# Are cats liquid? Answering this question won me an Ig Nobel

## Ig Nobels rewards unconventional research that first makes you laugh and then makes you think, showing the world that science can be funny (and fun) too.

A liquid is traditionally defined as a material that adapts its shape to fit a container. Yet under certain conditions, cats seem to fit this definition.

This somewhat paradoxical observation emerged on the web a few years ago and joined the long list of internet memes involving our feline friends. When I first saw this question it made me laugh, and then think. I decided to reformulate it to illustrate some problems at the heart of rheology, the study of the deformations and flows of matter. My study on the rheology of cats won the 2017 Ig Nobel Prize in Physics.

The prizes are awarded every year by Improbable Research, an organisation devoted to science and humour. The goal is to highlight scientific studies that first make people laugh, then think. A ceremony is held every year at Harvard University.

## What is a liquid?

At the centre of the definition of a liquid is an action: a material must be able to modify its form to fit within a container. The action must also have a characteristic duration. In rheology this is called the relaxation time. Determining if something is liquid depends on whether it’s observed over a time period that’s shorter or longer than the relaxation time.

If we take cats as our example, the fact is that they can adapt their shape to their container if we give them enough time. Cats are thus liquid if we give them the time to become liquid. In rheology, the state of a material is not really a fixed property – what must be measured is the relaxation time. What is its value and on what does it depend? For example, does the relaxation time of a cat vary with its age? (In rheology we speak of thixotropy.)

Could the type of container be a factor? (In rheology this is studied in “wetting” problems.) Or does it vary with the cat’s degree of stress? (One speaks of “shear thickening” if the relaxation time increases with stress, or “shear thinning” if the opposite is true.) Of course, we mean stress in the mechanical sense rather than emotional, but the two meanings may overlap in some cases.

## ‘Deborah number’ and the flow of mountains

What cats show clearly is that determining the state of a material requires comparing two time periods: the relaxation time and the experimental time, which is the time elapsed since the onset of deformation initiated by the container. For instance, it may be the time elapsed since the cat stepped into a sink. Conventionally, one divides the relaxation time by the experimental time, and if the result is more than 1, the material is relatively solid; if the result is lower than 1, the material is relatively liquid.

This is referred to as the Deborah number, after the biblical priestess who remarked that on geological time scales (“before god”) even mountains flowed. On shorter time scales one can see glaciers progressively flowing down valleys.

Even if the relaxation time is very large (days, years), the behavior can be that of a liquid if the Deborah number is small (compared to 1). Conversely, even if the relaxation time is very small (milliseconds), the behavior can be that of a solid if the Deborah number is large (compared to 1). This is the case if one observes a water balloon at the instant when it’s popped.

The Deborah number is an example of dimensionless number: Since we divide one time period by another, the ratio does not have any unit. In rheology, and in science more generally, there are many dimensionless numbers that can be used to determine the state or regime of a material or system.

## Measuring the flow of cake batter

For liquids there is another dimensionless number that can be used to estimate whether the flow will be turbulent, with vortices, or whether it will calmly follow the outline of the container (we say that the flow is laminar).

If the flow speed is V and the container has a typical size h perpendicular to the flow, then we can define the velocity gradient V/h. The inverse of this velocity gradient scales as a time.

Comparing this duration and the relaxation time produces the Reynolds number in the case of fluids dominated by inertia (like water), or the Weissenberg number for those dominated by elasticity (like cake batter). If these dimensionless numbers are large in comparison to 1, then the flow is likely to be turbulent. If they’re small in comparison to 1 the flow is likely to be laminar.

Asking the question of whether cats were a liquid allowed me to illustrate the use of these dimensionless numbers in rheology. I hope that it will make people laugh and then think.

Marc-Antoine Fardin, Chercheur en rhéologie, Université Paris Diderot – USPC.

This article first appeared on The Conversation.

# Relying on the power of habits to solve India’s mammoth sanitation problem

## Adopting three simple habits can help maximise the benefits of existing sanitation infrastructure.

India’s sanitation problem is well documented – the country was recently declared as having the highest number of people living without basic sanitation facilities. Sanitation encompasses all conditions relating to public health - especially sewage disposal and access to clean drinking water. Due to associated losses in productivity caused by sickness, increased healthcare costs and increased mortality, India recorded a loss of 5.2% of its GDP to poor sanitation in 2015. As tremendous as the economic losses are, the on-ground, human consequences of poor sanitation are grim - about one in 10 deaths, according to the World Bank.

Poor sanitation contributes to about 10% of the world’s disease burden and is linked to even those diseases that may not present any correlation at first. For example, while lack of nutrition is a direct cause of anaemia, poor sanitation can contribute to the problem by causing intestinal diseases which prevent people from absorbing nutrition from their food. In fact, a study found a correlation between improved sanitation and reduced prevalence of anaemia in 14 Indian states. Diarrhoeal diseases, the most well-known consequence of poor sanitation, are the third largest cause of child mortality in India. They are also linked to undernutrition and stunting in children - 38% of Indian children exhibit stunted growth. Improved sanitation can also help reduce prevalence of neglected tropical diseases (NTDs). Though not a cause of high mortality rate, NTDs impair physical and cognitive development, contribute to mother and child illness and death and affect overall productivity. NTDs caused by parasitic worms - such as hookworms, whipworms etc. - infect millions every year and spread through open defecation. Improving toilet access and access to clean drinking water can significantly boost disease control programmes for diarrhoea, NTDs and other correlated conditions.

Unfortunately, with about 732 million people who have no access to toilets, India currently accounts for more than half of the world population that defecates in the open. India also accounts for the largest rural population living without access to clean water. Only 16% of India’s rural population is currently served by piped water.

However, there is cause for optimism. In the three years of Swachh Bharat Abhiyan, the country’s sanitation coverage has risen from 39% to 65% and eight states and Union Territories have been declared open defecation free. But lasting change cannot be ensured by the proliferation of sanitation infrastructure alone. Ensuring the usage of toilets is as important as building them, more so due to the cultural preference for open defecation in rural India.

According to the World Bank, hygiene promotion is essential to realise the potential of infrastructure investments in sanitation. Behavioural intervention is most successful when it targets few behaviours with the most potential for impact. An area of public health where behavioural training has made an impact is WASH - water, sanitation and hygiene - a key issue of UN Sustainable Development Goal 6. Compliance to WASH practices has the potential to reduce illness and death, poverty and improve overall socio-economic development. The UN has even marked observance days for each - World Water Day for water (22 March), World Toilet Day for sanitation (19 November) and Global Handwashing Day for hygiene (15 October).

At its simplest, the benefits of WASH can be availed through three simple habits that safeguard against disease - washing hands before eating, drinking clean water and using a clean toilet. Handwashing and use of toilets are some of the most important behavioural interventions that keep diarrhoeal diseases from spreading, while clean drinking water is essential to prevent water-borne diseases and adverse health effects of toxic contaminants. In India, Hindustan Unilever Limited launched the Swachh Aadat Swachh Bharat initiative, a WASH behaviour change programme, to complement the Swachh Bharat Abhiyan. Through its on-ground behaviour change model, SASB seeks to promote the three basic WASH habits to create long-lasting personal hygiene compliance among the populations it serves.

This touching film made as a part of SASB’s awareness campaign shows how lack of knowledge of basic hygiene practices means children miss out on developmental milestones due to preventable diseases.

SASB created the Swachhata curriculum, a textbook to encourage adoption of personal hygiene among school going children. It makes use of conceptual learning to teach primary school students about cleanliness, germs and clean habits in an engaging manner. Swachh Basti is an extensive urban outreach programme for sensitising urban slum residents about WASH habits through demos, skits and etc. in partnership with key local stakeholders such as doctors, anganwadi workers and support groups. In Ghatkopar, Mumbai, HUL built the first-of-its-kind Suvidha Centre - an urban water, hygiene and sanitation community centre. It provides toilets, handwashing and shower facilities, safe drinking water and state-of-the-art laundry operations at an affordable cost to about 1,500 residents of the area.