Biomedical subjects

Jorrit Mellema

Publications and source records attributed to Jorrit Mellema.

3 recordsLinked to original sources

PubMed2006

Restructuring and break-up of two-dimensional aggregates in shear flow.

We consider single two-dimensional aggregates, containing glass particles, placed at a water/air interface. We have investigated the critical shear rate for break-up of aggregates with different sizes in a simple shear flow. All aggregates break-up nearly at the same shear rate (1.8 +/- 0.2 s(-)(1)) independent of their size. The evolution of the aggregate structure before break-up was also investigated. With increasing shear rate, the aggregates adopt a more circular shape, and the particles order in a more dense, hexagonal structure. A simple theoretical model was developed to explain the experimentally observed break-up. In the model, the aggregate is considered as a solid circular disk that will break near its diameter. The capillary and drag force on the two parts of the aggregate were calculated, and from this force balance, the critical shear rate was found. The model shows a weak size dependence of the critical shear rate for the considered aggregates. This is consistent with the experimental observations.

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PubMed2005

Capillary forces between spherical particles floating at a liquid-liquid interface.

We study the capillary forces acting on sub-millimeter particles (0.02-0.6 mm) trapped at a liquid-liquid interface due to gravity-induced interface deformations. An analytical procedure is developed to solve the linearized capillary (Young-Laplace) equation and calculate the forces for an arbitrary number of particles, allowing also for a background curvature of the interface. The full solution is expressed in a series of Bessel functions with coefficients determined by the contact angle at the particle surface. For sub-millimeter spherical particles, it is shown that the forces calculated using the lowest order term of the full solution (linear superposition approximation; LSA) are accurate to within a few percents. Consequently the many particle capillary force is simply the sum of the isolated pair interactions. To test these theoretical results, we use video microscopy to follow the motion of individual particles and pairs of interacting particles at a liquid-liquid interface with a slight macroscopic background curvature. Particle velocities are determined by the balance of capillary forces and viscous drag. The measured velocities (and thus the capillary forces) are well described by the LSA solution with a single fitting parameter.

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PubMed2001

Experimental Investigation of the Orthokinetic Coalescence Efficiency of Droplets in Simple Shear Flow.

The coalescence efficiency of two droplets in simple shear flow was experimentally investigated. The effects of the radius of the colliding droplets, the viscosity of the continuous medium, and the radii ratio on the coalescence efficiency were studied. The coalescence efficiency was determined from the critical angle, alpha(crit), above which the droplets coalesce and below which no coalescence takes place. It was found that the coalescence efficiency decreases and then increases with the flow number. The theories of the drainage models and the trajectory analysis were unable to predict the experimental results. This was attributed to the inadequacy of the existing expression used for the critical film thickness. On the basis of the experimental data obtained a new expression for the critical film thickness was proposed. The new expression predicts a decrease followed by an increase in the critical film thickness. The increase in the critical film thickness is, perhaps, due to instabilities that take place at the interfaces. Implementing the proposed expression for the critical film thickness, the experimentally measured coalescence efficiency, the capture angle and the contact time were successfully predicted. The experimental investigation revealed that the coalescence efficiency does not depend on the radii ratio in the range 0.7 to 1. The experimental investigation also showed that the coalescence efficiency increases as the viscosity of the continuous phase decreases. Measuring the capture angle showed that coalescence can take place either in the compression or in the extensional quadrant of the shear plane. A variation in the capture angle and the contact time was observed for a given fixed set of conditions. The change of the angle alpha with time was measured. The measured values were compared to those calculated from an equation derived by R. S. Allan and S. G. Mason (J. Colloid Interface Sci. 17, pp. 383, 1962). A good agreement was obtained when the droplets were of equal radii and small in size. When the droplets were large in size and of different radii, the agreement was not satisfactory. Measuring the distance between the centers of the colliding droplets showed that the two droplets vibrated when they came close to each other. Copyright 2001 Academic Press.

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