Client Citation Analysis
What the Dropometer did and did not do
The Dropometer captured 3 µL deionized water drops on untreated paper, on the closed gelatin valve and on the valve after it had been heated open. The angles were analysed in ImageJ with the Drop Analysis plugin. Pore structure (capillary flow porometer), morphology (SEM), valve timing and the malathion assay (desktop scanner and ImageJ) came from other methods.
Primary surface measurement reported
Static water contact angle on Whatman grade 1 paper. Untreated paper absorbed the drop with no measurable static angle, the closed valve read 99 ± 6° and the opened valve read 39 ± 1° (Fig. 5A to C).
Research question
Can gelatin, deposited straight onto a paper channel with no chemical modification, act as a low cost valve that stays closed through an assay step and opens on mild heating?
Headline finding
A 30 mg/mL gelatin valve gelled at 4 °C for 15 min stayed closed for the 35 min observation period and opened in about 20 s at 40 °C. Built into a single shot malathion test, it gave a calculated detection limit of 0.24 µM, below the Health Canada guideline of about 0.88 µM.
This study sits where paper microfluidics, temperature responsive materials and low cost water testing meet, with wettability as one of the checks on how the valve works.
Primary surface / interfacial measurement
Static water contact angle with 3 µL deionized water drops on untreated Whatman grade 1 paper, on the closed 30 mg/mL gelatin valve region and on the valve region after heating and fluid passage (Fig. 5A to C).
Supporting measurements
A capillary flow porometer measured accessible through pores in untreated and gelatin treated paper, and SEM imaged the fibre network. Dyed water tracked valve retention and opening time on a hot plate. For the assay, a desktop scanner and ImageJ measured blue colour intensity across malathion standards and spiked river water.
Dropometer (Droplet Lab Inc., Canada)
Water contact angle drop images on untreated paper, the closed valve and the opened valve
ImageJ, Drop Analysis plugin
Contact angle analysis of the drop images
Anton Paar Quantachrome 3Gzh capillary flow porometer
Accessible through pore distribution
Scanning electron microscope
Fibre network before and after gelatin treatment and heating
CO2 laser cutter
Channel patterning in the Parafilm heating and laser cutting method
Hot plate
Valve actuation at 37 to 60 °C
Canon Pixma TS5020 scanner with ImageJ 1.54p
Grayscale intensity of the assay detection zone
The paper states that water contact angles were measured with a Dropometer instrument from Droplet Lab Inc., Canada, using a 3 µL drop of deionized water. The drop images were analysed in ImageJ with the Drop Analysis plugin, not in the Dropometer app.
The Dropometer supplied the three wettability readings in Figure 5A to C. Untreated paper absorbed the drop too fast for a static angle, the closed valve read 99 ± 6°, and the valve read 39 ± 1° after it had been heated open and liquid had passed.
The authors read the high closed state angle as suppressing spontaneous wicking, and the lower angle after heating as consistent with restored wetting. Read together with porometry, they conclude that gelatin changes both wettability and the accessible pore pathways, without settling which effect dominates.
Valve timing, pore structure, morphology and all malathion results came from other methods listed on this page.
Contact angle gave the study a direct reading of how gelatin changed the paper surface in each valve state. Through the cos θ term in the Young Laplace relation the authors discuss, an angle above 90° on the closed valve corresponds to a capillary pressure that opposes wicking.
To see how a treatment changes a paper or other porous surface, measure water contact angle on each state with the Dropometer.
Gelatin turned absorbent paper water repellent
Untreated paper absorbed the 3 µL drop with no measurable static angle. The closed 30 mg/mL valve read 99 ± 6°, and after heating and liquid passage the region read 39 ± 1°: wettable enough to wick, but not back to untreated paper.
Concentration and cooling set the seal
Valves of 30 mg/mL and above stopped flow for the 35 min observation period, while 10 mg/mL valves failed within about 5 to 8 min. Only cooling at 4 °C for 15 min kept all four replicate valves closed; ambient cooling leaked within about 4 min.
Mild heat opens the valve
At 40 °C the chosen valve opened in about 20 s. Above 55 °C every tested concentration opened in under 20 s, while at 37 to 45 °C higher concentrations opened more slowly.
Storage affects sealing more than opening
Fresh and one week old valves stayed closed for 35 min. After one and two months, retention ranged from about 5 to 19 min depending on storage, but every stored valve still opened within about 1.5 min at 40 °C.
A single shot pesticide test
In a single shot enzyme (AChE) inhibition test for malathion, the valve held back the substrate during a 15 min incubation. The calculated detection limit was 0.24 µM, and spiked St. Lawrence River water gave recoveries of 74.9 to 86.1%. The authors present it as a proof of concept screening platform.
One figure in the paper contains Dropometer data: panels A to C of Figure 5 show water drops on untreated paper, the closed valve and the opened valve. The article is open access under a Creative Commons Attribution 4.0 licence.
Figure 5: Contact angle and porometry
Panels A to C show water drop images on untreated paper, the closed gelatin valve and the valve after heating, captured with the Dropometer and analysed in ImageJ. Panel D shows pore distributions from a capillary flow porometer. Credit: Mehrjou et al., RSC Adv. 2026, 16, 60239, CC BY 4.0.
Multistep paper assays need a reliable way to hold a reagent back and release it on cue. This study shows a valve made from off the shelf gelatin, and uses wettability as one of the checks on why it holds.
No synthesis or grafting
Gelatin was deposited straight into the paper channel, with no polymer synthesis, surface grafting or moving parts.
Wettability as a valve check
The closed valve read 99 ± 6° on paper that otherwise absorbed the drop, and 39 ± 1° once opened, which the authors link to stopping and then supporting capillary flow.
Fewer manual steps
A bypass channel let one 15 µL sample addition replace a second liquid addition, with the valve timing the substrate release.
Closed valve 99 ± 6°, opened valve 39 ± 1°
Opens in about 20 s at 40 °C
Calculated malathion detection limit of 0.24 µM
Authors call for humidity controlled storage tests and better packaging next
Measure every valve state
The study measured untreated paper, the closed valve and the opened valve, which shows both the barrier and how far wetting recovers.
Record when paper absorbs the drop
Untreated paper gave no static angle. Reporting that outcome is as useful as a number when comparing treated and untreated zones.
Expect some change to remain after opening
The opened valve read 39 ± 1°, not the near instant wetting of untreated paper, which the authors suggest may come from gelatin left on the fibres.
It captured images of 3 µL deionized water drops on untreated Whatman grade 1 paper, on the closed gelatin valve and on the valve after it was heated open. The angles were analysed in ImageJ with the Drop Analysis plugin. Pore structure, morphology, valve timing and the malathion assay came from other methods.
Untreated paper absorbed the drop and gave no measurable static angle. The closed 30 mg/mL valve read 99 ± 6°, and the valve read 39 ± 1° after heating and liquid passage (Figure 5A to C).
No. The authors combine the high closed state angle with porometry, which showed fewer accessible pore pathways, and SEM, which showed no complete filling of the fibre network. They conclude that wettability and pore pathways both change, but that their data do not show which one dominates.
The devices were made with the Parafilm heating and laser cutting method the lab reported in 2023, where Dropometer readings showed no measurable change in the Parafilm after the laser step. See our analysis of the Parafilm heating and laser cutting study.
Use the formal reference below when citing the original paper.
Formal Reference
Mehrjou, A., Safiabadi Tali, S. H., Ghiasi Tarzi, M., Sadiq, Z., & Jahanshahi-Anbuhi, S. (2026). Stopping, opening, and redirecting fluid flow in paper-based microfluidic devices using thermoresponsive gelatin valves. RSC Advances, 16, 60239–60249. https://doi.org/10.1039/d6ra07964d
Short Cite + DOI
Mehrjou et al., RSC Advances 2026, 16, 60239. DOI: 10.1039/d6ra07964d