Dinoflagellates are marine planktons that fluoresce blue light when subject to strain. They can be observed during the night time along beaches when waves break against the shore (Figure 1). In the past, these organisms have allowed scientists to track ships in the ocean from space and to develop weather forecasting models. We would like to use their strain-responsive fluorescence to measure pressure fields in fluid flows (e.g. a flow around an impeller). We have assembled a team from a mixed fluid dynamics, chemical and biological background to do so. Our aim is to develop a cost-effective: (i) incubator to grow the organisms and (ii) an underwater multi-camera system to acquire tomographic videos of the fluoresced light for three-dimensional pressure field reconstructions.
Measuring pressure fields
Pressure is a fundamental property of a fluid flow. Parcels of fluid exert a force on their surroundings and the distribution of these forces are interlinked with the movement of the fluid.
Traditionally, pressure is measured at single points within a flow using devices such as Pitot tubes or pressure transducers. While these measurements can achieve a high degree of accuracy and temporal frequency, they are often intrusive to the flow and cannot be used to understand how pressure is spatially distributed in the flow field.
Current three-dimensional techniques for measuring pressure are indirect in that they are calculated from the measurement of the velocity of seeding tracers in the flow field (known as tomographic PIV reconstructions). Getting pressure fields in such a way requires a high degree of accuracy for the velocity measurements which is often very hard to achieve. Furthermore, these techniques typically rely on the use of expensive (and dangerous-to-operate) high-power pulsating lasers.
Using strain-responsive plankton for measuring pressure would provide an alternative to these measurement techniques which would circumvent many of these problems associated to them, with the added benefit of being much more cost-effective. These organism can also be used in combination with UV lights to simultaneously measure velocities and pressures. We plan to develop open-source easily-deployable systems to grow these amazing “bio-pressure-sensors” and a multi-camera system to image them three-dimensionally.
The plankton are particularly sensitive to temperature, salinity and oxygen levels and require a 12-hour cycle of light and dark. Off-the shelf incubators are not designed with our particular application in mind and we are therefore limited to a number of commercially available products that are rather expensive.
Tomographic camera systems
Tomographic camera systems share the same burden of an excessive cost; often prohibitive to most researchers. Their cost has been choking both their development and deployment in the field. In recent years, flourishing alongside the fields of computer vision and robotics, low-cost electronics are making the design of complex acquisition system much more accessible. We would like to contribute to the community by showcasing a design for an ultra-low cost 3D imaging system (which could be used in a broad range of disciplines).
Various species of Dinoflagellates are bioluminescent, and they emit blue light upon physical stimulation. The bioluminescence behaviour is related to the circadian cycle of these organisms, and it only occurs during the night cycle. The two most studies species are Pyrocystis lunula and Lingulodinium polyedra. While they share some of the general features in their bioluminescence, they vary in their mobility and biophysical response to a stress stimulus. As they can be cultured in very similar conditions we will be growing both species and assessing their suitability as pressure sensors in the flow visualisation device.
We are currently developing a few designs for the incubator (figures 2 and 3). We will control the temperature within the recommended range of 20-22 °C in two ways. For heating, we will encase heating elements in a mat underneath the plankton-containing batches. Some dispersive elements will be used so that anything just adjacent to the mat does not exceed critical temperatures. For cooling, we will use a heat pump (a thermoelectric Peltier element) to cool the air above and around the batches. Tropical aquarium lights will be used for illumination, controlled by a simple timer switch.
We will be using ArduCAM cameras that will be controlled using a Raspberry Pi 3B+ board and ArduCAM shield (figure 4). This is a cheap way of developing a four-camera system and will allow us to control the cameras simultaneously. It also should allow us, in the long run, to integrate the camera-acquisition system into more complicated systems, in which the cameras can be moved in response to the flow measured.
We plan to achieve a system that could record at 1-5Megapixel resolutions (comparable to the 720-1080p High Definition) at frame rates of 20-30 fps.
We intend to open-source both the incubator hardware and control software design and the entirety of the multi-camera acquisition system. The project will lead to a number of benefits. It will massively increase the accessibility of a technique to measure a fundamental flow property (pressure). Both the incubator and the imaging system can be used in a wide range of different scientific applications.