Suppression of the Coffee-Ring Effect for OMVs Distribution of TEM Sampling via GA-Induced Marangoni Flow and...

The aggregation of suspended particles caused by the coffee-ring effect (CRE) disrupts uniform distribution and limits accurate transmission electron microscopy (TEM) observation. Here, we show that glutaraldehyde (GA) suppresses CRE in aqueous polystyrene microsphere droplets and enables more uniform outer membrane vesicles (OMVs) deposition for TEM. Specifically, the addition of GA (0%-15% v/v) to 1% w/v, 1 µm red polystyrene microsphere suspensions progressively reduced ring formation: The ratio of maximum-to-minimum gray values decreased from 2.40 ± 0.22 (0% GA) to 1.11 ± 0.06 (15% GA), and mean CRE width dropped from ∼97 to ∼3.2 µm. Microsphere radial velocity decreased from 4.25 ± 0.37 (0% GA) to 0.89 ± 0.08 µm/s (15% GA), whereas bulk solution viscosity increased concomitantly (from ∼4 to ∼232 mPa s at 15% GA), indicating viscosity-mediated suppression of radial capillary flow. Addition of sodium dodecyl sulfate (SDS) partly restored CRE, consistent with reduction of surface-tension gradients that drive Marangoni reflux. Applying GA (6% v/v) to OMVs TEM preparation produced homogeneous OMVs counts across center, middle, and edge regions, without observable morphological alteration. We conclude that GA suppresses CRE primarily by (i) increasing viscosity to inhibit radial capillary flow and (ii) establishing surface-tension-driven Marangoni backflow that redistributes particles away from the contact line. GA thus provides a practical, fixation-compatible approach to improve TEM sample uniformity for biological nanoassemblies.