Decision Flow:

1. Finding Focus
   • Rivers and Inland water bodies? Or Oceans?
     • First points of discharge of particulate pollutants into the water sphere
     • Direct entry into human food chain
     • Easier monitoring of effect on biosphere
     • Greater level of human coexistence with water - length of river banks >>> coastal shoreline; number of major cities on rivers > num of coastal cities; more concerned citizens and scientists
     • Hence rivers
   • Microplastic type
     • Focus on microfibers of textile origin? Or those in the size range of fish food?
     • Would need to narrow focus if an optical method is used (Field of view, magnification, resolution)
     • Can work with a larger range of sizes and plastic types (polymers) if a method like impedance spectroscopy is used
     • The size, type of microps - dictated by the method of sensing and the sensitivity of the instrument that we build (eg distance between electrodes limits particle size: larger particles need to be filtered out before the water stream enters the sampler)
     • Sample preparation: (guided by Prof @Walter Waldman)
       • Straightforward cutting of oxo degradable plastics with a scissor or knife (starting with this)
       • Sandpaper on brittle (sunlight aged) plastics, the sand particles separated by sedimentation later
       • Magnetic stirrer on plastic fragments + sand in a beaker - separated by sedimentation; measure- mass of plastic fragments added initially
       • Polypropylene can be thermally fatigued by refrigerating followed by boiling (Glass transition is close to 0 deg C)
2. Method of sensing - optical or impedance?
   • Fluorescence spectroscopy seems promising-
     • possible to characterize by polymer type
     • Doubles as a particle counter- a pass by instrument that would report the number and type of microplastics sampled per time unit (converted to per volume unit from the flow rate)
     • Evaluate the possibility of using the planktonscope
     • stuff to work out:
       • time required for a particle to be in the field of view to get a proper spectral signal
       • way to keep particles on the focal (flow) plane
3. Impedance Spectroscopy

   • Count and characterize each particle in a stream or make an estimate for a confined/ collected sample?

   • Impedance values in bio impedance measurements are reported for the sample - not its constituents. (But the relaxation times are unique to suspended particles? @Prasoon Awasthi)

   • Impedance due electrode contacts is regulated by using a standard PDMS "well" for samples to go in. Is it okay if the electrodes touch the water in the sample? Is it possible to have concentric cylinders as electrodes?

   • Sampler Geometry and Electrode configuration

     • Parallel plates - confinement of sample between parallel plates

     Parallel plates - proposed design

     Measurement Module as a Parallel Plate Capacitor

     • Co planar electrodes and cylindrical well for sample placement
     • Co axial probe - sample in the annulus

   • Oscilloscope

     • A soundcard oscilloscope was made
       • Frequency limited to 24 kHz

     Soundcard Oscilloscope

   • Is it enough to make measurements at a single frequency

     • with different waveforms?
       • No, the equivalent analog form will be a Fourier sum of several sinusoidal waves- this makes it too complicated to be understood meaningfully
     • with different electrode configurations?
       • Requires temporal averaging since the water sample is not static
     • with different levels of physical confinement?
       • Imagining this as a change in the aspect ratio of the electrode system, it works theoretically. But the measurement has to be done twice and the electrode system cannot be rigid.

   • [Fun project: Is there a song that detects microplastics well? (:D)

   • Network Analyser as a low cost alternative for impedance measurements:

     • What is the circuit?
4. Reporting microplastic concentration
   • We are working on a "microplastic index" similar to AQI and BOD which is relevant in its meaning and can potentially be used by policy makers

Can we create an index similar to AQI to inform the public and make policies on microplastics?