Researchers from the Indian Institute of Science (IISc), Bengaluru, have found that a cheap microscope connected to a smartphone camera could find wider application in a variety of research areas, and in some cases potentially replace more expensive equipment.
The Foldscope is a handheld microscope made mostly of paper that can be easily linked to a smartphone camera. It has a magnification of around 140x and can identify objects just 2 micrometres wide. It was created by researchers at Stanford University in 2014. Today, a Foldscope costs around Rs 400.
The researchers found that Foldscopes could capture the roundness and aspect ratio of an object to within 5% of those imaged by a state-of-the-art instrument called a scanning electron microscope (SEM), which costs more than Rs 50 lakh each.
They also report that based on their findings, Foldscopes can be used in pharmaceuticals (to inspect drug products), environmental science (to observe pollutants), and cosmetics (to observe powders and emulsions), among other fields.
P. Anbazhagan, associate professor in IISc’s Department of Civil Engineering and the paper’s corresponding author, also said in an email that Foldscopes can be used to study “soil particles’ morphology,” which can “help understand soil structure, nutrient availability, and plant growth” in agriculture.
Soil grains
The idea is the product of a study published in the journal Current Science on February 25, in which Dr. Anbazhagan and his former PhD student Kunjari Mog reported assessing the usefulness of a Foldscope to identify the shape of soil grains.
Currently, many soil classification schemes around the world don’t include soil grain shape “because of the complexities of accurately measuring grain shape and the limited availability of affordable image capturing instruments,” Dr. Anbazhagan said.
The Indian Standard Soil Classification System classifies soil as ‘coarse’, ‘fine’, and ‘peat/organic’ based on size, consistency, and susceptibility to deformation.
Shape is hard to determine because it requires bulky microscopes with high resolution. Yet it matters: Anindya Sarkar, professor and former head, Department of Geology and Geophysics, IIT Kharagpur, said in an email that it influences how the grains pack together. More gap between grains means the soil overall can hold more water than if there is less gap.
Also, “soils with angular or irregularly shaped grains tend to have higher interparticle friction, which makes them more resistant to deformation than soils with well-rounded or spherical grains,” Dr. Anbazhagan said.
Classification schemes work around the inability to quickly and affordably assess grain shape using other measures. In their new study, Dr. Anbazhagan and Dr. Mog studied whether Foldscope could bridge this gap.
Four samples, three attributes
They collected natural sand from four places in 2018: the beds of the Manu and the Brahmaputra rivers, sand unearthed by an earthquake in Tripura in 2017, and sand from a metre underground in Kalpakkam, Tamil Nadu. All four were coarse-grained soil with particles 0.32-0.47 mm wide.
They used two instruments – a Foldscope attached to a 64-MP smartphone camera and a scanning electron microscope (SEM) – to measure three attributes of each sample: roundness (the extent to which a particle is spherical), aspect ratio (how wide it is compared to how tall it is), and circularity (the extent to which it is circular in two dimensions).
In each case, they captured an image with the instrument and analysed it using a software tool called ImageJ.
They found that the Foldscope-based and the SEM-based readings differed by around 5% for roundness and aspect ratio for all sands except the Tripura sand, whose measures differed by 9%.
However, the circularity readings differed by 50% from the SEM reading. In their paper, the researchers attribute this to resolution issues associated with the image analyser. They add that future studies could find a solution, including by improving existing software like ImageJ.
Otherwise, however, they found that the Foldscope could augment studies of soil grain shape, including towards the feature’s inclusion in the soil classification system.
Pros and cons
The researchers found that each Foldscope couldn’t be used to image more than “150-200 particles” at a time because its focusing mechanism would wear out. But this was balanced by the instrument’s small size and low cost.
The researchers also noted that preparing a sample for study through a Foldscope took less than an hour, whereas the same process for an SEM was “tedious and time-consuming”.
“Foldscope is not designed to be a substitute for an SEM, which is a highly specialised and expensive instrument,” Dr. Anbazhagan said. However, it “allows for in-field soil analysis” and visualisation of “soil structure and properties quickly and easily in the digital era”.
