Every morning, you perform a small physics activity without realising it. You squeeze toothpaste out of a tube. You apply a force and the paste flows out like a liquid. When you stop squeezing, the toothpaste stays on the brush, holding its shape against gravity. It doesn’t drip, spread or collapse into a puddle.
This simple act confronts us with a fundamental question in physics: how do materials respond to forces and what makes something like toothpaste flow when pushed yet remain solid when left alone?
Solid or liquid?
In school, materials are neatly divided into solids and liquids. Solids resist deformation and maintain their shape whereas liquids flow under even the slightest force. But toothpaste, shampoo, gels, and creams do neither in the usual way. They belong to a broad class of materials that modern physics calls soft-matter materials: they can behave like solids or flow like liquids depending on the forces applied and the timescales involved.
What makes soft materials different lies in their internal structure. Their basic building blocks are far larger than atoms or simple molecules, yet far too small to see with the naked eye. In personal care products, these building blocks can be small droplets, microscopic clusters or long and flexible macromolecules suspended in a fluid. Because they are relatively large, even gentle actions like squeezing a tube, shaking a bottle or spreading with a brush or finger, can rearrange them.
Equally important are the forces holding these structures together. In hard solids, atoms are bound by strong interactions that lock them in place. In ordinary liquids, the interactions are weaker, allowing molecules to move freely. But the molecules themselves are small and their own structure doesn’t change under gentle forces. In soft materials, on the other hand, the forces between the building blocks are weak and easily disrupted. This makes their internal structure fragile but also highly adaptable. As a result, how soft materials behave depends not only on how strongly they are pushed but also on how quickly. The same force applied in different ways can produce very different outcomes. A slow, gentle stress may leave the internal structure largely intact while a rapid or sudden stress can rearrange it completely, allowing the material to flow.
This sensitivity to both force and time is a defining feature of soft matter. Squeeze a tube of toothpaste slowly and it may barely budge. But squeeze it suddenly and it flows out with ease. Shake a bottle of shampoo and it will pour out freely, but handle it gently and it will feel thick and resistant. In each case, the same material responds differently depending on the rate at which it is forced to move. In fact many shampoos contain long and flexible worm-like molecules dispersed in a liquid. At rest, these molecules are tangled together, forming a loose internal network that gives the shampoo its thickness. When you shake the bottle or pour it out, the molecules are stretched and become aligned with each other, which makes it easier for them to slide past one another.
Under the influence of this motion, the network — the connections between molecules — continually breaks and reforms. The worm-like micelles can for a brief period break into shorter segments and then reconnect, reducing the resistance to flowing. Thus the shampoo becomes more runny. Once the molecules stop moving past each other, they slowly coil back and become entangled with each other once more. This way the network is restored and the shampoo becomes thicker again.
Toothpaste behaves in a similar way, with microscopic structures that are rearranged under pressure and which reassemble when the force is removed. This behaviour is remarkable because the microscopic building blocks are constantly being reorganised, breaking apart, reconnecting, and reshaping themselves in a reversible way. This repeat making and unmaking allows soft materials to adapt smoothly to external forces.
In the end, what we usually call solids and liquids seems to depend on how a material responds to force over time. On very short time scales, even a liquid can resist deformation like a solid while a solid can slowly flow if enough force is applied or enough time is allowed.
The pitch drop experiment
At the University of Queensland in Brisbane, Australia, scientists have been conducting an experiment since 1927, known as the pitch drop experiment, to test the viscosity of pitch, or bitumen, which is a substance derived from tar. At room temperature, pitch seems like a normal solid and can be shattered with a hammer. But according to scientists, pitch is really a liquid with extremely high viscosity — around 200-billion-times that of water.
To demonstrate this, a physics professor named Thomas Parnell heated pitch and poured it into a sealed glass funnel in 1927. After leaving it undisturbed for three years so it could cool and stabilise, he removed a seal from the bottom of the container, allowing the pitch to drip under gravity. In the last almost 100 years, only nine drops have fallen, with the ninth drop in April 2014. Even though this is excruciatingly slow, the fact that it’s dropping at all shows that pitch is in fact a liquid. The next drop is expected by 2030. Similar experiments are underway in Ireland, Scotland, and Wales.
Physicists study this force and time-dependent behaviour in a field called rheology, which examines how materials deform and flow under applied stress. The roots of this study go back to the ancient Greek philosopher Heraclitus, who captured it in the aphorism “panta rhei”, meaning “everything flows”. In soft matter, this metaphor for continuous change, which is in fact a feature inherent to all matter, becomes tangible at the timescales and force levels we encounter in everyday life. Beyond their practical usefulness, soft materials offer insights into the deeper scientific principles of motion, force, flow, and change.
The next time you open your bathroom cabinet, remember you are not just handling personal care products. You are handling materials carefully engineered to balance force and flow, structure, and softness. In their gentle response to a squeeze or a shake lies a rich, hidden chapter of modern physics, playing out out of sight, every day.
Indresh Yadav is an assistant professor in the Department of Physics at IIT-Bhubaneswar.
Published – January 19, 2026 08:30 am IST
