Client Citation Analysis
The Droplet Lab pendant drop tensiometer measured the air and liquid surface tension of each suspension and surfactant mixture, with at least 13 drops per fluid. It did not measure rheology, conductivity, particle size or zeta potential. Viscosity came from a separate Fann 35A coaxial cylinder viscometer and conductivity from a Thermo Orion meter.
Surface tension in mN/m against surfactant concentration from 0 to 500 ppm in 50 ppm steps, for 1 wt% nanocrystal suspensions and 2 and 5 wt% fumed silica suspensions. Results are shown as plots; the only individual readings listed in the text are 13 repeats for one mixture.
Do low doses of an anionic (Stepanol, sodium lauryl sulfate based) or cationic (HTAB) surfactant change the flow behaviour of negatively charged nanocrystal and fumed silica suspensions, and can surface tension and conductivity locate where that change starts?
HTAB, which carries the opposite charge to both particles, raised the consistency index 20 to 70 fold at high doses. The surface tension break point shifted with the particle type and loading, and it lined up with the onset of the viscosity change and the conductivity break.
This paper uses pendant drop surface tension as a marker for where surfactant and particle interactions start to change suspension flow.
Air and liquid surface tension of each suspension and surfactant mixture, plus pure HTAB solutions, measured by pendant drop with a Young Laplace profile fit. No oil and water interfacial tension was measured. The authors read break points in the surface tension curves rather than reporting a cmc for each mixture.
Steady shear viscosity on a Fann coaxial cylinder viscometer, fitted with power law and Herschel Bulkley models. Electrical conductivity on a Thermo Orion meter. Hydrodynamic size by dynamic light scattering on a Malvern Zetasizer. These carry the rheology conclusions; the tensiometer did not.
Surface tension of all suspensions, surfactant mixtures and pure HTAB solutions, at least 13 drops per fluid.
Steady shear viscosity and shear stress from about 1.53 to 1021.38 s^-1 at about 22 °C; source of all power law and yield stress values.
Electrical conductivity of each mixture; used to find conductivity break points.
Hydrodynamic size of dilute nanocrystal and fumed silica suspensions by dynamic light scattering at 25 °C.
Dispensed the pendant drop through a 1.8 mm stainless steel needle using a screw driven plunger.
The surface tension data in this paper came from a Droplet Lab smartphone based pendant drop tensiometer, the instrument we sell as the Dropometer. The paper does not use the Dropometer name.
For every suspension and surfactant level, a drop hung from a 1.8 mm steel needle, was photographed with the phone camera, and fitted to the Young Laplace equation. Each fluid was measured at least 13 times and averaged.
Those curves gave the break points the authors used: about 300 ppm HTAB in 1 wt% nanocrystals, 229 ppm in 2 wt% fumed silica and 352 ppm in 5 wt% fumed silica. Stepanol curves fell steadily with no break.
The tensiometer did not measure viscosity, yield stress or conductivity. Rheology was measured on a separate Fann coaxial cylinder viscometer, and the 20 to 70 fold thickening comes from that instrument.
Surface tension was one of two independent checks, with conductivity, that the sharp viscosity change began near the same HTAB dose. It supports the interpretation but does not by itself prove the network mechanism, which the authors call speculative.
If you need to find where a surfactant starts to change your suspension, a pendant drop surface tension series is a quick place to start. See how the Dropometer handles this measurement.
With HTAB, surface tension breaks appeared near 300 ppm in 1 wt% nanocrystals, 229 ppm in 2 wt% fumed silica and 352 ppm in 5 wt% fumed silica, all measured on the Droplet Lab tensiometer.
Stepanol lowered surface tension steadily in every suspension with no break point, and conductivity rose linearly. Viscosity barely changed, consistent with like charges repelling.
Surface tension and conductivity of pure HTAB solutions gave a cmc of 300 ppm, against a literature value of 0.91 mM, about 332 ppm.
Thirteen drops of 2 wt% fumed silica with 400 ppm Stepanol read 50.26 to 54.82 mN/m, mean 50.97 mN/m, standard deviation 1.18 mN/m.
On the separate viscometer, HTAB raised the nanocrystal consistency index from 96.3 to 1790.8 mPa·s^n at 450 ppm, with a fitted yield stress of about 2 Pa at 500 ppm.
Four figures plot surface tension measured with the Droplet Lab tensiometer, and Figure 4 shows the instrument itself. The paper is open access under CC BY; credit Pal, Richards and Pal, Nanomaterials 2026.
Panel (a) shows surface tension of pure HTAB solutions with a cmc of 300 ppm, Droplet Lab data; panel (b) is conductivity from a separate meter. CC BY, credit Pal, Richards and Pal, Nanomaterials 2026.
Surface tension of 1 wt% nanocrystals with Stepanol (no break) and HTAB (break near 300 ppm), Droplet Lab data. CC BY, credit Pal, Richards and Pal, Nanomaterials 2026.
Surface tension of 2 wt% (Figure 32, HTAB break near 229 ppm) and 5 wt% (Figure 34, break near 352 ppm) fumed silica mixtures, Droplet Lab data. CC BY, credit Pal, Richards and Pal, Nanomaterials 2026.
Formulators thicken liquids with nanocrystals or fumed silica and add surfactants for stability and surface activity. This study shows a surface tension curve can flag the dose where a cationic surfactant starts to change flow.
A break in surface tension marked the same dose range where viscosity jumped on the viscometer, so a quick pendant drop series can narrow down where to run full rheology.
The HTAB break moved from about 229 ppm to 352 ppm as fumed silica rose from 2 to 5 wt%, so a cmc measured in water alone may not predict the break in a filled formulation.
The like charged anionic surfactant left both surface tension trends and viscosity without a break across the same 0 to 500 ppm range.
Same Waterloo rheology group as other Droplet Lab citations
Surface tension used alongside conductivity, not alone
Low surfactant range, 0 to 500 ppm
Open access, CC BY
Run the surface tension series in the filled suspension, not just the surfactant in water, because particles adsorb surfactant and shift the break.
Steps of 50 ppm were fine enough to place breaks within a few tens of ppm of the conductivity breaks.
The authors averaged at least 13 drops; their example set shows one high reading, which is why repeats matter.
No. All viscosity, power law and yield stress values came from a Fann 35A coaxial cylinder viscometer. The Droplet Lab tensiometer measured only surface tension by pendant drop.
It reports a cmc of 300 ppm for pure HTAB in water from surface tension and conductivity. For the suspensions it reports break points rather than a cmc. No oil and water interfacial tension was measured.
Only one set is tabulated: 13 readings for 2 wt% fumed silica with 400 ppm Stepanol, averaging 50.97 mN/m. All other surface tension results appear as plots against surfactant concentration.
The authors link it to weak interaction, since Stepanol and both particles are negatively charged. Conductivity and viscosity also showed no break with Stepanol.
Use the formal reference below when citing the original paper.
Pal, R., Richards, J., & Pal, A. (2026). Rheology of cellulose nanocrystal and fumed silica suspensions: Influence of ionic surfactants. Nanomaterials, 16(11), 676. https://doi.org/10.3390/nano16110676
Pal et al., Nanomaterials 2026, 16, 676. DOI: 10.3390/nano16110676