Pseudoplasticity (shear thinning): the gel that flows under force
Quick facts
- Definition
- Property of a fluid whereby its viscosity drops under shear force and recovers at rest.
- Also known as
- Shear thinning.
- Everyday example
- Ketchup and paint: they flow when shaken or squeezed and firm up when they stop.
- Relevance in facial harmonization
- Explains why a thick gel passes through the needle and behaves like a gel again in the tissue.
Pseudoplasticity is the property of certain fluids to become less viscous when force is applied and to recover viscosity when it stops. In English, the phenomenon is called shear thinning. It is one of the central concepts of rheology, the science that studies how materials flow and deform.
How does this happen in the gel?
A cross-linked hyaluronic acid gel is made of a network of interlaced chains. At rest, this network stays organized and the gel is thick. When a force passes through the gel, such as the plunger pushing it through a needle, the network aligns with the direction of flow and the particles slide more easily. The apparent viscosity drops and the material flows.
On leaving the needle, the force disappears, the network reorganizes and the gel again behaves like a soft solid. This cycle repeats within seconds and is what allows a firm product to be injected through a fine needle and stay where it was placed.
| Situation | State of the gel | Behavior |
|---|---|---|
| In the syringe, at rest | Intact network | Thick, barely flowing |
| Passing through the needle, under pressure | Aligned network | More fluid, with less resistance |
| In the tissue, at rest | Reorganized network | Firm, keeps shape and position |
Why does it matter in practice?
Two properties depend directly on pseudoplasticity: the extrusion force needed to push the product and the feel of passage through the needle or cannula. Highly pseudoplastic gels flow with less effort, while less pseudoplastic ones need more force to come out.
This adds to the needle gauge: the finer it is, the greater the resistance, and the gel's pseudoplasticity helps compensate for it. It also adds to viscosity and viscoelasticity, which describe how the gel responds to deformation at rest.
Is pseudoplasticity the same as firmness?
No. Firmness, measured by G prime, describes the gel's resistance to deformation at rest. Pseudoplasticity describes how the gel behaves while under flow. A gel can be firm at rest and still flow well through the needle, and that is what manufacturers aim for when designing the product.
Another related concept is cohesivity, the gel's ability to stay together without breaking into fragments. It determines how the product holds together in the tissue after injection.
Also called: shear thinning, non-Newtonian behavior.
Frequently asked questions
Why does hyaluronic acid gel pass through a fine needle even though it is so thick?
Because it is pseudoplastic: under the pressure of the plunger, viscosity drops and the gel flows. On leaving the needle and no longer being pressed, it becomes firm again.