Interparticle Force Measurements Research Articles

Solvents govern rheology and jamming of polymeric bead suspensions

Despite their apparent simplicity, suspensions of hard spheres in a Newtonian fluid show complex non-Newtonian behaviors and remain poorly understood. Recent works have pointed out the crucial role of interparticle contact forces in these behaviors. Here, we show that the same (polystyrene) particles, when immersed in different Newtonian solvents, show different behaviors at both the microscopic and macroscopic scales. Thanks to interparticle force measurements in each solvent together with rheological measurements, we show how the fine details of the pairwise particle interactions impact the macroscopic behavior. The rheological properties (shear thinning, shear thickening, jamming solid fraction value) of the suspensions, made up of same particles, are shown to depend on the nature of the solvent. Here, we highlight several mechanisms at the particle scale: the swelling of polymeric particles in an organic solvent, the role of colloidal repulsive forces and inertia for particles in a water solution, and the variation of the friction coefficient as a function of the load for particles immersed in silicone oils. Our study provides new quantitative data to test micromechanical models and simulations. It questions the interpretation of previous experimental works. Finally, it shows the need to systematically characterize the interparticle normal and tangential forces when studying a given suspension of hard spheres in a Newtonian fluid

Predicting physical stability in pressurized metered dose inhalers via dwell and instantaneous force colloidal probe microscopy

Colloidal probe microscopy (CPM) is a quantitative predictive tool, which can offer insight into particle behavior in suspension pressurized metered dose inhalers (pMDIs). Although CPM instantaneous force measurements, which involve immediate retraction of the probe upon sample contact, can provide information on inter-particle attractive forces, they lack the ability to appropriately imitate all critical particle pMDI interactions (e.g., particle re-dispersion after prolonged pMDI storage).In this paper, two novel dwell force techniques – indentation and deflection dwell – were employed to mimic long-term particle interactions present in pMDIs, using particles of various internal structures and a model liquid propellant (2H,3H perfluoropentane) as a model system. Dwell measurements involve particle contact for an extended period of time. In deflection dwell mode the probe is held at a specific position, while in indentation dwell mode the probe is forced into the sample with a constant force for the entirety of the contact time. To evaluate the applicability of CPM to predict actual pMDI physical stability, inter-particle force measurements were compared with qualitative and quantitative bulk pMDI measurement techniques (visual quality and light scattering).Measured instantaneous attractive (snap-in) and adhesive (max-pull) forces decreased as a function of increasing surface area, while adhesive forces measured by indentation dwell decreased as a function of dwell contact time for particles containing voids.Instantaneous force measurements provided information on the likelihood of floccule formation, which was predictive of partitioning rates, while indentation dwell force measurements were predictive of formulation re-dispersibility after prolonged storage. Dwell force measurements provide additional information on particle behavior within a pMDI not obtainable via instantaneous measurements.

Hydrodynamic effects in the measurement of interparticle forces in nematic colloids

We propose improved measurement methods of interparticle force between nematic colloids. Although various methods have been utilized for the force measurement, the comparison between the forces obtained by different methods has not been reported. In the frequently used method called the "free-release" method, the hydrodynamic interaction between moving particles has a serious influence on the measurement. In this study we modified those measurement methods by taking the long-ranged hydrodynamic interaction into account. The evaluated forces have been compared with that obtained by the dual beam "optical trap" method, which is free from the hydrodynamic effect. The agreement between them is quantitatively fairly good.

Experimental study on inter-particle acoustic forces

A method for the experimental measurement of inter-particle forces (secondary Bjerknes force) generated by the action of an acoustic field in a resonator micro-channel is presented. The acoustic radiation force created by an ultrasonic standing wave moves suspended particles towards the pressure nodes and the acoustic pressure induces particle volume oscillations. Once particles are in the levitation plane, transverse and secondary Bjerknes forces become important. Experiments were carried out in a resonator filled with a suspension composed of water and latex particles of different size (5-15 μm) at different concentrations. Ultrasound was generated by means of a 2.5 MHz nominal frequency transducer. For the first time the acoustic force generated by oscillating particles acting on other particles has been measured, and the critical interaction distance in various cases has been determined. Inter-particle forces on the order of 10(-14) N have been measured by using this method.

Measurement of inter-particle forces by an interfacial force microscope

An interfacial force microscope (IFM) was employed to measure the inter-particle forces between two individual glass beads with diameters varying from 8 to 20 μm. With the feedback function of IFM turned off, attractive forces were obtained. The forces varied in the range of 0.1–0.34 μN, and their validity was confirmed by a theoretical analysis of the van der Waals force between the same glass beads. With the feedback function switched on, no attractive forces between particles were detected by the IFM when the probe approached the sample substrate. This may be attributed to the dramatic change of the attractive forces within a very short separation distance and/or the relatively poor signal-to-noise ratio of the IFM.

Equilibrium Force Isotherms of a Deformable Bubble/Drop Interacting with a Solid Particle across a Thin Liquid Film

Comparison of experimental force−distance isotherms obtained for a deformable drop/bubble and a solid particle interacting across a thin liquid film with colloidal theories, for example, the DLVO theory, currently relies on two incompatible and nonphysical assumptions. These are approximating the bubble/drop as a purely elastic solid and, contradictorily, as a nondeformable solid. To avoid these aphysical assumptions, we have developed a more rigorous method for interpreting the experimental force−distance isotherms. Equilibrium shapes of the bubbles/drops are calculated from the augmented Young−Laplace equation pertinent to the relevant geometry of the experiments. The overall bubble/drop−solid interaction force is then calculated using a macroscopic force balance. We find that the nondeformable and the elastic-deformation models for the bubble/drop are rather inaccurate. The developed methodology is of particular relevance to interparticle force measurements using an atomic force microscope (AFM). To th...

Formation Mechanisms and Aggregation Behavior of Borohydride Reduced Silver Particles

In this work, we examine the formation mechanisms of nanoscale silver particles produced by the reduction of silver perchlorate with sodium borohydride. Evidence is presented that the reaction pathway does not follow classical nucleation and growth theory, but is dominated by colloidal interactions. Upon injection of silver into a sodium borohydride solution, a molecular species absorbing at 220 nm is produced in less than 1 s. We suggest that this species contains borohydride and small particles of reduced silver. The reaction mixture is initially dark as the result of the aggregation of the small silver particles into larger particles which have broad absorption spectra. During an “intermediate” stage, transmission electron microscopy and absorbance data show that even larger (∼6−10 nm) particles grow at the expense of the “monomeric” silver particles. Later in the reaction, electrochemical potential measurements show that the borohydride concentration suddenly decreases. Direct measurement of interparticle forces demonstrate that this change in the solution conditions drives the particle surface potential toward zero and results in increased adhesive forces. The resulting aggregation manifests itself in a darkening of the solution color. At low temperatures this corresponds to a 10 times increase in the particle size whereas, at high temperatures, the increase is minimal. This effect can be linked to the number of monomeric silver particles remaining during the final transition.

The Unusual Colloid Stability of Gibbsite at High pH

The colloid stability of gibbsite dispersions has been investigated at high pH, near and under conditions which reflect the Bayer process for the production of alumina. These dispersions display unusual colloid stability at high pH and electrolyte concentration, as shown by coagulation rate and rheological and interparticle force measurements. There is a repulsive, non-DLVO contribution to the yield value at pH values between 10 and 12. At high concentrations of NaOH and dissolved Al (cf. Bayer conditions), strong repulsive forces are evident. For particles of colloidal dimensions, the forces are of two types, long-range hydrodynamic and shorter range steric or structural forces. A steric layer thickness of about 20 nm at the gibbsite–solution interface is indicated.

Use of atomic force microscopy in particle science and technology research

The atomic force microscopy, because of its unique features, can be used for a variety of applications and provides excellent research opportunities in the area of particulate science and technology. Most importantly, AFM is emerging as a major analytical tool for the characterization of interfacial behavior by measuring interparticle and intermolecular forces for a wide range of particulate and fibrous materials that are often encountered in mineral processing, ceramic processing, colloid and polymer science, and in biological systems. In this paper, the principles and applications of the AFM in the area of particulate processing are illustrated. Results from interparticle force measurements involved in selected systems are presented. Also, the application of AFM for the determination of the point of zero charge and for the characterization of polymer surfaces is discussed.

Interactions between dissimilar surfaces in high ionic strength solutions as determined by atomic force microscopy

The recent non-equilibrium electrophoretic mobility measurements by laser-Doppler electrophoresis coupled with the stability and prevalence of collector colloids in soluble salt flotation systems suggest that the selective flotation of alkali halides is due to the adsorption of oppositely charged collector colloids by heterocoagulation. Previously reported flotation results confirm this surface charge/collector colloid adsorption model. However, the nature of the interparticle forces responsible for heterocoagulation of such oppositely charged particles at high ionic strengths remains to be determined. In this regard, atomic force microscopy was used for interparticle force measurements at high ionic strengths. Model systems such as polystyrene/quartz and silica/sapphire were studied to measure the particle interaction forces prevalent at high ionic strengths. Results from this study indicate that while repulsive hydration forces exist between similarly charged hydrophilic particles, attractive forces exist between oppositely charged particles in high ionic strength solutions. The repulsive forces between similarly charged hydrophilic surfaces at high ionic strengths (2–4 m) are described by a double exponential function with decay lengths of 0.17 and 1.4 nm, depending upon the surfaces involved. On the other hand, attractive forces were observed between oppositely charged hydrophilic surfaces at high ionic strengths (2–4 m) and were described by a single exponential function with decay lengths of up to 9 nm, depending upon the surfaces involved.

Recent contributions to the analysis of soluble salt flotation systems

Abstract The selective separation of alkali halide salts such as KCl from NaCl by flotation in saturated brines (ionic strength ∼ 4 M) is of considerable industrial significance and has been studied by flotation chemists for many decades. Electrokinetic information coupled with the stability and prevalence of certain collector colloids in these soluble salt flotation systems suggest that the selective flotation of alkali halide minerals is due to the adsorption of oppositely charged collector colloids by heterocoagulation. Recent contributions have revealed the nature of the interparticle forces responsible for heterocoagulation of oppositely charged particles at high ionic strengths and the reasons for the deviations of the surface charge of some alkali halides from theory. Because of the progress made in interparticle force measurements at high ionic strengths by atomic force microscopy (AFM), more information is now available and results from recent AFM measurements are reported. Also extension of the simplified lattice ion hydration theory by considering partial hydration of the surface state allows for the prediction of the sign of the surface charge of all alkali halides from first principles. Thus the interactions at high ionic strengths and the deviations of the surface charge of some alkali halides may be explained by structural forces as measured by AFM and the extended lattice ion hydration theory, respectively.

Direct Measurement of Interparticle Forces by the Optical Trapping Technique

The intrinsic interactive forces between two free colloidal particles suspended in aqueous solutions have been measured as a function of the separation by applying the optical trapping technique to a monodispersed polystyrene latex of a mean diameter 2.13 μm. For this purpose, specific technology for the precise measurement of the surface-to-surface separation and force strength has been developed. The results were in excellent agreement with the prediction of the DLVO theory in a variety of electrolyte concentrations.

Measurement of inter-particle forces from the osmotic pressure of partially frozen dispersions

When an oil-continuous dispersion is frozen two microdomains are formed: one, the pure oil solvent which is selectively solidified (I), the other, a concentrated `liquid' dispersion of particles in oil (II). These two domains are intimately mixed within the frozen colloid and exist in a state of equilibrium determined by the system pressure and temperature. The position of equilibrium controls the proportion of the solvent which is solidified, and thereby the concentration of particles within the fluid microdomains (II). Combined with SANS measurements, to determine the inter-particle separation in these microdomains, an analysis based on osmotic pressure provides a measure of the inter-particle repulsion forces presented by the surfactant layers.

Recent developments in the measurement of interparticle forces

In the last twenty years there has been an explosion of experimental work devoted to determining the forces between colloidal particles. There have been considerable experimental and theoretical advances. In this review we shall concentrate on the experimental aspects. Currently there is no technique which directly measures the interaction between two individual particles as a function of their separation. Particle - particle forces can only be inferred. In this paper we review the experimental procedures which have been used to determine the nature and range of interparticle forces in this indirect way. The many different experimental techniques which have been developed are outlined and typical data presented. The relative merits and demerits of each technique is discussed and the way forward to measuring particle - particle interactions directly is proposed.

Interparticle force measurements on ferromagnetic steel balls

Interparticle force measurements have been performed upon various configurations of linear chains and planar layers of chrome steel balls immersed in a steady uniform applied magnetic field. These force measurements, made for particles in contact and at various separations, reveal that the attractive forces are enhanced in the case of particle chains parallel to the applied magnetic field and slightly decreased in the case of perpendicular layers. The magnetic saturation exhibited by the steel balls has only a modest effect on the force. Experimental data obtained for two-particle chains correlate reasonably well to force calculations based upon a linear multipolar expansion using a relative particle permeability value of μr=100. It is also found that data obtained with longer linear chains as well as with planar particle arrays do not deviate drastically from the predictions of the two-sphere problem.