Why does liquid stay in a horizontally turned straw, but not in a glass?

A new paper, co-authored by Oxford University’s Prof Dirk Aarts, presents a precise mathematical criterion determining whether the liquid in a capillary of arbitrary cross-section will remain in it, or will flow out, when the capillary is at a horizontal position.

Have you ever noticed that liquid stays inside a straw when it’s held horizontally? Or that the same thing doesn’t happen when you turn a glass on its side?

A team of researchers including Professor Dirk Aarts from Oxford University’s Department of Chemistry has been investigating this phenomenon – one that’s ‘surprisingly difficult’ to explain from a scientific point of view.

Professor Aarts worked with colleagues Carlos Rascón from Universidad Carlos III de Madrid in Spain and Andrew Parry from Imperial College London for the study, which is published in the journal PNAS. Professor Aarts said: ‘We considered the following seemingly simple question: why does the liquid spill out when I hold a glass – say, of beer – horizontally, but stays in a straw when I do the same thing?

‘This question is actually surprisingly difficult, especially when considering non-circular cross-sections of the capillaries, or tubes.

‘For a liquid trapped between two parallel walls, and for a liquid trapped in a circular capillary like a straw, the answer is one that we would intuitively expect: the liquid wants to flow out because of gravity, but is trapped due to the surface tension.’

The ‘capillary action’ described here is the ability of a liquid to flow in narrow spaces, often in opposition to external forces such as gravity. For example, if you zoom in on the surface of water in a glass, you’ll see that it curves upwards by a couple of millimetres at the wall. This curve is known as the meniscus.

Professor Aarts said: ‘The competition between gravity and surface tension leads us to the capillary length, which sets the height to which a meniscus will climb at a wall. Indeed, if the separation between the two walls is less than roughly the capillary length, or if a circular capillary has a diameter less than roughly the capillary length, the liquids will stay put. If not, the liquids will flow out.

‘However, if you change the cross-sectional shape of the capillary – for example, making it a triangle – the situation may change completely, and for certain geometries the liquid may spill out at any length scale, well below the capillary length.

‘We figured out how to calculate this behaviour for general cross-sectional shapes, although the actual numerical calculations, carried out by Carlos Rascón, took almost seven years to complete. One of the reasons for this was that the spilling out may occur via different pathways, and the crossovers between those pathways were hard to understand.’

The researchers were able to solve the problem by reducing it down to an equivalent two-dimensional problem, which is numerically more accessible. The paper shows how ’emptying diagrams’ can be created by calculating the energy of the problem in 2D. As soon as the energy became smaller than zero, no 3D solution for the meniscus exists, and the liquid empties.
Professor Aarts added: ‘The surprising result here is that a capillary may empty even at lengths much smaller than the capillary length. This has implications for any technologies where liquids are used or are present on small scales, such as microfluidics, biomedical diagnostics, oil recovery, inkjet printing and so on.’

This article was written by Stuart Gillespie and originally appeared in the Oxford Science Blog.

Why there’s a handpump in an Oxford car park.


An introduction to Oxford University’s innovative Smart Handpump research.

A crowd gathered at Oxford University’s School of Geography and the Environment on 17 October to find out about the peculiar looking red handpump in the car park. The so-called ‘Smart Handpump’ is part of a bold research initiative that connects novel technology, computational informatics, institutional design, sustainable finance and policy reform, to improve poor people’s access to safe, reliable water.

Lack of finance for maintenance keeps Africa behind
A staggering 270 million rural Africans still live without access to safe water. Over $1 billion per year is needed just to maintain Africa’s existing rural water supply infrastructure. Government and donors commonly finance and build new infrastructure, before handing it over to the users who are expected to manage and maintain it. But the reality is that rural Africans seldom pay regularly for water. And when finance dries up, broken infrastructure doesn’t get fixed and waterpoints are left abandoned.

‘If we’re serious about meeting the Sustainable Development Goal for water then financial sustainability must be addressed’, said Dr Rob Hope of Oxford University. It’s also essential that solutions that can be implemented at scale – a handpump by handpump approach isn’t going to deliver universal access to water by 2030.

The birth of the Smart Handpump
The Smart Handpump story started in Zambia back in 2010 when Oxford University researcher Patrick Thomson wondered: what if a handpump could tell you if it was working or not? He designed a simple and low-cost accelerometer which is fitted to the handpump handle and measures its movement and subtle vibrations. The device sends data through the mobile phone network which means it can be monitored remotely. Not only can the Smart Handpump tell you if it’s working, but the data is proving useful in a number of ways.

First, the handpump provides hourly data on water usage.

Second, researchers at the Department for Engineering Science are using machine learning techniques to detect deterioration of pump mechanics and actually predict failure before it happens. This could enable predictive maintenance and result in uninterrupted handpump services.

Lastly, it was by surprise that the researchers discovered they could model the data to estimate the depth of the groundwater below the pump.

From data to decisions
Successful implementation of Smart Handpumps in in two counties in Kenya (Kwale and Kitui) has led to the creation of a professional maintenance company Fundifix Ltd (‘Fundi’ means ‘mechanic’ in Swahili). The company receives data from over 300 community handpumps serving over 50,000 rural Kenyans and has reduced the downtime of broken pumps from over 30 days to less than 3 days.

Connecting multiple waterpoints into one network for maintenance improves services and reduces cost.

Performance-based finance
Handpumps typically don’t break down often, but when they do repairs can be expensive. To prevent water users being hit with a large bill, the Fundifix model requires them to pre-pay monthly with mobile money. In return they are guaranteed a high quality service.

But user payments alone will never be enough to cover full operation and maintenance costs, especially in poor rural areas.

A Water Services Maintenance Trust Fund has been set up to pool additional finance from government and investors. Crucially, the Trust Fund only releases finance to the maintenance company if agreed performance indicators are met, such as rapid repair, collection of user payments, water quality monitoring and sound financial management.

Mobile communications allow information about handpump functionality and payments to be monitored easily and cheaply.

This performance-based approach has been successfully trialled in parts of Kwale and Kitui counties, and the next step is scaling up county-wide, and expanding beyond handpumps to include all types of rural water infrastructure, including piped systems.

The Smart Handpump travels to Asia
While the research to date has focused on Afridev pumps in Africa, the pump outside the School of Geography and the Environment is a Samrat from Bangladesh. Researchers are analysing data from this different type of pump, to ensure their models are robust enough to work across different types of handpumps.

As part of our research on universal drinking water security in Bangladesh, we plan to install transmitters on handpumps in the rural area of Matlab Bazaar in order to understand water use and support the government’s coordination, investment and delivery of water services.

From Kenya to Bangladesh and beyond, by equipping government, communities and investors with better information and monitoring systems, they can work together to keep water flowing in communities, schools and clinics.

See the presentation from the handpump launch event

This post originally appeared on the REACH website.

Equitable access is key to meeting water, sanitation and hygiene targets

Oxford University’s Dr Katrina Charles explores the challenge of realising the Sustainable Development Goals for WASH.


A woman carries water she has collected from the Turkwel River near Lodwar in Turkana County, north-west Kenya. Rob Hope/REACH

The UN’s millennium development goal target of halving the amount of people with access to safe drinking water has been met. The same is sadly not true of the sanitation target. And the transition to the Sustainable Development Goals for water and sanitation has created even more ambitious targets. These will require real change within this sector to achieve them by the 2030 deadline.

Goal 6 of the sustainable development goals, released in 2015, involves ensuring availability and sustainable management of water and sanitation for all. The indicators which will be used to track progress were only agreed in March 2016. It’s early days, so changes and shifts might not be visible to those outside the sector.

I’m happy to report that there are shifts towards greater equity in access – which is important because, as research has previously shown, progress in the provision of water and sanitation tends to benefit wealthier populations. The poor are left out in the cold.

The Sustainable Development Goals aim to provide access to all, but to achieve this will take major changes in the sector.


Targets for access to water, sanitation and hygiene: then and now

Equality in access

The Millennium Development Goals focus of halving the number of people without access to water meant that the target could be achieved without helping the poorest. By 2012, the Joint Monitoring Programme, or JMP, analysed progress toward the targets by wealth. This highlighted how progress was often greatest for the wealthiest, while there was little change for the poorest.

Senegal is an example how different progress can be for the richest and poorest in a country which met the MDG target on water. The progress the country made was unequal. In urban areas, access to improved water sources decreased for the poorest between 1995 and 2012. In rural areas, rapid progress for the second wealthiest group still left them 17 years behind that of the wealthiest.

Across the sector there is now a focus on how to extend access to water, sanitation and hygiene services to those who are most marginalised, but also to those who are least able to afford to pay. The target is to make water affordable for all, but this is the one area not currently captured in the SDG indicators. Extending sustainable services to all will require different financing models to address both construction and maintenance, and this remains a key topic under discussion.


Senegal met the MDG target for water, but progress was unequalWHO/UNICEF Joint Monitoring Programme

A better level of access

There are three key critical areas in which the bar for what is considered access is being raised: safer water quality, integration of hygiene, and safe management of sanitation.

By the end of the MDG period it was clear that improved water sources did not equate to safe drinking water. A rapid assessment of drinking-water quality in five countries – Ethiopia, Jordan, Nicaragua, Nigeria and Tajikistan – demonstrated the gap between improved water sources and safe water. Over half of protected dug wells did not provide safe water and nor did around a third of protected springs and boreholes.

These results showed that in Nigeria the proportion of the population with access to safe water was 15%, or 22 million people lower than estimated based on the MDG indicator. Similar results were found for 4 of the 5 countries included in the study, with a 7-16% decrease in access when water quality was taken into consideration.

Going forward, the SDG indicator for safely managed drinking water services is defined as:

a basic drinking water source which is located on premises, available when needed and free of faecal and priority chemical contamination.

A basic drinking water source is an improved drinking water source with a round trip collection time of no more than 30 minutes including queuing. Where existing data is available, there will be reports against this indicator in the coming year.

But data is not widely available. This will be one of the major outcomes from the SDG for water: millions more people across the globe will have their water sources monitored, with increasing pressure on those that provide water services to ensure water isn’t just available, that it is also safe to drink. The area is already seeing progress with the implementation of water quality testing being expanded in household surveys.

How this data will be made available to water users and decision makers at a local level is not yet clear. But it is essential that this is addressed in the coming years to help deliver safe accessible drinking water for all.

The hygiene gap

There is often limited attention given to hygiene. The inclusion of hygiene in target 6.2 is the result of sustained advocacy and research work within the sector.

About 28 countries in sub-Saharan Africa have been included in surveys demonstrating that, on average, only 13% of the population have access to a handwashing facility at home with soap and water. That is around half the population that had access to sanitation in those same countries, and about one fifth of those with access to water. The inclusion of hygiene in the sustainable development goals will ensure the sector continues to build on this important work.

Safe sanitation

The emphasis in the millennium development goals was on toilet infrastructure only. This has left what has been described as the second generation sanitation challenge: how to remove excreta building up in pit latrines and septic tanks and how to treat it?

Many toilets aren’t accessible to emptying trucks, or are at risk of collapse if they are emptied. Where equipment is available for desludging, waste is still commonly dumped into waterways as treatment works do not have the capacity. Raising awareness of these issues and communicating them through shit flow diagrams is crucial. The sector is changing how it works to address the whole faecal sludge management chain.

The sustainable development goals add new dimensions to evaluating access to drinking water and sanitation, and now hygiene. In the millennium development goals infrastructure was a focus, but with the sustainable development goals it will expand to include management and behaviour change. Progress against the SDG targets for water, sanitation and hygiene may appear slow as these are incorporated into such initiatives.The Conversation

Katrina Charles, Lecturer and course director in Water Science, Policy and Management, University of Oxford

This article was originally published on The Conversation. Read the original article.

Studying water processes at NASA’s Jet Propulsion Laboratory

Oxford DPhil student, Homero Paltan shares his experience of a summer spent at NASA’s Jet Propulsion Laboratory in California.

The Von Karman Auditorium, at NASA’s Jet Propulsion Laboratory in Pasadena, California is lined with artefacts chronicling the history of space science; these include replicas of the Mars Rover, the Explorer 1 and the Cassini-Huygens spacecraft. It was in this room I received an induction to the JPL back in May. Following the talk, I chanced upon a large screen listing known habitable planets: 17 at that time, drawn from few thousand already identified. In these early encounters, I was immediately struck by the transcendence and history of the place where I would spend the rest of my summer.


Induction day at the JPL – jet-lagged.

I arrived at the JPL on the invitation of Dr Duane Waliser, Chief Scientist of the JPL’s Earth Science and Technology Directorate. The aim of my visit was to expand our understanding of the physical drivers of global hydrological extremes. In particular, we were keen to identify catchments where atmospheric rivers make landfall and become relevant to water resource managers. Yes, there are also rivers in the atmosphere! These are channels where moisture moves between the tropics and mid to high latitudes. A convergence of moisture in these atmospheric rivers can lead intense precipitation and flooding on the ground. Conversely, improving the understanding of the low flows maintained by atmopsheric rivers could be vital for managing drought.

While at Oxford, I’d worked with my supervisor, Prof Simon Dadson, to test and implement a hydrological framework connecting atmosphere, land surface interactions, hydrological variables and rivers dynamics. At the JPL, we would make use of NASA’s computing power, and most importantly the atmospheric rivers database developed by Dr Waliser and Dr Bin Guan, to run series of sensitivity experiments.

I met some brilliant minds at the JPL, and came across a number of interesting projects; these included an evaluation of the future SWOT (Surface Water & Ocean Topography) mission that will detect the change of water bodies over time at finer scales; the ongoing GRACE (Gravity, Recovery and Climate Experiment) mission that measures the change in water masses; and projects that improve cloud detection in order to make better climate projection models. I also had the opportunity to receive technical support from the group led by former Oxford post-doc, Dr Josh Fisher. This group uses satellite and modelling techniques to study global ecosystems.


Where satellites and spacecraft are assembled.

As a traditional ‘water person’ this was the first time I’d met actual rocket scientists. When engaging in conversation with those outside my division, I learnt how important it was to emphasize where I was doing my study. I’d often introduce myself with a short explanation of my topic – “I study water sources and water extremes”. This was invariably met with the following response: – ‘Oh, cool! On what planet?!’.

Other highlights included a series of interesting talks about missions to Mars, and how JPL technology is improving the understanding of the California drought. However these were eclipsed by a visit to the JPLs laboratories and its Space Flight Operation Facility. It was impressive witnessing engineers making the satellites that I will use in future to understand the water cycle. The Space Flight Operation Facility is one of the most iconic rooms at JPL, and yes it is as impressive as it looks (see picture below).


Space Flight Operation Facility

After 3 months of data crunching, my time at the JPL came to an end. By this point we’d got some exciting findings, some of which we presented at a conference at Scripps Institute of Oceanography, San Diego. It was great meeting and working with such interesting people, experiencing the JPL’s facilities and developing a collaboration that has made a tangible contribution to the exploration of the complexities of water. We believe our work marks a significant step towards managing water resources in areas highly exposed to hydrological extremes.

Introducing the REACH Junior Global Advisory Panel


REACH announces a new advisory group of young water professionals that will provide critical input to the programme.

The Oxford-led REACH programme recently welcomed 15 outstanding young professionals to its inaugural Junior Global Advisory Panel (JGAP). The panel is a two-way exchange whereby REACH benefits from the panel’s experience and networks, while JGAP members broaden their knowledge of water security and work alongside our (senior) Global Advisory Panel of globally recognised experts.

The panel will bring fresh eyes to the programme, providing critical review of activities and strategic advice. Members have an impressive range of expertise and experience, in terms of geography (working in Africa, Asia, South America, Europe and USA), professional area (research, civil society, government and practitioner) and academic background (engineering, social and natural sciences). As ambassadors for the programme, they’ll be working creatively to promote our science outputs and support our goal of improving water security for the poor

abbas-shabana2Shabana Abbas
Junior Programme Manager, Aqua for
Shabana Abbas is Junior Programme Manager for the VIA Water programme at Aqua for All, a non-profit organisation based in the Hague, Netherlands. The programme is funded by the Dutch Ministry of Foreign Affairs and aims to support innovative ideas and solutions to pressing water-related needs in cities of seven African countries.  Shabana has an MSc in Environmental Planning and Management.

brown-colin2Colin Brown
Advisor to the United Nations Special Rapporteur on the human right to safe drinking water and sanitation, Oswaldo Cruz Foundation
Colin has experience working on research projects related to water supply and sanitation services in rural Brazilian communities. He has a Master’s degree in Social and Human Sciences with a specialisation in Environmental Policies and Social Practices.

carole-bonguen-onouck-rolandeBonguen Onouck Rolane Carole
Assistant, Ministry of Environment, Protection of Nature and Sustainable Development, Cameroon
Bonguen is interested in sustainable water availability in developing countries and transboundary water resources management. She has an academic background in law, strategic studies and political science.

farrow-tylerTyler Farrow
International Programme Officer, Water Witness International
Tyler is a specialist in novel water security interventions including social accountability monitoring and private sector water stewardship, and sits on the Technical Committee of the Alliance for the Water Stewardship. He has an MSc in Water Security and International Development.

gain-animesh-kumarDr Animesh Kumar Gain
Research Fellow, GFZ German Research Centre for Geosciences
Animesh has expertise in the interdisciplinary field of water resources management including water security, the water-energy-food (WEF) security nexus and climate change adaptation. He has an MSc in Water Resources Development and a PhD on ‘science and management of climate change’.

guta-eyassuEyassu Guta
Technical and Program Support Officer, Ministry of Water, Irrigation and Electricity, Ethiopia
Eyassu is a Soil and Water Conservation Engineer providing technical support to the Water Sector Working Group Secretariat in Ethiopia’s Ministry of Water, Irrigation and Electricity. In his current role he has supported the development of an integrated water resource management framework, the national urban sanitation strategy, the water sector disaster risk management strategy, and a manual for accelerating self supply.

jones-stephenDr Stephen Jones (Co-Chair)
Director of DRC WASH Consortium, Concern Worldwide
Stephen is a WASH specialist with experience across programme management, technical advice, policy analysis and applied research. He currently directs the DRC WASH Consortium, a £30m DFID-funded rural WASH programme of five INGOs in DRC. He has a PhD on the sustainability and financing of rural water supplies in Mali.

luseka-euphresiaEuphresia Luseka
WASH Governance Specialist, USAID – KIWASH Project, contracted by DAI
Euphresia is a social entrepreneur in the WASH sector specialising in the institutional strengthening of water utilities in both urban and rural areas. She currently works for the Kenya—Integrated Water, Sanitation, and Hygiene Project (KIWASH). She has an MSc in Education for Sustainability and a BSc in Environmental Studies and Community Development.

moore-scottDr Scott Moore
Young Professional, World Bank Group
Scott is a political scientist whose research focuses on environmental politics and policy reform, especially related to climate change and water scarcity. He currently works at the World Bank’s Water Global Practice, primarily on transboundary water security and climate change in East and South Asia. Scott holds a PhD in Politics and an MSc in Environmental Change and Management.

ojomo-edemaDr Edema Ojomo (Co-Chair)
Postdoctoral Researcher, University of North Carolina and Chapel Hill
Edema’s expertise is in evaluating and shaping the enabling environment for WASH programmes. She works on public health projects and conducts research related to WASH with a focus on low-income countries. Edema has degrees in Chemistry, Chemical Engineering and Environmental Sciences and Engineering.

ortigara-angelaDr Angela Renata Cordeiro Ortigara
Programme Officer, UNESCO World Water Assessment Programme
Angela contributes to the production of UNESCO’s World Water Development Reports and has recently coordinated the Capacity Building Programme on Water and Sustainable Development. She holds a PhD in Environmental Engineering and has expertise in wastewater management and the linkages between water use and sustainable development.

saiyasith-ounheuanOunheuan Saiyasith
Senior Program Officer for Water Resources, Department of Foreign Affairs and Trade, Australia
Ounheuan has a Master’s degree in International Development focusing on livelihoods adaptation, rural development and resettlement in Laos. Currently, he is based in Laos working on water resources management in river basins of mainland south-east Asia.

shemie-danielDaniel Shemie
Strategy Director for Water Funds, The Nature Conservancy
Daniel serves as Strategy Director for Water Funds at The Nature Conservancy. In this role, he manages efforts to accelerate the adoption and financing of natural infrastructure in water supply. Before joining the Conservancy, Daniel was a partner at mWater, a non-profit that develops mobile water monitoring technology. He has an MSc in Water Science, Policy and Management.

umupfasoni-lylioseLyliose Umupfasoni
Independent consultant, Experts D’afrique
Lyliose holds a bachelor’s degree in Soil and Environment Management, and is currently pursuing a Master’s programme (MBA) in Leadership and Sustainability. She has experience in environment, and water and sanitation development activities, mainly focusing on policy development, law formulation, planning, and monitoring and evaluation. Lyliose recently worked as a Program Officer for the African Ministers’ Council on Water (AMCOW).

wultetawu-abera-workuDr Wultetawu Abera Worku
Postdoctoral Researcher, Trento University
Wuletawu has a PhD in Environmental Engineering and expertise in water budget modelling, hydrological modelling, eco-hydrology, hydro-geomorphology, remote sensing and GIS for hydrological applications. His research aims to improve estimations of all the water budget components (precipitation, evapotranspiration, runoff and discharge) and error quantification of the estimation procedures.

A version of this post first appeared on the REACH website.