Making a Splash in Liquid Science

Agnes Pockels’s kitchen experiments advance the science of surface tension. 

American chemist and physicist Irving Langmuir won the Nobel Prize for Chemistry in 1932 for discovering that a monolayer of elongated molecules sits on the surface of liquids; oil films on water. What few people know is that his work was made possible by the late-1800s kitchen experiments of a young German woman with no formal scientific training.

Agnes Pockels was doing the dishes one day when she noticed how fast the greasy water moved when the soap first hit it. This sparked an experiment that would lead to her making the very first direct measurements of the surface tension of a liquid. 

“This is really true, and no joke or poetic license: what millions of women see every day without pleasure and are anxious to get rid of, i.e. the greasy washing-up water, encouraged this girl to make observations and eventually do … scientific investigations,” her relative Elizabeth Pockels observed.

First, Agnes filled up a long, rectangular tin tray with water. Next, she laid a strip of tin across the liquid’s surface to divide the tray and act as a moveable partition. Finally, she attached a button to a balance to act as a float, to measure the changes in tension of the water's surface. Now her experiments could begin. She saw currents stream when she added salts, the surface tension change if she added oil or alcohol and slid the partition. The surface tension of a strongly contaminated water surface varies with the size of the surface.

The year is 1880 and 18-year-old Agnes is longing to be heading off to university to begin a physics degree, but women, as a rule, were not yet admitted to higher education institutes. Besides, she was needed at home to care for her ill parents and manage the household. 

“I had already developed a passionate interest in the natural sciences, especially in physics, and would have liked to become a student, but at that time women were not accepted for higher education and later on, when they started to be accepted, my parents nevertheless asked me not to do so,” Agnes wrote in her diary. “Like a soldier I stand firm at my post caring for my aged parents.”

Agnes’s parents were from Germany, but she was born in Venice, Italy while her father was serving in the Austrian army. Her brother, Friedrich, also known as Fritz, was born three years later. Living in malaria-infected Northern Italy meant that before long, the whole family was experiencing chronic health issues. They returned to Brunswick, Germany in 1871 upon her father’s early retirement, for medical reasons. 

Back in Germany, Agnes began attending Municipal High School for Girls. She found it infuriating that sciences were not part of the curriculum until senior year. The boys, on the other hand, got to study chemistry, physics, and mathematics for several years. She learned all she could by borrowing her brother’s textbooks. 

“I attempted to continue my education by my own devices, first of all by the use of a small text book by Pouillet-Müller and since 1883 by means of a book provided by my brother, Friedrich,” Agnes wrote. “However, this type of training did not take me far in respect of the mathematical approach to physics, so that I much regret to have but little knowledge of theoretical matters.”

It just so happened that 1883 was the year Fritz began to study for the degree Agnes could not: physics. He first attended a local school, then moved to the University of Göttingen to continue on for a PhD. Agnes took this opportunity to send her findings to a physics professor there, but they showed little-to-no interest in the at-home tinkering of an untrained woman. 

Fritz is credited with having discovered what’s known as the Pockels electro-optic effect: as light passes through them, some crystals will rotate its plane of polarization in an electric field. He found a post as physics professor at Heidelberg University. 

For ten years, Agnes worked in her family’s kitchen studying the properties of liquid surface tension. She carefully repeated her experiments over and over, and kept fastidious notes of all of her observations. 

“Agnes Pockels' research, developed almost entirely independently, are meritorious by any standard. They show a clarity of thought and observation, and strictness of scientific approach remarkable for a girl of her years who had no formal training. When examined however against the background of her life they become truly astonishing,” a 1971 article in Chemistry and Industry proclaimed. “Her diary illustrates the difficulties she faced in trying to maintain her own health, the health of her parents and continue her scientific research at the same time.”

In 1890, Agnes began subscribing to the German science journal Naturwissenschaftliche Rundschau. This was just in time to read an abstract of a paper on the subject of liquid surface films originally published in English science journals by physicist Lord Rayleigh. She immediately wrote to him about her work. Luckily, Lady Rayleigh was fluent in German and quickly translated the letter for her husband. 

“My lord, Will you kindly excuse my venturing to trouble you with a German letter on a scientific subject? Having heard of the fruitful researches carried on by you last year on the hitherto little understood properties of water surfaces, I thought it might interest you to know of my own observations on the subject. For various reasons I am not in a position to publish them in scientific periodicals, and I therefore adopt this means of communicating to you the most important of them.”

Rayleigh was thoroughly impressed by her research. He wrote back to Agnes asking her to clarify a few points. He determined to get her discoveries the attention they deserved. Rayleigh forwarded Agnes’s original letter to Nature for publication with the accompanying note: 

“I shall be obliged if you can find space for the accompanying translation of an interesting letter which I have received from a German lady, who with very homely appliances has arrived at valuable results respecting the behaviour of contaminated water surfaces. The earlier part of Miss Pockels’ letter covers nearly the same ground as some of my own recent work, and in the main harmonize with it. The later sections seem to me very suggestive, raising … many important questions. I hope soon to find opportunity for repeating some of Miss Pockels' experiments.” 

Nature published Agnes’s letter on March 12, 1891 under the concise title “Surface Tension.” 

Through her own chronic pain, Agnes went on to publish a dozen scientific papers and translate a textbook. Still, caring for her family took precedence. “Since my time was much in demand for home nursing, I was only rarely able to conduct experiments after 1902,” she wrote. When her brother died in 1913 she lost her one connection to the world of German physics and up-to-date publications. “In the end I completely lost contact with research in my field, the deterioration in my eyesight and my health altogether being a contributing factor,” Agnes lamented. 

Still, her methods made a splash in the study of liquid physics. 

“The surface balance technique Pockels developed became useful in physical chemistry for determining the size and shape of organic molecules at a time when X-ray diffraction was not yet available,” noted M. Elizabeth Derrick in the 1982 book Women in Physics and Chemistry. “Her surface film balance technique … is still used. Pockels's description of how she introduced water-insoluble compounds to the water surface by dissolving them in an organic solvent, applying drops of the solution, and then allowing the solvent to evaporate is now the standard technique used. Clean surfaces are a major problem for surface experimentation. The technique she developed for ensuring a clean surface has become standard procedure.”

Luckily, she lived long enough to see the impact of her research: “I learned to my great joy that my work is being used by others for their investigations.”

She also lived long enough to achieve some long-overdue recognition. In 1931, she won the Laura R. Leonard Prize of the German Colloid Society “for her quantitative investigation of the properties of interfaces and surface films, and for the methods she used, which have since become fundamental in modern colloid science.” The following year, on her 70th birthday, she became the first woman to receive an honorary doctorate from the Technical University of Braunschweig. She died in 1935, so she also lived long enough to see Langmuir win the Nobel Prize for his work that was made possible by her own.

Further Reading:

Pockels' Trough,” by Andrea Sella, Chemistry World, May 25, 2015.

Who was Agnes Pockels?” Technical University of Braunschweig.

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