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We've got a good feeling about the future

People use many words such as creamy, sticky, silky, greasy etc. to describe how face cream it feels on their skin. Yet the feel of the cream on skin is picked up by only a small number of mechanically and thermally sensitive skin receptors.

What do words actually mean?

Our goal is to understand how descriptive words such as creamy, sticky, silky, greasy relate to actual frictional forces; insights that will help us continually improve our products.

A team of scientists and engineers led by Andy Hopkinson of our Measurement and Behavioural Sciences team at Unilever R&D Port Sunlight, developed a plate that can measure the position, pressure and friction generated by a finger on a surface as it ‘explores’ a cream or other fluid on a flat surface.

The plate consists of a board attached to 6 strain gauges which allow us to determine 3 forces and 3 torques. From these, we calculate the movement of the finger.

Getting into people’s minds

We took a set of fluids and examined the correlation between the friction force during finger stroking and the pressure and speed of the finger. We then used the same fluids in psychophysical experiments by Professor Greg Essick at the University of North Carolina’s Centre for Neurosensory Disorders. Armed with a list of attributes, the test group rated the fluids depending on how oily, smooth or slippery they were to the touch.

Making connections

From this we are starting to learn how different tactile perceptions are driven by the properties of friction. The experiments show that the frictional forces applied depend more on the fluid than the person. And we found links between the descriptive words and physical measurements. For instance, there was a strong connection between how watery and bulky a fluid felt and measures of the friction and pressure applied.

Digging deeper

When asked about next steps in this project, Andy said, “We can visualise other relationships between sensory perception and the physical tactile stimulus, and we’re especially interested in silkiness. Work is ongoing to find the most appropriate statistical approach to tease out the more complex relationships, yet in a way that still gives us simple insights that a materials scientist can act upon”.

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