PubMed Health⌕ Search

Biomedical subjects

J Mellema

Publications and source records attributed to J Mellema.

18 recordsLinked to original sources
PubMed2006

Plastic-to-elastic transition in aggregated emulsion networks, studied with atomic force microscopy-confocal scanning laser microscopy microrheology.

In this paper, we demonstrate how the simultaneous application of atomic force microscopy (AFM) and confocal scanning laser microscopy (CSLM) can be used to characterize the (local) rheological properties of soft condensed matter at micrometer length scales. Measurement of AFM force curves as a function of the indentation amplitude and speed (magnitude and direction) can produce a "mechanical fingerprint" that contains information about material stiffness, hysteretic losses, and time scales for stress relaxation and/or network recovery. The simultaneous CSLM visualization of changes in the material's structure provides complementary information about how the material accommodates the indentation load. Since these experiments are done on areas of O(100 microm2) on materials having a surface of O(1 cm2), the measurements can be repeated on "fresh" material many times, contrary to traditional rheometers where the whole sample is loaded at once. As a particular example, we consider the case of a network of aggregated water-in-oil (W/O) emulsion droplets, in which the mechanical behavior changes drastically over time. Whereas the freshly prepared material shows a soft plastic behavior, after a time lapse of several weeks, the very same sample shows a much stiffer and elastic response. This drastic change in behavior is clearly reflected both in the signature of the AFM force curves and in (the reversibility of) the structural deformations observed with CSLM. The fact that these drastic mechanical changes take place without significant changes in the structure of the material (before loading) indicates that the stiffening of the droplet network is caused by an increase in the strength of the bonds between droplets. A remarkable finding for the elastic droplet network is that, while the structure recovers completely after the indenter is taken out, there is still an appreciable hysteresis in the force curves, indicating that dissipation also occurs. This hysteresis was not found to depend on the indentation speed.

Journal Article↗
PubMed2006

Microrheology of aggregated emulsion droplet networks, studied with AFM-CSLM.

We studied the mechanical behavior of densely packed (up to approximately 30% v/v), sedimented layers of (1 microm) water-in-oil W/O emulsion droplets, upon indentation with a (10 microm) large spherical probe. In the presence of attractive forces, the droplets form solid like networks which can resist deformation. Adding a polymer to the oil phase was used to control droplet attraction. The droplet layers were assembled via normal gravity settling. Considering that both the network structure and the droplet interactions play a key role, we used a combination of atomic force microscopy (AFM) and confocal scanning laser microscopy (CSLM) to characterize the mechanical behavior. Here the AFM was used both as indentation tool and as force sensor. Indentation experiments were performed via a protocol consisting of approach, waiting, and retract stages. CSLM was used to observe the network structure at micron resolution in real time. Use of refractive index matched fluorescent droplets allowed the visualization of the entire layer. Upon compression with the probe, a markedly nonhomogeneous deformation occurred, evidenced by the formation of a dense corona (containing practically all of the displaced droplets) in the direct vicinity of the probe, as well as more subtle deformations of force-chains at larger distances. Upon decompression, both the imprint of the indenter and the corona remained, even long after the load was released. The force-distance curves recorded with the AFM correspond well to these observations. For each deformation cycle performed on fresh material, the retract curve was much steeper than the approach curve, thus corroborating the occurrence of irreversible compaction. Contrary to classic linear viscoelastic materials, this hysteresis did not show any dependence on the deformation speed. Our force-indentation approach curves were seen to scale roughly as F approximately delta(3/2). The pre-factor was found to increase with the polymer concentration and with the density of the network. These findings suggest that this new AFM-CSLM method could be used for rheological characterization of small volumes of "granular networks" in liquid. Our hypothesis that the mechanical resistance of the networks originates from interdroplet friction forces, which in turn are set by the interdroplet potential forces, is supported by the predictions from a new mechanical model in which the interdroplet bonds are represented by stick-slip elements.

Journal Article↗
PubMed2005

Coalescence in semiconcentrated emulsions in simple shear flow.

The coalescence frequency in emulsions containing droplets with a low viscosity (viscosity ratio approximately 0.005) in simple shear flow has been investigated experimentally at several volume fractions of the dispersed phase (2%-14%) and several values of the shear rate (0.1-10 s(-1)). The evolution of the size distribution was monitored to determine the average coalescence probability from the decay of the total number of droplets. Theoretically models for two-droplet coalescence are considered, where the probability is given by P(c)=exp(-tau(dr)tau(int)). Since the drainage time tau(dr) depends on the size of the two colliding droplets, and the collision time tau(int) depends on the initial orientation of the colliding droplets, the calculated coalescence probability was averaged over the initial orientation distribution and the experimental size distribution. This averaged probability was compared to the experimentally obtained coalescence frequency. The experimental results indicate that (1) to predict the average coalescence probability one has to take into account the full size distribution of the droplets; (2) the coalescence process is best described by the "partially mobile deformable interface" model or the "fully immobile deformable interface" model of Chesters [A. K. Chesters, Chem. Eng. Res. Des. 69, 259 (1991)]; and (3) independent of the models used it was concluded that the ratio tau(dr)tau(int) scales with the coalescence radius to a power (2+/-1) and with the rate of shear to a power (1.5+/-1). The critical coalescence radius R(o), above which hardly any coalescence occurs is about 10 microm.

Journal Article↗
PubMed2005

Influence of bulk elasticity and interfacial tension on the deformation of gelled water-in-oil emulsion droplets: an AFM study.

We used atomic force microscopy (AFM) to study the deformation and wetting behavior of large (50-250 microm) emulsion droplets upon mechanical loading with a colloidal glass probe. Our droplets were obtained from water-in-oil emulsions. By adding gelatin to the water prior to emulsification, also droplets with a bulk elasticity were prepared. Systematic variations of surfactant and gelatin concentrations were made, to investigate their effect on the deformation and wetting behavior of the droplets and to identify the contributions of interfacial tension, bulk elasticity, and expelled water. The AFM experiments were performed in force--distance mode and showed on approach a repulsive regime which in many cases was terminated by a jump-in of the probe. In the case of pure water (i.e. gelatin-free) droplets, the repulsive part of the curve showed a good linearity, thus allowing the extraction of an effective droplet spring constant. This quantity was found to decrease on raising the surfactant concentration from below the critical micelle concentration (cmc) to well above the cmc, and its numerical values were found to correspond remarkably well to literature values for the interfacial tension. Our findings indicate that, on gelatin increase inside the droplets, the bulk elasticity gradually becomes dominant and the droplets' stiffness does not depend anymore on surfactant concentration. Also the stability of the droplet interface against wetting, as measured by the force at which the jump-in instability occurs, was enhanced by gelatin. For gelatin concentrations of > or =15 wt %, the droplets were found to behave like purely elastic bodies. Both gelatin and surfactant contribute positively to the stability against interface breakup.

Journal Article↗
PubMed2004

Flow electrification in nonaqueous colloidal suspensions, studied with video microscopy.

Flow electrification in nonaqueous suspensions has been scarcely reported in the literature but can significantly affect colloidal stability and (phase) behavior, perhaps even without being recognized. We have observed it in shear flow experiments on concentrated binary suspensions of hydrophobized silica particles in chloroform. In this low-polarity solvent, electrical charges on the large-particles' surfaces manifest themselves via long-ranged forces, because hardly any screening can take place through counterions. By shearing the suspension for a prolonged time, we could demonstrate that the effective interactions between the large particles change from weakly attractive (due to the small particles) to strongly repulsive (due to acquired Coulomb interactions). One of the conditions required for flow electrification was the presence of a glass surface in the shear cell. A spectacular manifestation of the phenomenon was observed with confocal video microscopy. First, the formation of large-particle aggregates was seen, while subsequently (over a long shearing time) the aggregates disintegrated into small entities, mostly primary particles. The spatial distribution of these entities in the quiescent state after stopping the flow showed evidence for acquired long-range repulsion. The occurrence of flow electrification was further corroborated by control experiments, where no flow was imposed, antistatic agent was added, or the glass bottom was coated with a conducting (indium tin oxide, ITO) layer: here, the aggregates kept growing until they became very large. To further diagnose the phenomenon, we have also done experiments in which an external electric field was applied (via the ITO layer) to an aggregated suspension. When the lower electrode was given the lowest potential, the aggregates were found to move away from the bottom and disintegrate. The qualitative similarity hereof with the flow electrification experiment suggests that in the latter, the glass acquired negative charges. After prolonged application of an external electric field, we observed segregation into regions enriched in large particles and regions completely depleted of them. In the quiescent fluid these regions exist as isolated units, but in shear flow they merge into bands, a behavior which resembles shear banding.

Journal Article↗
PubMed2004

Hierarchical networks of casein proteins: an elasticity study based on atomic force microscopy.

2D- and 3D-atomic force microscopy (AFM) experiments were performed on single casein micelles (CM) in native state, submerged in liquid, using a home-built AFM instrument. The micelles were immobilized via carbodiimide chemistry to a self-assembled monolayer supported on gold-coated slides. Off-line data analysis allowed the extraction of both surface topography and elastic properties. Relative Young moduli (E*) were derived from force-vs-indentation curves, using the Hertz theory. The obtained E* values were found to increase with CM diameter, following a straight line dependence. The data showed that temperature, via its influence on both the protein-protein interactions and the composition of the micelle, has a clear effect on the mechanical properties of the CMs: higher temperatures and lower serum casein concentrations result in stiffer micelles. For pH < or = 5.6, effecting calcium phosphate release from the micelles by decreasing the pH does not have a large effect on CM stiffness. On decrease of the pH below 5.0, particulate gels and multilayers were obtained. Their measured elasticity (expressed by an equivalent G'AFM) agrees remarkably well with the storage moduli as measured with a conventional rheometer. Compared to single micelles, gels from nonheated CM suspensions are about 3 orders of magnitude softer. The "softness" of these gels (measured under compression or shear) therefore must come from the microscopic and/or mesoscopic links rather than the micelles themselves.

Caseins↗
PubMed2004

Aggregation and breakup of colloidal particle aggregates in shear flow, studied with video microscopy.

We used video microscopy to study the behavior of aggregating suspensions in shear flow. Suspensions consisted of 920 nm diameter silica spheres, dispersed in a methanol/bromoform solvent, to which poly(ethylene glycol) (M = 35.000 g) was added to effect weak particle aggregation. With our solvent mixture, the refractive index of the particles could be closely matched, to allow microscopic observations up to 80 microm deep into the suspension. Also the mass density is nearly equal to that of the particles, thus allowing long observation times without problems due to aggregate sedimentation. Particles were visualized via fluorescent molecules incorporated into their cores. Using a fast confocal scanning laser microscope made it possible to characterize the (flowing) aggregates via their contour-area distributions as observed in the focal plane. The aggregation process was monitored from the initial state (just after adding the polymer), until a steady state was reached. The particle volume fraction was chosen at 0.001, to obtain a characteristic aggregation time of a few hundred seconds. On variation of polymer concentration, cP (2.2-12.0 g/L), and shear rate, gamma (3-6/s), it was observed that the volume-averaged size, Dv, in the steady state became larger with polymer concentration and smaller with shear rate. This demonstrates that the aggregate size is set by a competition between cohesive forces caused by the polymer and rupture forces caused by the flow. Also aggregate size distributions were be measured (semiquantitatively). Together with a description for the internal aggregate structure they allowed a modeling of the complete aggregation curve, from t = 0 up to the steady state. A satisfactory description could be obtained by describing the aggregates as fractal objects, with Df = 2.0, as measured from CSLM images after stopping the flow. In this modeling, the fitted characteristic breakup time was found to increase with cP.

Journal Article↗
PubMed2001

Measuring shear-induced self-diffusion in a counterrotating geometry.

The novel correlation method to measure shear-induced self-diffusion in concentrated suspensions of noncolloidal hard spheres which we developed recently [J. Fluid Mech. 375, 297 (1998)] has been applied in a dedicated counterrotating geometry. The counterrotating nature of the setup enables experiments over a wider range of well-controlled dimensionless time (gamma;Deltat in the range 0.03-3.5, compared to 0.05-0.6 in previous experiments; here gamma; denotes the shear rate and Deltat the correlation time). The accessible range of timescales made it possible to study the nature of the particle motion in a more detailed way. The wide radius geometry provides a well-defined flow field and was designed such that there is optical access from different directions. As a result, shear-induced self-diffusion coefficients could be determined as a function of particle volume fraction straight phi (0.20-0.50) in both the vorticity and velocity gradient direction. A transition could be observed to occur for gamma;Deltat of O(1), above which the particle motion is diffusive. The corresponding self-diffusion coefficients do not increase monotonically with particle volume fraction, as has been suggested by numerical calculations and theoretical modeling of Brady and Morris [J. Fluid Mech. 348, 103 (1997)]. After an exponential growth up to straight phi=0.35, the diffusion coefficients level off. The experiments even suggest the existence of a maximum around straight phi=0.40. The results are in good agreement with experimental literature data of Phan and Leighton [J. Fluid Mech. (submitted)], although these measurements were performed for much larger values of the dimensionless time gamma;Deltat.

Journal Article↗
PubMed1999

Linear viscoelasticity of an inverse ferrofluid.

A magnetorheological fluid consisting of colloidal silica spheres suspended in an organic ferrofluid is described. Its linear viscoelastic behavior as a function of frequency, magnetic field strength, and silica volume fraction was investigated with a specially designed magnetorheometer. The storage modulus G' is at least an order of magnitude larger than the loss modulus G" at all magnetic field strengths investigated. G' does depend only weakly on frequency, and linearly on volume fraction. A model is presented for the high frequency limit of the storage modulus G'(infinity). In the model our system is treated as a collection of single noninteracting chains of particles. Assuming a dipolar magnetic interaction, theory and experiment show reasonable agreement at high frequencies.

Journal Article↗
PubMed1988

Rheological properties of human saliva.

From measurements with a Couette-type viscometer provided with a guard ring it was shown that at the saliva-air interface a protein layer is adsorbed. Measurements of the surface shear modulus of this layer on saliva of 7 healthy subjects were performed at a frequency of about 70 Hz and a temperature of 25 degrees C. For a surface age of about 1.5 h the surface shear modulus and the surface viscosity were in the order of 1 Nm-1 and 10(-3) Nm-1 s, respectively. From ellipsometric measurements it was found that the thickness of the protein layer was approx. 100 nm and, using this value, it could be concluded that the shear modulus and the dynamic viscosity were in the order of 10(7) Pa and 10(4) Pa s, respectively. The layer appeared to be fragile. Even shear deformation amplitudes of 4 x 10(-5) are too high to assure linearity. The complex viscosity (eta = eta' - i eta") of the bulk liquid of human submandibular saliva below the absorbed layer was measured in the frequency range 70 Hz-200 kHz with 3 torsional resonators, each for a different frequency, and a Ni-tube resonator. It was concluded, that the real part of the complex viscosity (eta') decreases from 1.1 mPa s at 70 Hz to a value of 0.95 mPa s at high frequencies. Except at the lowest frequency (70 Hz), the value of eta" was too small to be detected.

Adult↗
PubMedDate not supplied

Rheological aspects of mucin-containing solutions and saliva substitutes.

In this study rheological properties of aqueous solutions of mucin, albumin and mucin-albumin have been investigated in search for saliva substitutes. They were compared with commercially available saliva substitutes on the one hand and natural human saliva on the other hand. For the latter a few measurements on saliva are reported here in addition to previously reported measurements done in our laboratory. Proteins absorb at the interface and saliva proteins do so strongly and rapidly. Therefore rheological measurements were carried out on the interface and on the bulk underneath the layer. In both cases the flow curve and the complex viscosity was determined. The results show that specific mucin-albumin solutions were rheologically similar to human whole saliva with respect to both bulk liquid and surface properties. The rheological properties of commercial saliva substitutes were essentially different from those of human saliva. It is concluded that mucin-albumin solutions have good perspectives as saliva substitutes.

Albumins↗