Biomedical subjects

Raymond R Dagastine

Publications and source records attributed to Raymond R Dagastine.

8 recordsLinked to original sources

PubMed2006

Dynamic forces between two deformable oil droplets in water.

The understanding of static interactions in colloidal suspensions is well established, whereas dynamic interactions more relevant to biological and other suspended soft-matter systems are less well understood. We present the direct force measurement and quantitative theoretical description for dynamic forces for liquid droplets in another immiscible fluid. Analysis of this system demonstrates the strong link between interfacial deformation, static surface forces, and hydrodynamic drainage, which govern dynamic droplet-droplet interactions over the length scale of nanometers and over the time scales of Brownian collisions. The results and analysis have direct bearing on the control and manipulation of suspended droplets in soft-matter systems ranging from the emulsions in shampoo to cellular interactions.

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PubMed2005

Direct comparison of atomic force microscopic and total internal reflection microscopic measurements in the presence of nonadsorbing polyelectrolytes.

We have investigated the structural and depletion forces between silica glass surfaces in aqueous, salt-free solutions of sodium poly(styrene sulfonate). The interaction forces were investigated by two techniques: total internal reflectance microscopy (TIRM) and colloid probe atomic force microscopy (AFM). The TIRM technique measures the potential energy of interaction directly, while the AFM is a force balance. Comparison between the data sets was used to independently calibrate the AFM data since the separation distances cannot be unequivocally determined by this technique. Oscillatory structural forces are excellent for this work since they give multiple reference points against which to analyze. Comparison of the data from the two techniques highlighted significant uncertainties in the AFM data. At low polymer concentrations, a significant uncertainty in the apparent zero separation distance was seen as a result of the AFM cantilever reaching an apparent constant compliance region prior to any real contact between the surfaces. Further complications arising from the number and position of the measured minima were also seen in the dilute polymer concentration regime as a result of hydrodynamic drainage between the approaching surfaces in the AFM perturbing the delicate structural components in the fluid.

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PubMed2005

Measurement of dynamical forces between deformable drops using the atomic force microscope. I. Theory.

Recent experimental developments have enabled the measurement of dynamical forces between two moving liquid drops in solution using an atomic force microscope (AFM). The drop sizes, interfacial tension, and approach velocities used in the experiments are in a regime where surface forces, hydrodynamics, and drop deformation are all significant. A detailed theoretical model of the experimental setup which accounts for surface forces, hydrodynamic interactions, droplet deformation, and AFM cantilever deflection has been developed. In agreement with experimental observations, the calculated force curves show pseudo-constant compliance regions due to drop flattening, as well as attractive pull-off forces due mainly to hydrodynamic lubrication forces.

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PubMed2005

Interaction forces between oil-water particle interfaces--non-DLVO forces.

The interaction force between a rigid silica sphere and a butyl or octyl acetate droplet was measured in an aqueous environment using atomic force microscopy (AFM). The force measurements were performed without added stabilizers and the observed force behavior was found to be dependent on the type of inorganic electrolyte present, where the interfacial tension was constant over the electrolyte concentration range used. Force measurements in the presence of sodium nitrate showed repulsion at all concentrations. Force measurements in the presence of calcium nitrate or sodium perchlorate exhibited an initial repulsion followed by an attraction resulting in a mechanical instability in the AFM cantilever, termed jump-in. The force behavior observed was independent of the water solubility of the organic liquid, in that the same force-distance characteristics were obtained for slightly water soluble butyl acetate and the water sparingly soluble octyl acetate droplets. Modeling of the drop profile during particle-droplet interactions for this type of AFM measurement showed that the force-distance data for the sodium nitrate system obeys typical DLVO interactions. The disagreement between the DLVO predictions for the sodium perchlorate and calcium nitrate systems is attributed to a specific ion effect at the liquid-liquid interface, which gives rise to an attraction force that is greater than the electrostatic double layer repulsion over the length scale of 5 to 10 nm.

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PubMed2004

Dynamics of the interaction forces at the silver/solution interface during amine adsorption.

The forces of interaction between a silver-coated particle and a flat silver surface in an aqueous medium were measured in the presence of a series of organic amines of varying concentrations. Atomic force microscopy (AFM) was used to quantify the replacement rate of adsorbed citrate molecules on the silver surfaces by a variety of amines, under conditions where the time scale of the amine adsorption was significantly slower than the time scale of the AFM measurements. The decay length of the electrostatic double-layer interaction between the silver surfaces was found to be time independent; thus, the change in surface change density (determined from the interaction forces) was used to quantify the replacement rate of adsorbed citrate by amine. In the absence of amine, the interaction force between the silver surfaces exhibited evidence of a multilayer structure of adsorbed citrate molecules on each silver surface. Upon addition of the amine, a decrease in the interaction force was always observed, where the dynamics of the force were dependent on both concentration and the molecular structure of the amine. These results are discussed with respect to formation of colloidal particles in synthesis routes where particle aggregation has a significant impact on the control of particle morphology and size.

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PubMed2004

Forces between two oil drops in aqueous solution measured by AFM.

The surface and hydrodynamic forces between individual oil droplets in solution can provide insight into both emulsion stability and processes such as drop coalescence in liquid-liquid extraction. We present the first measurements of the interaction forces between alkane droplets in aqueous solution using atomic force microscopy. The radii of the droplets were well below the capillary lengths for the system, thus gravity effects are negligible, and interfacial tension and interaction forces governed the system behavior. The effects of modulating electrostatic double-layer interactions and interfacial tension through the presence of an anionic surfactant are demonstrated. Challenges in interpretation of the force data due to drop deformation are also discussed. A range of drop approach and retract speeds was used to determine the regime where hydrodynamic drainage effects had significant impact on the measurement.

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PubMed2004

Forces between a rigid probe particle and a liquid interface. III. Extraction of the planar half-space interaction energy E(D).

The deformation of a liquid drop (radius R0) under the probe particle (radius a) greatly complicates the interpretation by atomic force microscopy. For rigid interfaces, F(DeltaX) can be directly related to the interaction energy E(D) per unit area between planar half-spaces of probe material and drop material across a thickness D of the liquid medium by the Derjaguin approximation, [formula in text], where D(0) is the intersurface separation distance on the line of the centers of the bodies and DeltaX0 is a constant set by the somewhat arbitrary choice of origin for the separation distance DeltaX between the stage on which the drop rests and the lowest point on the probe particle. The problem of absolute intersurface separation distance is common to all surface force measurement techniques. For rigid interfaces, DeltaX0 may be established by bringing the surfaces into close (essentially hard) contact and making measurements in the constant compliance regime. For deformable interfaces, this is not possible and a general method of extracting the absolute separation distance has yet to be devised. In this paper we discuss a general algorithm for extracting E(D+DeltaX0) from F(DeltaX) data. We apply the method to constructed data to investigate the effect of data noise and to a set of real data for a sessile tetradecane droplet in water with an anionic surfactant and a bare silica probe.

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PubMed2002

Calculations of van der Waals forces in 2-dimensionally anisotropic materials and its application to carbon black.

We present calculations of the van der Waals force for carbon black dispersions in both aqueous and nonaqueous media using Lifshitz theory. The microstructure and composition of carbon black are complex, but an initial approximation to the shell-like microstructure of carbon black allows the local interaction of carbon black particles to be approximated as oriented domains of graphite. The dielectric spectra for graphite, which has a 2-dimensional anisotropy due to its the layered microstructure, is required for the Lifshitz theory van der Waals force calculations. The anisotropic dielectric spectra of graphite (which behaves as a semiconductor) was constructed by modeling the conduction or free charge response separately from the polarization or bound charge response. The free charge response was modeled using the Drude model, while the dielectric spectra for the bound charge response was constructed from the spectroscopic data directly according to the Kramers-Kronig relation for the dielectric function, epsilon(omega). The expressions for calculating the fully retarded van der Waals force for half spaces with 2-dimensional dielectric anisotropy were derived as well as. The construction for the dielectric spectra of polystyrene from recent spectroscopic data from the literature according to the method outlined in (Dagastine, R. R., Prieve, D. C., and White, L. R., J. Colloid Interface Sci.231, 351 (2000)) is also presented.

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