Otto Lehmann (physicist)

Otto Lehmann around 1907 in his laboratory at the TH Karlsruhe

Otto Lehmann (born January 13, 1855 in Konstanz , † June 17, 1922 in Karlsruhe ) was a German physicist and spiritual father of liquid crystal research.

Life dates

Lehmann studied natural sciences at the University of Strasbourg between 1872 and 1877 and then did his doctorate under Paul Heinrich von Groth , the founder of the Zeitschrift für Mineralogie und Kristallographie (1877), on physical isomerism ( isomers are molecules of the same sum formula but different structure). First he was a teacher of physics , mathematics and chemistry at the middle school of Mulhouse in Alsace , before he became a lecturer in physics on October 1, 1883 at the then Kgl. Technical University in Aachen was employed.

In 1888 Lehmann was appointed professor of electrical engineering at the Kgl. Saxon Polytechnic called to Dresden . But just one year later he became Heinrich Hertz's successor at the Physics Institute of the Technical University of Karlsruhe with a teaching position for physics and electrical engineering. He remained associated with this university, whose rector he was in 1900 and 1901, until the end of his life in 1922.

After 1912, Lehmann was proposed several times for the Nobel Prize, but never received it. In 1891 he was elected a member of the Leopoldina . In 1909 he became an associate member of the Heidelberg Academy of Sciences and in December 1912 a corresponding member of the Académie des Sciences in Paris. Lehmann was an honorary member of the Karlsruhe fraternity Tulla .

His son Karl Otto (1903–1976) studied electrical engineering in Karlsruhe and later became a professor there.

Liquid crystals

Lehmann's scientific life's work can be summarized with the title of his main work published in Leipzig in 1904 : Liquid Crystals . Since the 1870s Lehmann systematically investigated crystal growth and modification changes in crystalline substances. Among other things, he used a crystallization microscope that he developed in 1877 and improved in 1884 and 1890 . He also used this microscope to examine the internal structure of bodies. His Molecular Physics , published in Leipzig in 1888/89, summarizes the entire knowledge of the physics of matter at that time. Lehmann showed, among other things, “that if one goes beyond the elastic limit with the forces acting on a solid body, changes in shape of the body occur which are essentially viewed as overcoming the rigidity, as a slow flow similar to the flow of liquid can. "He had already observed in 1877 that the" viscous modification of iodized silver may have an octahedral shape. "

In 1888 the Austrian botanist and chemist Friedrich Reinitzer (1857–1927) from the German Technical University in Prague reported that he had observed splendid color phenomena and the simultaneous appearance of crystals and amorphous molten mass when two substances were heated and then cooled in polarized light. Reinitzer had discovered cholesterol benzoate and found that this strange substance melts at 145 ° C, but only becomes a clear liquid at temperatures above 179 ° C. At temperatures above 145 ° C and below 179 ° C, the fabric looks cloudy and milky. Lehmann examined these so-called Reinitzer preparations and recognized that cholesterol benzoate and the silver iodine he was investigating also have a third phase between the liquid and solid and show identical behavior in this intermediate phase, such as strong birefringence under the polarization microscope . Lehmann began with the systematic investigation of the substances and subsequently found more than 100 substances with similar behavior. Lehmann called these substances "flowing crystals".

In 1891 Lehmann founded the crystal analysis named after him . He compared a known compound with the one under investigation for identical properties. For this he used the crystallization microscope he had constructed. According to Lehmann, crystalline bodies differ from amorphous bodies in that amorphous bodies have no direction of growth. In 1904 Lehmann published his work Liquid Crystals as a summary of his research results . Until his death in 1922, liquid crystals remained a central theme of his research. With this topic Otto Lehmann left behind a chapter in physics that was almost completely researched for the time. But initially there was no technical application for the phenomena he discovered. The “liquid crystals” and their discoverer were more or less forgotten for almost sixty years.

LCD

It was not until the late 1960s that people began to work intensively with Lehmann's liquid crystals. In 1971 the Swiss chemical company F. Hoffmann-La Roche produced the first prototypes of liquid crystal displays based on the principle of the nematic rotary cell ( LCD = Liquid Crystal Display ) with organic liquid crystals ( Schadt-Helfrich cell . Liquid crystals are characterized by the fact that they are within a temperature range ( about −20 ° C to +70 ° C) between the normal liquid and the solid phase there is a liquid state in which the molecules of these substances are in at least one spatial direction (nematic phase) or additionally in layers ( smectic or cholesteric phase The individual layers can be moved and rotated against each other and within themselves. The entirety of these phases between isotropic liquid and solid is called the mesophase . As in a crystal, the order of the molecules here leads to optical effects (birefringence, polarization of light, etc.) With the LCD, the molecular stream is generated by applying electric fields Structure in the mesophase is influenced in such a way that the light refraction of the liquid crystal changes reversibly, i.e. the liquid crystal returns to its original state after the electrical field has been switched off. This behavior has an optical effect in polarized light, for example, as a brightness contrast, which makes it possible to display numbers or letters. The advantage of this liquid crystal display is the extremely low electrical power in the range of a few µW / cm², which is required to operate such a display. After the problems resulting from the electrical control in so-called “passive matrix displays” (e.g. low contrast, reduced viewing area, long switching times) had been solved by the active matrix control, liquid crystal displays in the middle appeared The 1970s first appeared as a technical application in digital watches . They are now widely used in all kinds of technical applications. They are inexpensive and hardly require any electrical energy. According to the way in which they create their respective images or pixels (= picture elements ), a distinction is made between TN (= Twisted Nematic ) -, STN (= Super Twisted Nematic ) -, D-STN (= Double Super Twisted Nematic ) - Displays or so-called TFT (= Thin Film Transistor ) displays. With TFT-LCDs, each pixel is controlled by its own transistor .

Otto Lehmann Foundation

Since 1998, an Otto Lehmann Foundation has awarded a prize named after the important physicist for the promotion of scientific work by young professionals in the field of liquid crystal displays. The triumphant advance and spread of portable computers in notebook format with liquid crystal displays and the fact that today hardly any machine, electrical device or technical application can do without an LCD is the most obvious proof of the enormous technical and economic importance of Otto Lehmann's discoveries.

Works

• Self-made physical apparatus . Leipzig 1885.
• Molecular physics . 2 volumes (1888/89). Leipzig.
• The crystal analysis . Leipzig 1891.
• Electricity and Light . Braunschweig 1895.
• Liquid crystals . Leipzig 1904.
• The apparently living crystals . Esslingen 1907.
• The most important terms and laws of physics . Berlin 1907.
• The crystallization microscope and the discoveries made with it, especially those of liquid crystals. From the Festschrift of the Fridericiana for the 53rd birthday of Sr. Königl. Highness of the Grand Duke Friedrich II of Baden . Vieweg, Braunschweig 1910.
• Liquid crystals and their apparent life. Research results presented in a movie . Voss, Leipzig 1921.

literature

• Peter M. Knoll, Hans Kelker: Otto Lehmann - researcher of liquid crystals. A biography with letters to Otto Lehmann . Private publication, Ettlingen 1988.
• Klaus Ricking: Otto Lehmann - Liquid Crystals and their Apparent Life . In: Peter Johannes Droste (Ed.): Made in Aachen. Contributions to the regional technical, economic and social history . Aachen 2000, p. 56-59 .
• Timothy J. Sluckin, David A. Dunmur, Horst Stegemeyer (eds.): Crystals that flow. Classic papers from the history of liquid crystals . Taylor & Francis, London / New York 2004, ISBN 0-415-25789-1 .

Web links

Commons : Otto Lehmann  - Collection of images, videos and audio files

Wikisource: Otto Lehmann  - Sources and full texts

• Otto Lehmann Foundation
• Literature by and about Otto Lehmann in the catalog of the German National Library

Individual evidence

1. ^ List of members since 1666: Letter L. Académie des sciences, accessed on January 11, 2020 (French). 
2. ↑ Karlsruhe Burschenschaft Tulla 1893–1993 . Karlsruhe 1993 p. 29
3. ↑ Otto Lehmann: Liquid crystals: as well as plasticity of crystals in general, molecular rearrangements and changes in the state of aggregation . Wilhelm Engelmann, Leipzig 1904 ( limited preview in the Google book search).
4. ↑ Details from Gerhard H. Buntz: Twisted Nematic Liquid Crystal Displays (TNLCDs) - An invention from Basel goes around the world. (PDF) (No longer available online.) In: “Information” No. 118 of INTERNATIONAL TREUHAND AG, Basel. October 2005, archived from the original on May 9, 2006 .