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Pitch drop experiment at the University of Queensland, Australia. Image licensed under CC BY-SA 3.0.

You are watching: Which liquid is the most viscous?

Viscosity is a measure of how strongly a fluid resists gradual deformation. Fluids with higher viscosity such as honey are thought of as “thicker” than less viscous fluids like water. The range of viscosities in liquids is enormous. Superfluids, such as Helium-II, have zero viscosity. Honey is between 2,000 and 10,000 times more viscous than water. Some fluids are so viscous they appear to be solid and yet, over time, slowly flow.

One of the most viscous liquids known is pitch, also known as bitumen, asphalt, or tar. Demonstrating its flow and measuring its viscosity is the subject of the longest continuously running scientific experiment, begun in 1927 at the University of Queensland in Australia. Prof. Thomas Parnell heated pitch (which dramatically decreases its viscosity), then poured it into a funnel with a sealed bottom. After three years (to allow the pitch to settle), the bottom of the funnel was removed and the funnel placed in a bell jar with a beaker below it. The pitch slowly flows out of the funnel, forming a large drop. About every decade a drop falls off into the beaker.

While each drop takes a decade to form, it only takes about a tenth of a second to fall. In the 89 years the experiment has been running, nine drops have fallen so far. Nobody has yet observed a drop falling. Since the 1990s the experiment has been continuously monitored by a Web camera, but when the eighth drop fell in November 2000 the camera was offline and the event was not recorded. In April 2014 the ninth drop was broken off while replacing the beaker beneath the funnel, which had been filled sufficiently by previous drops to interfere with the experiment. There is a Web site devoted to watching for the tenth drop to fall.

From the rate of flow over the duration of the experiment, it has been estimated that the pitch in the funnel has a viscosity 230 billion times greater than water. It is estimated that enough pitch remains in the funnel for the experiment to continue for another century.

Here is a time-lapse video of the descent of the ninth drop and the forming of the tenth drop covering the period from April 2012 through April 2015.

This Chemical Heritage Foundation video includes a description of the experiment by its long term custodian, the late Prof. John Mainstone.

A pitch drop experiment was begun at Trinity College in Dublin, Ireland in 1944. The experiment laid forgotten on a shelf for decades as several drops formed and fell. In April of 2013, as another drop was forming, a Web camera was set up to continuously monitor the experiment, and on July 11, 2013 the drop was observed to fall, the first time such an event had been recorded. Here is a news report from RTE about the experiment showing the drop falling.

Here is the full time-lapse of the Dublin drop falling.

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If you"ve reached that stage of life where the heart-pounding excitement of watching paint dry is too much to take, it"s easy to set up your own pitch drop experiment. All you need is a glass funnel and some pitch (you can buy high-grade optical pitch from GotGrit.com). I"d put a rubber stopper in the bottom of the funnel to initially seal it. Warm the pitch gently on a stove until it pours easily (use a disposable pan, since you"ll never be able to remove all the pitch), then pour it into the funnel. Wait a few weeks until the pitch has entirely cooled, then remove the stopper, set the funnel on a stand with something to catch the eventual drops, and away you go. Pitch comes in various grades ranked by “hardness” (actually, viscosity), and it"s better to choose one of the softer (lower numbered) grades. A pitch drop experiment was reportedly begun Aberystwyth University in Wales in 1914, but to date has yet to produce a single drop.