Investigation of transport phenomena inside a microcapsule

Microcapsules are used in many chemical or biochemical processes. They consist of semi-permeable membranes, which enclose absorbing solutions or vital biomaterials. In general the capsules have a spherical shape with a diameter of about 3 mm. The microcapsules are surrounded by a fluid, which contains the nutrients and other components required for the process. A convective flow of this fluid transport nutriments and metabolic products to and from the outer membrane. These substances pass the membrane of the microcapsule by diffusion. The transport within the microcapsule also depends on diffusion. However, the diffusion is a slow process, which can render the whole process uneconomic.

With a new approach the transport process within the microcapsule is accelerated by introduction of a convection. This convection conveys substances from the membrane into the center of the microcapsule and vice versa. This is achieved by a movable body inside the microcapsule. It has a different density as the fluid within the microcapsule. It can be moved relatively to the microcapsule by application of an external force through a magnet field or through an acceleration/deceleration. Two methods were applied to investigate the flow field generated by a motion of the body and the effect of transport improvement by this flow field. One method is the Computational Fluid Dynamics (CFD) and the other the Particle Image Velocimetry (PIV).

First, a CFD method was used to investigate the effect of transport improvement inside the capsule by convection. The CFD flow program packet FLUENT was used, which creates a structured or unstructured computational mesh and is well suited for incompressible flow and transport problems. A simplified model was applied: a spherical body was placed in the capsule, which has also a spherical shape. Only transversal motion of the body along the capsule axes was considered. Such simplification results in the solution of two-dimensional axial-symmetrical problem. Three numerical models with different diameters of the body were generated with geometrical pre-processor Pre-BFC. Afterwards, a steady convective flow field inside the microcapsule was generated and a non-stationary transport process inside the capsule was calculated. A constant concentration of the diffusing substance outside the capsule was defined as a boundary condition. During this non-stationary simulation the position of the body was kept constant. Péclet number Pe = L⋅V / D_f is a non-dimensional parameter, which defines a relation between convection and diffusion. L is the diffusion path, V is the averaged velocity magnitude of the flow field and D_f is the diffusion constant of the transported material. Two parameters of the Pe number were varied in our investigation: The diffusion path by use of different body diameters and different values of the averaged velocity in the capsule. The measurement of the transport improvement effect was made by the comparison of the times, which are needed to achieve an 80 % saturation in the capsule. Further the relation between velocity magnitude of the body motion and the average velocity in the microcapsule was investigated. This was done experimentally. An enlarged microcapsule model was build (see figure 1). The flow field in the capsule was measured with the PIV method. The flow was assessed with a CCD camera (1024 × 1024 Pixel). The light sheet was generated with an Argon laser Exel 3000 (2 W). A cross-correlation method implemented in the Visiflow software was used to analyze the velocity field.

Figure 1: Experimental setup

Figure 1: Schematic drawing of the experimental set up. The rotational movement of the motor is converted in the linear motion of the body in the fixed capsule. The capsule made of transparent polysterene is filled with water containing particles Vestosint® 7182 and placed in the transparent water bath.

The results of CFD simulation show that a transport process can be improved only due to introduction of the movable body. This is a result of a diffusion path length reduction. The convection in the microcapsule results in further acceleration of the transport. Figure 2 shows the flow induced by the microcapsula measured by DPIV. Figure 3 shows some examples of the generated flow field.

Figure 2: Concentration distributions

Figure 2: Concentration distributions in the capsule model with convective flow field (left) and with only diffusive transport (right) after the same period.

Figure 3: Velocity field

Figure 3: Velocity field generated by the motion of the spherical body in the capsule.

The results of the investigation show that a significant improvement of the material transport in the microcapsule can be achieved by the generation of the convective flow field inside them. The effect was investigated as a function of Péclet number.