QSense QCM‑D measures swelling of thin films by tracking changes in resonance frequency (Δf) and dissipation (ΔD) of a quartz sensor as the film takes up or releases solvent. The water or solvent uptake is sensed as an apparent mass increase and a change in viscoelastic properties of the layer at the surface, recorded in real time.
Summary
- Many materials rely on their ability to take up and release liquid (e.g., hydrogels, thickeners, edible films, paper, membranes).
- Swelling in QCM‑D appears as Δf↓ (apparent mass increase) and typically ΔD↑ (softer, more hydrated film).
- QCM‑D can monitor how much the film swells, how fast, and whether the swelling is reversible under changing humidity or solution conditions.
- These insights are useful in biointerface science, pharma, food formulations, chemicals and cleaning, and water/environmental technologies.
Introduction
The function and properties of many natural and man‑made materials depend on their ability to take up and release water or solvent. Examples include hydrogels in biomaterials and tissue engineering, thickeners and emulsifiers in food and cosmetics, hygroscopic films and coatings in the chemicals industry, and filtration and separation devices, all relying on controlled hydration and dehydration.
In research and product development, it is therefore highly relevant to study swelling processes of these materials, for example to understand mechanical stability, permeability, dissolution, fouling or cleaning behavior. Because swelling is an interfacial phenomenon that changes both the mass and mechanical properties of a film, it is well suited for study with QSense QCM‑D, a surface‑sensitive and label‑free technology which can be used to characterize and optimize water uptake and release processes in thin films.
Monitoring solvent uptake and swelling of thin films
The amount of solvent, for example water, in a thin film can be very high, sometimes >95%, depending on the molecules present and their conformation at the surface. As an illustrative example, consider elongated molecules that adsorb flat on the surface compared to the same molecules adsorbing in a more upright fashion, Fig. 1.:
- A layer of molecules lying flat will couple only a small amount of solvent.
- If the molecules instead stand up or reorganize into a more open structure, more solvent can penetrate and couple to the film.
The hydration and dehydration of molecular layers, and the transitions between more collapsed and more hydrated states, can be characterized with QCM‑D. In such measurements, water uptake and swelling are sensed as an apparent mass increase at the surface (Δf↓) together with a change in dissipation that reflects the softer, more hydrated character of the swollen film.
Figure 1 (conceptual). Molecules adsorbed flat onto a surface (left) couple less solvent than when adsorbed in an upright or more open conformation (right). The latter conformation allows more water to enter the film, contributing to swelling.
Example: moisture‑induced swelling of a starch film
Starch is an important ingredient in, for example, the paper industry and in the design of edible films. In both areas, the effect of ambient humidity on material strength and quality is a central question:
- For paper and packaging, the solubility and swelling should be low over the relevant humidity range so that mechanical and barrier properties remain stable.
- For edible materials, in contrast, the material is often intended to soften or dissolve; here the solubility and swelling need to be sufficiently high above a certain humidity threshold.
In the example below,1 a thin film of native potato starch (NPS) on a QSense QCM‑D sensor is exposed to air at different humidity levels while the change in film thickness is monitored, Fig. 2. The thickness is obtained by modelling the measured Δf and ΔD response with an appropriate viscoelastic model.
- The film thickness at the start of the measurement is set to 100%. The measurement starts at 11% relative humidity.
- The humidity is then increased stepwise, and the thickness change is monitored. The thickness of the film increases as the humidity increases, showing moisture‑induced swelling.
- Notably, when the humidity is changed back from 100% to 11%, the starch film collapses back to the original thickness within seconds.
This measurement illustrates how humidity induces swelling of the film and affects its thickness, and how the material responds when conditions are reversed. The results provide insight into how the material behaves under different humidity conditions, which is directly relevant for product performance in use. 
Figure 2. Analyzing vapor uptake and thin film swelling using QCM-D. Thickness of a thin film of native potato starch as a function of air humidity1 showing vapor uptake, swelling and collapse as humidity is cycled.
What swelling looks like in QCM‑D data
When a film swells due to solvent uptake, QCM‑D typically records:
- Δf decreases (Δf↓) – apparent mass increase, arising from the coupled solvent.
- ΔD increases (ΔD↑) – more hydrated, softer, more viscoelastic film.
When the film collapses or dehydrates (for example, when humidity is lowered again):
- Δf increases (Δf↑) – apparent mass decreases as solvent leaves the film.
- ΔD decreases (ΔD↓) – film becomes denser and mechanically stiffer.
By following these signals at different overtones, QCM‑D provides information on:
- Degree of swelling (relative or absolute, if modelled)
- Dynamics of swelling and collapse
- Reversibility
- Changes in viscoelastic properties of the film
Practical protocol: swelling and collapse experiments with QCM‑D
A generic protocol to study swelling of thin films with QCM‑D can look like this:
1. Prepare the thin film on the sensor
- Form the film (e.g., polymer, hydrogel, starch, coating) on a suitable QCM‑D sensor.
- Ensure film thickness and preparation are relevant to the application.
2. Establish an initial reference state
- For humidity‑controlled experiments: equilibrate the film at a defined relative humidity.
- For liquid experiments: equilibrate in a defined buffer or solvent.
3. Induce swelling
- Change the environment to a condition that promotes swelling (e.g., higher humidity, different solvent quality, pH, ionic strength, temperature).
- Monitor Δf and ΔD over time as the film takes up solvent and swells.
4. Induce collapse / deswelling
- Return to the initial condition or to a condition that promotes collapse (e.g., lower humidity, different solvent/pH/temperature).
- Record how the film thickness and viscoelastic response change as it collapses.
5. Analyze the data
- Evaluate the magnitude and dynamics of swelling and deswelling.
- Compare behaviour across conditions (humidity levels, solvents, pH, etc.).
- Use viscoelastic modelling where appropriate to extract quantitative thickness and mechanical parameters.
Other situations where swelling is relevant
Measurements of vapor uptake, solvent uptake and thin film swelling are relevant in many areas where hygroscopic or responsive materials are used. Examples include:
- Biomaterials and biointerfaces – swelling of hydrogels, polymer brushes and bio‑inspired coatings used in implants, and tissue engineering.
- Drug encapsulation and delivery – swelling and dissolution of polymer or hydrogel carriers and edible films under different humidity or solvent conditions.
- Humidity‑sensitive materials and devices – coatings and films in humidity‑resistant devices, where changes in ambient humidity should not compromise performance.
- Cleaning processes – soil and deposit layers that swell in contact with surfactants and cleaning formulations before removal.
- Smart and stimuli‑responsive materials – films and hydrogels that swell or collapse in response to pH, temperature, ionic strength or specific analytes.
In all these contexts, understanding how and how much a material swells, and whether that process is reversible, is important for designing robust and high‑performing products and processes.
Common pitfalls and practical tips
- Ignoring viscoelastic effects: Swollen films are often soft and highly hydrated. Relying only on rigid‑film (Sauerbrey) assumptions can underestimate thickness and misinterpret mass. Use viscoelastic modelling where ΔD is significant.
- Not controlling environment carefully: Swelling is sensitive to humidity, temperature, pH and ionic strength. Use well‑controlled environmental conditions and, where possible, reference measurements.
- Overlooking hysteresis: Swelling and collapse may not follow the same path (hysteresis). Always check behaviour on both increasing and decreasing humidity/solvent quality.
- Insufficient equilibration times: Allow enough time for the film to reach equilibrium at each step; swelling dynamics can vary widely between materials.
- Inadequate documentation: Record conditions (humidity, temperature, buffer/solvent, pH, ionic strength) and film preparation details for reproducibility and comparison.
Summary
QSense QCM‑D provides a straightforward and sensitive way to monitor swelling and collapse of thin films. By following the mass (Δf) and dissipation (ΔD) in real time, you can quantify how much a film swells, how fast it responds, how reversible the swelling is, and how the mechanical properties of the layer change as it hydrates or dehydrates. This information is valuable across a wide range of research and development contexts where interfacial hydration, film stability and solvent response play a critical role in product and process performance.
Download the overview to read more about what phenomena you can study and what information you can obtain with QSense QCM-D.
