Zero-Discharge Fluid-Dynamic Gauging for Studying the Swelling of Soft Solid Layers

Abstract

A bench-top device fluid dynamic gauging device to study the swelling or shrinking of soft solid layers immersed in a liquid environment in situ and in real time is demonstrated. A particular feature is that the volume of liquid is isolated, hence the name Zero net discharge Fluid Dynamic Gauging (ZFDG), which renders ZFDG suitable for aseptic operation. For the 1.78 mm nozzle diameter used here, calibration tests gave a resolution of ± 5 µm and uncertainty of ± 10 µm. Computational fluid dynamics simulations indicated that the shear stress imposed on a layer being gauged differed between the successive suction and ejection stages in ZFDG. The swelling of polyvinyl acetate (PVA) layers (about 1 mm dry thickness) and gelatin films (50-80 µm dry thickness) in aqueous solutions is reported as demonstration of a ZFDG application. There was good agreement with more cumbersome gravimetric methods. Gelatin swelled noticeably faster at high pH, above the pKa values of proline and hydroxyproline. Fitting the gelatin swelling data to a power law model indicated 'sub-Fickian' behaviour with a 'diffusion index' which increased with pH.

Aseptic Zero Discharge Fluid Dynamic Gauging for Measuring the Thickness of Soft Layers on Surfaces

“…ZFDG measurements are based around a convergent nozzle with throat diameter d t , through which liquid is withdrawn or ejected at a controlled rate by means of a syringe pump. The nozzle geometry and dimensions for this device are shown in Figure 1: its hydrodynamics in ejection and suction mode were studied experimentally and the results compared with computational fluid dynamics (CFD) simulations by Wang and Wilson [4].…”

“…The aseptic ZFDG test rig is similar to that used by Wang and Wilson [4] but features a smaller, (cylindrical) liquid reservoir, an x-y stage which allows the substrate and layer being tested to be translated for scanning, and a method for isolating the contents from the surroundings. Figure 2 is a schematic of the apparatus while Figure 3 shows photographs of the system and key components.…”

“…FDG does not require knowledge of physical and chemical properties of the solution and the layer (they can be opaque) except of the presence of a locally stiff surface. The forces exerted by the FDG flow can be estimated with confidence from computational fluid dynamics simulations (Chew et al [2,3]; Wang and Wilson [4]). The technique has been used to study the growth (fouling) and removal (cleaning) of a range of layer materials, and a scanning version of the device allowed different regions of a layer or substrate to be investigated (Gordon et al [5]).…”

“…In this mode, the flow rate is set by a syringe pump and the pressure drop phases, so that no liquid enters or leaves the system. The operating principles of ZFDG were demonstrated by Yang et al [7] and a system for studying layers on flat substrates was presented by Wang and Wilson [4]. Lemos et al [8] bespoke ZFDG device to measure the thickness of Pseudomonas fluorescens biofilms on cylinders with a ZFDG unit.…”

Fluid‐dynamic gauging for studying the initial swelling of soft solid layers

“…FDG devices have been used to monitor the growth of layers on initially clean substrates (e.g., 9,10 ). Where a layer is present at the start of a test, FDG measurements to date have been subject to an initial delay between the times when the sample is first immersed to when the first reliable measurement is made, associated with zeroing the nozzle position and establishing the flow pattern.…”