Couette cell gap at various distances from the inner wall. We first demonstrate the technique with measurements on a dilute colloidal dispersion in the Newtonian liquid glycerol, where the expected linear velocity profiles are recovered. We then employ the technique to map the shear-rate-dependent velocity profiles of a shear-thinning dispersion of nematically ordered Gibbsite platelets. The nonlinear velocity profiles of the Gibbsite dispersion include a narrow slip region adjacent to the outer wall and a band with a small velocity gradient in the interior of the gap that evolves into a region increasingly resembling plug flow with increasing shear rate. Variations in the velocity profile along the vorticity direction indicate an instability in the interface between this region of small velocity gradient and a region of high velocity gradient near the inner wall. The analysis of the small-angle scattering patterns provides information about the spatial and temporal variations in the nematic order of the Gibbsite dispersion and their coupling to the velocity profile. Additional potential applications of this XPCS-based technique and comparisons with established methods for characterizing velocity profiles are discussed.

Analytical Model for Transport of Dilute Colloidal Silica Dispersions through Porous Media

AbstractThe purpose of this paper is to present a simple analytical model to describe the flow of a dilute colloidal silica dispersion through a porous medium column. The model presents the movemen...

Photonic water dynamically responsive to external stimuli.

Fluids that contain ordered nanostructures with periodic distances in the visible-wavelength range, anomalously exhibit structural colours that can be rapidly modulated by external stimuli. Indeed, some fish can dynamically change colour by modulating the periodic distance of crystalline guanine sheets cofacially oriented in their fluid cytoplasm. Here we report that a dilute aqueous colloidal dispersion of negatively charged titanate nanosheets exhibits structural colours. In this ‘photonic water', the nanosheets spontaneously adopt a cofacial geometry with an ultralong periodic distance of up to 675 nm due to a strong electrostatic repulsion. Consequently, the photonic water can even reflect near-infrared light up to 1,750 nm. The structural colour becomes more vivid in a magnetic flux that induces monodomain structural ordering of the colloidal dispersion. The reflective colour of the photonic water can be modulated over the entire visible region in response to appropriate physical or chemical stimuli.

Force-induced diffusion in suspensions of hydrodynamically interacting colloids

We study the influence of hydrodynamic, thermodynamic and interparticle forces on the diffusive motion of a Brownian probe driven by a constant external force through a dilute colloidal dispersion. The influence of these microscopic forces on equilibrium self-diffusivity (passive microrheology) is well known: all three act to hinder the short- and long-time self-diffusion. Here, via pair-Smoluchowski theory, we explore their influence on self-diffusion in a flowing suspension, where particles and fluid have been set into motion by an externally forced probe (active microrheology), giving rise to non-equilibrium flow-induced diffusion. The probe’s motion entrains background particles as it travels through the bath, deforming the equilibrium suspension microstructure. The shape and extent of microstructural distortion is set by the relative strength of the external force $F^{\mathit{ext}}$ to the entropic restoring force $kT/a_{\mathit{th}}$ of the bath particles, defining a Péclet number $\mathit{Pe}\equiv F^{\mathit{ext}}/(2kT/a_{\mathit{th}})$; and also by the strength of hydrodynamic interactions, set by the range of interparticle repulsion ${\it\kappa}=(a_{\mathit{th}}-a)/a$, where $kT$ is the thermal energy and $a_{\mathit{th}}$ and $a$ are the thermodynamic and hydrodynamic sizes of the particles, respectively. We find that in the presence of flow, the same forces that hinder equilibrium diffusion now enhance it, with diffusive anisotropy set by the range of interparticle repulsion ${\it\kappa}$. A transition from hindered to enhanced diffusion occurs when diffusive and advective forces balance, $\mathit{Pe}\sim 1$, where the exact value is a sensitive function of the strength of hydrodynamics, ${\it\kappa}$. We find that the hindered to enhanced transition straddles two transport regimes: in hindered diffusion, stochastic forces in the presence of other bath particles produce deterministic displacements (Brownian drift) at the expense of a maximal random walk. In enhanced diffusion, driving the probe with a deterministic force through an initially random suspension leads to fluctuations in the duration of probe–bath particle entrainment, giving rise to enhanced, flow-induced diffusion. The force-induced diffusion is anisotropic for all $\mathit{Pe}$, scaling as $D\sim \mathit{Pe}^{2}$ in all directions for weak forcing, regardless of the strength of hydrodynamic interactions. When probe forcing is strong, $D\sim \mathit{Pe}$ in all directions in the absence of hydrodynamic interactions, but the picture changes qualitatively as hydrodynamic interactions grow strong. In this nonlinear regime, microstructural asymmetry weakens while the anisotropy of the force-induced diffusion tensor increases dramatically. This behaviour owes its origins to the approach toward Stokes flow reversibility, where diffusion along the direction of probe force scales advectively while transverse diffusion must vanish.

Magnetic Field Induced Ultrasound from Colloidal Superparamagnetic Nanoparticles

Generating ultrasound remotely by means of magnetic fields is an important technological development to circumvent the drawbacks of the traditional means of ultrasound generation techniques. In this report, it is demonstrated that ultrasound is generated from colloidal superparamagnetic nanoparticles when exposed to pulsed and alternating magnetic fields. Comparison of the inhomogeneous and homogeneous magnetic fields indicates that both homogeneous and inhomogeneous magnetic fields could be important for efficient ultrasound generation; however, the latter is more important for dilute colloidal dispersion of magnetic nanoparticles. In strong magnetic fields, the ultrasound generated from the colloidal magnetic nanoparticles shows reasonable agreement with the magnetostriction effect commonly observed for bulk ferromagnetic materials. At low magnetic fields, the colloidal magnetic nanoparticle dispersion produces considerable amount of ultrasound when exposed to ac magnetic fields in the 20–5000 kHz frequ...

Solidification of a Charged Colloidal Dispersion Investigated Using Microfluidic Pervaporation.

We investigate the dynamics of solidification of a charged colloidal dispersion using an original microfluidic technique referred to as micropervaporation. This technique exploits pervaporation within a microfluidic channel to extract the solvent of a dilute colloidal dispersion. Pervaporation concentrates the colloids in a controlled way up to the tip of the channel until a wet solid made of closely packed colloids grows and invades the microfluidic channel. For the charged dispersion under study, we however evidence a liquid to solid transition (LST) preceding the formation of the solid, owing to the presence of long-range electrostatic interactions. This LST is associated with the nucleation and growth of domains confined in the channel. These domains are then compacted anisotropically up to forming a wet solid of closely packed colloids. This solid then invades the whole channel as in directional drying with a growth rate which depends on the microfluidic geometry. In the final steps of the solidification, we observed the occurrence of cracks and shear bands, the delamination of the wet solid from the channel walls, and its invasion by a receding air front. Interestingly, this air front follows specific patterns within the solid which reveal different microscopic colloidal organizations.

Bimodal distribution of the magnetic dipole moment in nanoparticles with a monomodal distribution of the physical size

High-frequency applications of magnetic nanoparticles, such as therapeutic hyperthermia and magnetic particle imaging, are sensitive to nanoparticle size and dipole moment. Usually, it is assumed that magnetic nanoparticles with a log-normal distribution of the physical size also have a log-normal distribution of the magnetic dipole moment. Here, we test this assumption for different types of superparamagnetic iron oxide nanoparticles in the 5–20 nm range, by multimodal fitting of magnetization curves using the MINORIM inversion method. The particles are studied while in dilute colloidal dispersion in a liquid, thereby preventing hysteresis and diminishing the effects of magnetic anisotropy on the interpretation of the magnetization curves. For two different types of well crystallized particles, the magnetic distribution is indeed log-normal, as expected from the physical size distribution. However, two other types of particles, with twinning defects or inhomogeneous oxide phases, are found to have a bimodal magnetic distribution. Our qualitative explanation is that relatively low fields are sufficient to begin aligning the particles in the liquid on the basis of their net dipole moment, whereas higher fields are required to align the smaller domains or less magnetic phases inside the particles.

Active microrheology: Fixed-velocity versus fixed-force

In active microrheology, a probe particle is driven by an external force through a complex medium and its motion studied in order to infer properties of the embedding material. It is conducted in two limiting forms: either the probe is propelled by a fixed force, as with magnetic tweezers, or it is driven at a fixed velocity, as with optical tweezers. Recent work has shown that the mean probe motion can be interpreted as an effective material viscosity, but that this viscosity depends on whether the fixed-force or fixed-velocity mode is employed. We compute the effective viscosity probed by fixed-velocity active microrheology of a dilute colloidal dispersion. A comparison is made between this new result and the effective viscosity probed in the fixed-force mode. In the absence of hydrodynamic interactions, the particle-phase contributions to the effective viscosity for the two modes differ by exactly a factor of two. A simple scaling argument has been previously advanced to rationalize this difference: in the fixed-force mode, the probe is free to diffuse, and thus the relaxation time scale is set by the relative diffusivity between probe and bath. However, in the fixed-velocity mode, thermal motion of the probe particle is “frozen out” because the probe cannot diffuse; the relaxation rate is thus halved. The ratio of the two rates is independent of how quickly the probe particle is driven through the suspension—the extent and shape of microstructural deformation is the same for the two cases. In contrast, when the suspended particles interact hydrodynamically, the distortions to the suspension microstructure in the fixed-velocity versus fixed-force modes differ. We show that, depending on the strength of the hydrodynamic interactions, the ratio of the fixed-velocity to the fixed-force microstructural contributions to the effective viscosity may be as small as 1.3, and only approaches 2.0 when hydrodynamic interactions among the particles are negligibly weak. While this ratio varies both as a function of the strength of the deformation imposed and of the strength of hydrodynamic interactions, the fixed-velocity effective viscosity agrees qualitatively with that already measured for the fixed-force mode: the colloidal dispersion thins in the limit of weak hydrodynamic interactions; and it first thins and then thickens in the limit of strong hydrodynamic interactions, as the strength of deformation increases, recovering characteristics of shear-(force-) thinning and thickening well known in colloidal dispersions. The agreement between the two, and with traditional macrorheological approaches, shows that both fixed-force and fixed-velocity provide a useful tool for the interrogation of complex fluids.

Cluster Formation and Rheology of Photoreactive Nanoparticle Dispersions

We show how photocrosslinking of small nanoparticles within a very dilute colloidal dispersion leads to the formation of large fractal particle clusters, which have a strong influence on the viscosity of the dispersion although the overall solid content is well below 5 wt %. Furthermore, the solvent plays an important role because of its function as an optical filter, for example, in toluene only photocrosslinking but no photocleavage takes place. Therefore, a diffusion-controlled cluster growth mechanism, leading to clusters with low fractal dimension, is expected; on the other hand, in tetrahydrofuran the photoreaction is partially reversible. Therefore, the cluster growth in this case is reaction controlled, leading to more compact clusters with higher fractal dimension, which therefore only have a negligible effect on the rheological properties of the solvent. In this context, we will briefly discuss the possibility to use our nanoparticle system as opto-rheological switch.

Effect of typical sugars on the viscosity and colloidal stability of apple juice

Cloudy apple juice may be modeled as a dilute colloidal dispersion of solid particles in a solution of pectins, sugars, organic acids, and salts. In order to evaluate the effect of sugars on the viscosity and stability of the juice, it was diafiltered by ultrafiltration to remove the native soluble solids, and controlled amounts of sugars (glucose, sucrose, and maltose) were added afterwards. The addition of sugars produced a linear increase of the specific viscosity at decreasing water activities. The rates of increase (slopes) were proportional to the hydration capacity of each sugar. The specific viscosity of a colloidal dispersion of solid particles in sugar solution depends on three types of interactions: particle–particle (p–p), particle–water (p–w), and particle–sugar (p–s). P–p and p–w interactions were estimated from the extended DLVO theory in terms of the energy barrier between pairs of particles, and found to decrease at increasing sugar concentrations. Then, the increase of the specific viscosity was attributed to an increase of p–s interactions. The total energy barrier of the system (a measure of its stability) was modeled to be the sum of a p–p (including p–w) contribution plus a p–s contribution, and estimated from specific viscosity vs. particle volume fraction data at different sugar concentrations. The p–s contribution was estimated by difference. It was found to be positive (which was attributed to hydration repulsion) and higher than the p–p contribution at sugar concentrations ⩾0.04 mol/mol.

Transport Coefficients and Orientational Distributions of a Dilute Colloidal Dispersion composed of Hematite Particles (Analysis for an Applied Magnetic Field parallel to the Angular Velocity Vector of Simple Shear Flow)

粘度や拡散係数といった輸送係数に対する磁場，せん断流，ランダム力の影響，および希釈コロイド分散系中のヘマタイト粒子の配向分布について研究を行った．ヘマタイト粒子は粒子軸と垂直な方向の磁気モーメントを持つ回転楕円体としてモデル化した．本解析では，これらの粒子が単純せん断流および磁場中で回転ブラウン運動を行っている問題を取り扱っている．配向分布関数の基礎方程式をトルクの釣り合いから求め，数値解析法により解を求めた．得られた結果を要約すると以下のようになる．非常に強い磁場の場合，粒子の磁気モーメントは磁場方向に拘束されるので，棒状粒子の運動はせん断面に垂直な面に限られる．比較的強い磁場の状況下において，せん断流が小さい場合は，棒状粒子は磁場方向の周りに自由に回転できる．対照的に，せん断流が大きな場合は，磁気モーメントが磁場方向を向くとともに，粒子は流れ場の方向に傾く傾向を持つ．配向分布関数は粒子軸を沈降方向に一致させながら沈降方向に動く場合と，粒子軸を沈降方向に対して垂直な方向にとりながら粒子が動く場合の，ニつの特徴的な運動に支配されているため，大きな拡散係数の値を得ることができない．

Orientational Distributions of Ferromagnetic Spherocylinder Particles Influenced by External Magnetic Field

We have analyzed the time-dependent behaviors of the orientational distributions of a dilute colloidal dispersion composed of the ferromagnetic spherocylinder particles influenced by the external magnetic field. A basic equation is derived for the orientational distributions of the particles in steady two dimensional flow by using the perturbation expansion method. An exact solution of the basic equation is obtained theoretically by considering the inverse matrix composed of the functions for the solvable condition. By using the solution, the relevant statistical quantities for an ensemble of the particles are calculated. It is shown that the steady flow causes a distortion of the orientational distribution and can not bring about the steady state of the distribution without the external magnetic field. However, we find the external magnetic field can not always create the steady state of the distribution. We have derived the condition that the steady state of the distribution is made by the external magnetic field and the steady flow. In order to estimate the response of the particles for the magnetic field, the reference time is obtained from the rate of the convergence to the steady state.

Orientational Distributions of Ferromagnetic Spherocylinder Particles Influenced by External Magnetic Field in Trhee Dimensional Flow

The behaviors of the orientational distributions of a dilute colloidal dispersion, which is composed of the ferromagnetic spherocylinder particles influenced by the external magnetic field, have been investigated theoretically. A series of basic equations is derived for the orientational distributions of the particles in steady three dimensional flow by using the perturbation expansion method. Exact solutions of the basic equations are obtained theoretically. By using the solutions, we have discussed the time-dependent behaviors of the particles. We have derived the condition that the steady state of the distribution is realized by the external magnetic field, the steady flow and the rotational Brownian motion of the particles. In order to estimate the response of the particles for the magnetic field, the reference time is obtained from the rate of the convergence to the steady state. It is shown that the steady flow causes a distortion of the orientational distribution under the typical three types of the magnetic direction.

Rheological Properties of Ferromagnetic Spherocylinder Particles Influenced by External Magnetic Field in Three Dimensional Flow

We have analyzed the time-dependent behaviors of the rheological properties of a dilute colloidal dispersion composed of the ferromagnetic spherocylinder particles influenced by the external magnetic field. A series of basic equations is derived for the orientational distributions of the particles in steady three dimensional flow by using the perturbation expansion method. Exact solutions of the basic equations are obtained theoretically. By using the solutions, we have discussed the viscosity induced by the external magnetic field. The viscosity is mainly described by the simple fractional functions of the magnetic field. The coefficients of the fractional functions make a classification of the behaviors of the viscosity. The reference time obtained from the rate of the convergence to the steady state of the viscosity is shorter for the stronger magnetic field, because the orientations of the particles are arranged by the strong magnetic fields. We have derived the condition to generate the viscosity by the magnetic field, and the condition to increase the viscosity with the increased magnetic field.

Transport coefficients and orientational distributions of spheroidal particles with magnetic moment normal to the particle axis: (Analysis for an applied magnetic field normal to the shear plane)

We have investigated the influence of the magnetic field strength, shear rate, and rotational Brownian motion on transport coefficients such as viscosity and diffusion coefficient, and also on the orientational distributions of rodlike particles of a dilute colloidal dispersion. The rodlike particle is modeled as a magnetic spheroidal particle which has a magnetic moment normal to the particle axis; such a particle may typically be a hematite particle. In the present study, an external magnetic field is applied in the direction normal to the shear plane of a simple shear flow. The basic equation of the orientational distribution function has been derived from the balance of torques and solved numerically. The results obtained here are summarized as follows. Although the orientational distribution function shows a sharp peak in the shear flow direction for a very strong magnetic field, such a peak is not restricted to the field direction alone, but continues in every direction of the shear plane. This is due to the characteristic particle motion that the particle can rotate around the axis of the magnetic moment in the shear plane, although the magnetic moment nearly points to the magnetic field direction. This particle motion in the shear plane causes negative values of the viscosity due to the magnetic field. The viscosity decreases, attains a minimum value, and then converges to zero as the field strength increases. Additionally, the diffusion coefficient is significantly influenced by such characteristic particle motion in the shear plane for a strong magnetic field.

G801 ヘマタイト粒子からなる希釈コロイド分散系の配向分布と輸送係数に関する研究 : せん断流の回転角速度ベクトル方向に磁場が作用する場合の解析(2)(GS-8 管内流,混相流,粒子系,一般セッション)

G801 ヘマタイト粒子からなる希釈コロイド分散系の配向分布と輸送係数に関する研究 : せん断流の回転角速度ベクトル方向に磁場が作用する場合の解析(2)(GS-8 管内流,混相流,粒子系,一般セッション)

G801 ヘマタイト粒子からなる希釈コロイド分散系の配向分布と輸送係数に関する研究 : せん断流の回転角速度ベクトル方向に磁場が作用する場合の解析(1)(GS-8 管内流,混相流,粒子系,一般セッション)

磁気モーメントが粒子軸と直交するようなヘマタイト粒子からなる分散系の場合,磁場方向と垂直な方向に粒子が配向し,従来と異なる粒子挙動を示すことが期待される.従って,このような棒状粒子を懸濁した機能性流体を開発できれば,応用面での非常に魅力的で発展性のある機能性流体となる可能性が大きい.本研究では粒子軸に垂直な方向に磁気モーメントを有する棒状粒子からなる希釈分散系を対象とし,単純せん断流の回転角速度ベクトルと平行な方向に外部磁場を印加した場合の,粒子配向やレオロジー特性,拡散係数を検討することを目的とする.

Contribution of colloidal forces to the viscosity and stability of cloudy apple juice

Cloudy apple juice (CAJ) was considered to be a dilute colloidal dispersion of electrically charged, hydrophilic particles in an electrolyte solution (serum). Experimental data of relative (CAJ/serum) viscosity as a function of particle volume fraction, η r( ϕ), was modeled as the sum of a ‘hard-sphere’ contribution ( η r hs ) plus a ‘colloidal forces’ contribution ( η r cf ) . Theoretical values of η r hs ( ϕ) were obtained with Einstein's equation for dilute suspensions of non-interacting, rigid spheres. Semi-empirical values of ( η r cf ) were found to be proportional to ϕ 1.22, lower than the theoretical ϕ 2. The difference was attributed to the effect of the energy barrier or activation energy between pairs of particles ( U Max). The value of U Max at each ϕ was obtained from the maximum of total interaction potential curves as function of inter-particle distance, U( x). In its turn, U( x) was modeled with the extended DLVO theory as the balance between attractive Van der Waals, repulsive electrostatic, and repulsive hydration energies. The term U Max was found to be a function of ϕ and the hydration pressure constant ( P 0), which was unknown for CAJs particles. This function was introduced in an empirical model proposed in this work, η r cf = α ( U Max / k B T ) ϕ , and correlated with semi-empirical values, giving α=0.483 and P 0=2.45×10 6 N m −2. According to this result, hydration forces (even reduced by hydrophobic interactions between pectin molecules) played the main role in the stability of CAJ particles.

Transport coefficients and orientational distributions of rodlike particles with magnetic moment normal to the particle axis under circumstances of a simple shear flow

We have investigated the influences of the magnetic field strength, shear rate, and random forces on transport coefficients such as viscosity and diffusion coefficient, and also on the orientational distributions of rodlike particles of a dilute colloidal dispersion. This dispersion is composed of ferromagnetic spheroidal particles with a magnetic moment normal to the particle axis. In the present analysis, these spheroidal particles are assumed to conduct the rotational Brownian motion in a simple shear flow as well as an external magnetic field. The basic equation of the orientational distribution function has been derived from the balance of the torques and solved numerically. The results obtained here are summarized as follows. For a very strong magnetic field, the rodlike particle is significantly restricted in the field direction, so that the particle points to a direction normal to the flow direction (and also to the magnetic field direction). However, the present particle does not exhibit a strong directional characteristic, which is one of the typical properties for the previous particle with a magnetic moment parallel to the particle axis. That is, the particle can rotate around the axis of the magnetic moment, although the magnetic moment nearly points to the field direction. The viscosity significantly increases with the field strength, as in the previous particle model. The particle of a larger aspect ratio leads to the larger increase in the viscosity, since such elongated particles induce larger resistance in a flow field. The diffusion coefficient under circumstances of an applied magnetic field is in reasonable agreement between theoretical and experimental results.

Magnetization of cobalt nanodiscs

We have studied the magnetization and AC susceptibility of a dilute colloidal dispersion of ε -Co nanodiscs. The temperature dependence of the magnetization implies that the nanodisc moments become largely static below a blocking temperature which exceeds room temperature. The related anisotropy energy is significantly larger than the magnetocrystalline anisotropy for bulk ε -Co, which may result either from surface modifications to the crystalline anisotropy, or perhaps from shape anisotropy. The saturation moment is found to be temperature independent, over a wide range of temperatures and also close to the blocking temperature, although the field at which saturation occurs varies considerably with temperature. A coercive field of 1700 Oe is found at low temperature, indicating that magnetization reversal involves the coordinated rotation of the moments in individual nanodiscs. Aging effects are observed in the AC susceptibility, implying that the spatial coherence of the room temperature dispersions increases over time, leading to static short-range order.