Comparison of velocity profiles for different flow chamber designs used in studies of microbial adhesion to surfaces

Abstract

Flow chambers are commonly used to study microbial adhesion to surfaces under environmentally relevant hydrodynamic conditions. The parallel plate flow chamber (PPFC) is the most common design, and mass transport occurs through slow convective diffusion. In this study, we analyzed four different PPFCs to determine whether the expected hydrodynamic conditions, which control both mass transport and detachment forces, are actually achieved. Furthermore, the different PPFCs were critically evaluated based on the size of the area where the velocity profile was established and constant with a range of flow rates, indicating that valid observations could be made. Velocity profiles in the different chambers were calculated by using a numerical simulation model based on the finite element method and were found to coincide with the profiles measured by particle image velocimetry. Environmentally relevant shear rates between 0 and 10,000 s(-1) could be measured over a sizeable proportion of the substratum surface for only two of the four PPFCs. Two models appeared to be flawed in the design of their inlets and outlets and allowed development of a stable velocity profile only for shear rates up to 0.5 and 500 s(-1). For these PPFCs the inlet and outlet were curved, and the modeled shear rates deviated from the calculated shear rates by up to 75%. We concluded that PPFCs used for studies of microbial adhesion to surfaces should be designed so that their inlets and outlets are in line with the flow channel. Alternatively, the channel length should be increased to allow a greater length for the establishment of the desired hydrodynamic conditions.

FIG. 1.

Dimensions of the flow chambers (l₀, w₀, and h₀) and distances in the different directions (l, w, and h). The arrow indicates the direction of the fluid flow.

FIG. 2.

Modeled velocity profiles in longitudinal cross section from the inlet to the center of each PPFC at different volumetric flow rates (2.333, 0.710, 0.020, and 0.333 ml s⁻¹ for flow chambers A, B, C, and D, respectively).

FIG. 3.

Flow velocities at a volumetric flow rate of 0.003 ml s⁻¹ in different directions, as determined from a numerical simulation for PPFC C as a function of the dimensionless length at the center of the flow chamber (0.5h₀ and 0.5w₀) (A), as a function of the dimensionless width at the center (0.5l₀ and 0.5h₀) (B), and as a function of the dimensionless height at the center (0.5l₀ and 0.5w₀) (C). Symbols: •, velocities in the direction of the channel (l); ▴, sideward (w) velocities; ▪, velocities in the direction of the height (h).

FIG. 4.

Dimensionless surface areas (A/A₀) for which valid observations can be made as a function of the volumetric flow rate. Lines are drawn for clarity only and do not indicate any mathematical dependence. The open symbols indicate the flow rate for which an absolute surface area for valid observations of 1 cm² remains. A, B, C, and D refer to the flow chambers (see Table 1).

FIG. 5.

Modeled shear rates as a function of the volumetric flow rate for flow chambers A (•), B (▾), C (▪), and D (♦) up to the flow rate at which no uniform flow develops in the chambers. The lines indicate the shear rates calculated by using equation 4.

FIG. 6.

Fluid velocities as a function of the dimensionless height (h/h₀) for flow chambers A (○ and •) and B (□ and ▪) at a volumetric flow rate of 0.050 ml s⁻¹, as measured by particle image velocimetry (open symbols) and as calculated with the simulation model (solid symbols). Fluid velocities are valid for the center of each flow chamber (i.e., the middle between the inlet and the outlet).

FIG. 7.

Inlet velocity profiles for PPFCs A and D for flow rates yielding uniform flow (left panels) and nonuniform flow (right panels) in each flow chamber. For the left panels low flow rates of 0.0013 ml s⁻¹ (PPFC A) and 0.003 ml s⁻¹(PPFC D) corresponded to shear rates of 6.6 and 2.1 s⁻¹, respectively. For the right panels high flow rates of 1.25 ml s⁻¹ (PPFC A) and 0.33 ml s⁻¹ (PPFC D) corresponded to shear rates of 659 and 219 s⁻¹, respectively.