Bart Hallmark

The fluid flow and heat transfer performance of thermoplastic microcapillary films

This paper is concerned with the evaluation of microcapillary films (MCFs) for microfluidic applications. MCFs are a novel type of low cost plastic film containing continuous arrays of microcapillaries that are extruded from thermoplastics where the capillaries within these films can be round or elliptical with diameters between 30 and 500 µm. The hydrodynamic response of MCFs has been investigated in a series of experiments where the flow within each capillary was laminar with Reynolds numbers up to a maximum of 1800. Pressure drop measurements were consistent with standard laminar flow predictions. A set of experiments involving single- and two-flow systems were conducted to characterize the heat transfer performance of MCFs and the efficacy of heat transfer was found to rank close to that of metallic microfluidic devices. The experimental heat transfer measurements were compared to finite-element model predictions for the MCF geometry and the modelling results were in good agreement with experiment. The overall results demonstrate the viable performance of MCFs for low cost application to examples such as flow within capillaries where temperature profiling is required along the length of the capillaries.

A portable extensional rheometer for measuring the viscoelasticity of pitcher plant and other sticky liquids in the field

BackgroundBiological fluids often have interesting and unusual physical properties to adapt them for their specific purpose. Laboratory-based rheometers can be used to characterise the viscoelastic properties of such fluids. This, however, can be challenging as samples often do not retain their natural properties in storage while conventional rheometers are fragile and expensive devices ill-suited for field measurements. We present a portable, low-cost extensional rheometer designed specifically to enable in situ studies of biological fluids in the field. The design of the device (named Seymour) is based on a conventional capillary break-up extensional rheometer (the Cambridge Trimaster). It works by rapidly stretching a small fluid sample between two metal pistons. A battery-operated solenoid switch triggers the pistons to move apart rapidly and a compact, robust and inexpensive, USB 3 high speed camera is used to record the thinning and break-up of the fluid filament that forms between the pistons. The complete setup runs independently of mains electricity supply and weighs approximately 1 kg. Post-processing and analysis of the recorded images to extract rheological parameters is performed using open source software.ResultsThe device was tested both in the laboratory and in the field, in Brunei Darussalam, using calibration fluids (silicone oil and carboxymethyl cellulose solutions) as well as Nepenthes pitcher plant trapping fluids as an example of a viscoelastic biological fluid. The fluid relaxation times ranged from 1 ms to over 1 s. The device gave comparable performance to the Cambridge Trimaster. Differences in fluid viscoelasticity between three species were quantified, as well as the change in viscoelasticity with storage time. This, together with marked differences between N. rafflesiana fluids taken from greenhouse and wild plants, confirms the need for a portable device.ConclusionsProof of concept of the portable rheometer was demonstrated. Quantitative measurements of pitcher plant fluid viscoelasticity were made in the natural habitat for the first time. The device opens up opportunities for studying a wide range of plant fluids and secretions, under varying experimental conditions, or with changing temperatures and weather conditions.

A portable and affordable extensional rheometer for field testing

Support for a summer internship for NP from ENSTA, a PhD studentship for MPB from Sandvik Hyperion and Ceratizit, and a PhD studentship for OMM from Chemours.

Using Hollow Microcapillaries to Explore the Extrusion Rheology of Polymer Films

It is possible to extrude a molten polymer film that contains a multitude of microcapillaries [1,2]; this has been termed a microcapillary film or MCF. The presence of the microcapillaries can act as passive markers in the MCF [3] and be used to identify the nature of post extrusion processing deformation; for example from the form of the deformed capillary shape it is possible to identify regions of uniaxial or biaxial deformation. In this paper we report experimental results and some matching modeling to show how external deformation and rheology can influence the final hole size and shape of the capillary array within the MCF. This paper also explores ways in which voidage and hole size can be controlled by manipulation of both processing parameters and polymer rheology. Typical capillary diameters that can be achieved range from 800 μm–5 μm with voidages ranging between 10% and 70%.The melt rheology of the polymer was studied to provide rheological parameters that could be used both to gain an underst...