🔬 condensed matter

Instabilities in Drying Colloidal Films: Role of Surface Charge and Substrate Wettability

This study experimentally demonstrates that the interplay between silica nanoparticle surface charge and substrate wettability critically governs the evaporation dynamics, crack morphology, and delamination behavior of drying colloidal films, revealing distinct patterns for negatively versus positively charged particles across varying concentrations and surface types.

Original authors: A. Madhav Sai Kumar, A. Hari Govindha, Ranajit Mondal, Kirti Chandra Sahu

Published 2026-01-27

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: A. Madhav Sai Kumar, A. Hari Govindha, Ranajit Mondal, Kirti Chandra Sahu

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a drop of muddy water sitting on a table. As the water evaporates, the mud doesn't just dry into a flat, perfect circle. Instead, it often curls up, cracks like a dried riverbed, or peels off the table entirely. This paper is a deep dive into why that happens, specifically looking at two "hidden" forces: the electrical charge of the tiny particles in the mud and how "sticky" or "slippery" the table surface is.

Here is the story of their findings, broken down into simple concepts.

The Cast of Characters

The researchers used two types of tiny silica (sand-like) particles suspended in water:

The "Negatives" (Ludox TM50): These particles have a negative electrical charge. Think of them as magnets that repel other negatives.
The "Positives" (Ludox CL30): These particles have a positive charge. They are attracted to negatives.

They dropped these mixtures onto three different "tables" (substrates):

Glass: Very wettable (water spreads out easily). It has a negative charge.
Polystyrene: Somewhat wettable.
PTFE (Teflon): Very hydrophobic (water beads up). It is non-sticky.

The Main Event: How the Drop Dries

When a drop dries, the water at the edges evaporates faster than in the middle. This creates a current that pushes particles toward the edge, like a crowd rushing to the exit. This usually creates a "coffee ring" (a ring of dirt around the edge).

However, the researchers found that what happens next depends entirely on the chemistry of the particles and the table:

1. The "Traffic Jam" vs. The "Free-for-All"

On Glass with Negative Particles (TM50): Since both the particles and the glass are negatively charged, they repel each other. It's like trying to park cars on a lot where every car has a magnet on its bumper facing the ground—they don't stick well.
- Result: The particles form very neat, organized radial cracks (like spokes on a wheel). Because they don't stick tightly to the glass, once the film gets thick enough, the whole thing peels off (delaminates) in large, curling strips, like a sticker being pulled off a wall.
On Glass with Positive Particles (CL30): Here, the positive particles are attracted to the negative glass. It's like Velcro; they stick tight.
- Result: Because they are stuck so firmly, they can't move freely to organize. Instead of neat spokes, they form messy, random cracks. They also don't peel off as easily because the "Velcro" holds them down.

2. The "Table" Matters

Glass (Wettable): The drop spreads out wide and thin. When it dries, the film is large and thin, making it prone to peeling up if the particles are repelled (like the TM50 case).
PTFE (Slippery): The drop stays in a tight ball. The drying happens differently, often leading to a "doughnut" shape or just a few big cracks, rather than a full network of small ones.

The "Crack" Rules

The researchers discovered some cool patterns in how the cracks behave:

More Mud, Bigger Cracks: If you use a thicker, mud-denser drop, the cracks that form are wider and longer. It's like drying a thick layer of mud; the tension builds up more, so when it finally breaks, the break is bigger.
Timing: With thin drops, cracks only appear at the very end. With thick drops, cracks start early and race toward the center.
The Peeling Point: The film only peels up (delaminates) if it gets thick enough to generate enough internal stress to overcome the glue holding it to the table. On the slippery glass, this happens easily with thick drops. On the "sticky" polystyrene, the film is thicker in a smaller area, so it peels up even with thinner drops.

The "Why" (The Mechanism)

The authors propose a simple mechanical story:

The Pull: As water leaves, the particles get squeezed together, creating a pulling force (tension) trying to shrink the film.
The Anchor: The bottom of the film is stuck to the table.
The Snap: If the pulling force gets stronger than the "glue" (adhesion) holding the film to the table, the film snaps.
- If the particles repel the table (Negative on Negative), the glue is weak. The film snaps cleanly and peels up in organized strips.
- If the particles stick to the table (Positive on Negative), the glue is strong. The film can't peel up easily, so it just shatters into a messy pile of cracks instead.

The Bottom Line

This paper shows that you can't just look at a drying drop and guess what will happen. You have to know the personality of the particles (do they like or hate the surface?) and the nature of the surface (is it sticky or slippery?).

Repelling particles on a wet surface = Neat cracks and big peeling sheets.
Attracting particles on a wet surface = Messy cracks and no peeling.
Thicker drops = Bigger, earlier cracks and more peeling.

It's a reminder that even in something as simple as a drying drop of water, invisible electrical forces and surface textures are choreographing a complex dance of cracking and curling.

Technical Summary: Instabilities in Drying Colloidal Films: Role of Surface Charge and Substrate Wettability

Problem Statement
The drying of colloidal suspensions is a critical process in applications ranging from paints and coatings to microelectronics and lithography. However, this process often leads to complex deposition patterns accompanied by mechanical instabilities such as cracking, buckling, and delamination. While previous studies have extensively investigated individual factors—such as substrate properties, particle size, shape, and environmental conditions—there is a need to understand the coupled influence of particle surface charge and substrate wettability. Specifically, the interplay between electrostatic interactions (particle–substrate and particle–particle) and capillary forces during the evaporation of sessile droplets remains a key area requiring systematic exploration to explain the resulting deposition morphology and crack patterns.

Methodology
The study employs an experimental approach to investigate the evaporation dynamics and final deposition of silica nanoparticle suspensions.

Materials: Two types of colloidal silica nanoparticles were utilized: negatively charged Ludox TM50 (25 nm diameter, $\zeta = -33$ mV) and positively charged Ludox CL30 (12 nm diameter, $\zeta = +44$ mV). Suspensions were prepared at concentrations ranging from 0.1 wt.% to 5.0 wt.%.
Substrates: Experiments were conducted on three substrates with distinct wettabilities: hydrophilic glass ( $\theta \approx 22^\circ$ ), polystyrene ( $\theta \approx 77^\circ$ ), and hydrophobic polytetrafluoroethylene (PTFE) ( $\theta \approx 102^\circ$ ).
Experimental Setup: 2 $\mu$ L sessile droplets were deposited and dried under controlled ambient conditions ( $24.0 \pm 1.0^\circ$ C, $58 \pm 10\%$ relative humidity).
Imaging and Analysis: A dual-view imaging system was used. Side-view shadowgraphy with a CMOS camera captured the temporal evolution of droplet height, contact diameter, and contact angle. Top-view digital microscopy (50 $\times$ magnification) recorded the final deposition patterns.
Data Processing: Custom Matlab-based solvers and ImageJ were employed to extract droplet profiles, quantify crack spacing ( $\lambda$ ) and length ( $l$ ), and measure the extent of delamination ( $\delta$ ) by tracking the deflection height of film strips.

Key Results

Evaporation Dynamics: The evaporation mode is primarily dictated by substrate wettability rather than particle charge.
- Glass: Droplets predominantly follow a Constant Contact Radius (CCR) mode, where the contact line remains pinned until late in the drying process.
- PTFE: Droplets exhibit a prolonged Constant Contact Angle (CCA) mode, with the contact line receding early.
- Polystyrene: A mixed mode is observed, transitioning from CCR to a receding contact line.
- Particle charge had a negligible effect on the overall evaporation kinetics but influenced the duration of contact line pinning slightly due to electrostatic interactions.
Deposition Morphology and Cracking:
- Ludox TM50 (Negatively Charged): On glass and polystyrene, these droplets form classic "coffee-ring" deposits with radial cracks. As concentration increases ( $\ge 3.0$ wt.%), cracks propagate inward toward the center. On PTFE, deposits are more uniform with fewer, single prominent cracks.
- Ludox CL30 (Positively Charged): These droplets exhibit irregular, randomly oriented cracks, often with transverse connections perpendicular to radial cracks. This is attributed to attractive electrostatic interactions between the positively charged particles and the negatively charged glass substrate, leading to uneven particle settlement.
- Power-Law Relationships: For TM50 on glass, both crack spacing ( $\lambda$ ) and crack length ( $l$ ) follow power-law relationships with particle concentration ( $\lambda \propto \phi^{0.63}$ and $l \propto \phi^{0.4}$ ). Higher concentrations result in wider and longer cracks.
Delamination Behavior:
- Delamination is strongly influenced by particle concentration and substrate wettability.
- Glass Substrates: Delamination occurs only at higher concentrations ( $\ge 3.0$ wt.%) due to the formation of thicker, laterally extended films that generate significant in-plane tensile stress.
- Polystyrene Substrates: Despite lower initial spreading, delamination is observed at lower concentrations ( $\ge 1.0$ wt.%). This is attributed to the formation of a thicker local deposit at the same concentration compared to glass, creating a larger bending moment.
- PTFE Substrates: Delamination is suppressed due to reduced spreading and lower stress buildup.
- The extent of delamination ( $\delta_{avg}$ ) increases with particle concentration and is more pronounced on glass than on polystyrene.

Significance and Mechanistic Framework
The paper proposes a mechanistic framework to explain the observed instabilities, emphasizing the competition between capillary forces, interfacial interactions, and particle–substrate forces.

Crack Initiation: As the solvent evaporates, particles consolidate, and the film shrinks. Unbalanced tensile stress develops because the upper part of the deposit shrinks faster than the region adhered to the substrate.
Role of Charge: While evaporation kinetics are largely independent of charge, the nature of the cracks is charge-dependent.
- Repulsive Interactions (TM50 on Glass): Like-charged systems (negative particle/negative substrate) promote repulsion, leading to well-ordered radial cracks and eventual complete delamination when the capillary force exceeds the frictional force.
- Attractive Interactions (CL30 on Glass): Oppositely charged systems promote strong adhesion and particle mobility limitations, resulting in disordered, irregular cracks and a suppression of delamination.
Delamination Threshold: Delamination occurs when the elastic strain energy stored in the drying film exceeds the film–substrate adhesion energy. The study demonstrates that this threshold is modulated by the initial spreading (determined by wettability) and the resulting film thickness.

Conclusion
This study systematically elucidates the coupled influence of particle surface charge and substrate wettability on the drying of colloidal films. The findings reveal that while substrate wettability governs the macroscopic evaporation mode and deposit spread, particle charge dictates the microscopic ordering of cracks and the propensity for delamination. The results provide a quantitative basis for predicting crack spacing and delamination extent based on concentration and substrate type, offering insights into controlling deposition patterns in colloidal processing.