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

Abstract

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.Electronic supplementary materialThe online version of this article (doi:10.1186/s13007-015-0059-5) contains supplementary material, which is available to authorized users.

Characterization of extensional rheological filament stretching with a dual‐mode Giesekus model

“…Sometimes it is not possible to test the extensional response of a delicate fluid in a laboratory: for example, if a biological fluid found in a plant in a remote geographic location degrades rapidly it may neither be possible nor economically viable to transport it to a laboratory quickly. To solve this problem, a portable extensional rheometer, named Seymour, was developed at the University of Cambridge in 2014; a photograph of this device is shown in Figure . The operation of Seymour differs from most extensional rheometers in that only one of the pistons move, and is actuated by a simple solenoid: this is shown schematically in Figure .…”

“…The surface tension of the PEO solution was set to be 63 mN/m, replicating the value measured by Tirtaatmadja et al. The surface tension of the pitcher fluid was assumed to be 72 mN/m; the surface tension of the particular pitchers tested as part of the field trial is unknown, and this represents a potentially large source of error in the rheological parameters reported in Table .…”

“…Gauri and Koelling reported that PEO solutions exhibit Giesekus behavior in extension, and that a single mode Giesekus model could predict the observed filament profile on a fiber‐spinning line . Previous work has also identified that a single mode Giesekus model can predict the filament thinning of some pitcher fluid samples. Figure shows the locus of points corresponding to the solution of Eq.…”

“…The similarity of the two parameter sets indicates that the solution algorithm is functioning robustly since the filament thinning curves for these fluids shown in Figure b are also similar. Anecdotally, this result is of interest since the pitcher plant literature suggests that pitcher fluid relaxation times decrease with time after the opening of a pitcher; however, previous studies have only been able to measure an overall relaxation time. More analysis of pitcher fluid data, along with determination of a representative value for surface tension, is required to examine whether or not this observation applies to both the short and long relaxation modes presented here and as to whether the initial regime of filament thinning correlates to a physiological requirement of the pitcher plant.…”

“…Previous work has demonstrated that a single mode Giesekus model can adequately describe the filament thinning behavior of some experimental systems, including the oft‐observed Newtonian‐like approach to filament breakup. Despite finite extensibility effects not being explicitly incorporated within the Giesekus model, the extensional viscosity is bounded due to the inclusion of quadratic stress terms.…”