Author Archives: Andrew Sun

About Andrew Sun

I am interested in rheology.

The dynamic modulus of water

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Recently I tried to measured the dynamic modulus of water using a rotational rheometer (AR-G2, TA Instruments).

Water is a Newtonian fluid at room temperature (T = 25°C). Using a cone-and-plate geometry of 60 mm/1° made of hard aluminium, I found that the lowest torque measurable is 1e-7 N·m, ten times higher than the claimed 1e-8 N·m on the instrument’s specification. The lowest shear rate that can generate a measurable torque is about 1.8 s-1 under steady flow test. The steady state viscosity was measured to be 1.02 mPa·s.

Steady flow test of water

Steady flow test of water

To measure the dynamic modulus, the strain amplitude was set to 500%, and the data were averaged over 5 cycles. Meaningful results can be obtained at the range of ω = 1.0 ~ 10.0 rad/s.

Dynamic modulus of water

Dynamic modulus of water

As a Newtonian fluid, the storage modulus of water G’ = 0, and the loss modulus G” = ηω. The solid line in the figure is a fit of such relationship and the viscosity η = 1.14 mPa·s.

The dynamic modulus of water can also be measured by particle tracking microrheology or optical tweezer microrheology (see arXiv:1102.3035 [cond-mat.soft]).

Dynamic modulus of water by microrheology

Dynamic modulus of water by microrheology

Some history of LAOS

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Detailed historical account of LAOS rheometry can be found else where. I add here only my own interest in obsolete instrument designs.

DAQ from a Weissenberg rheogoniometer R16

DAQ from a Weissenberg rheogoniometer R16 (Bogie 1966)

The history of LAOS as a test condition is as old as that of oscillatory shearing rheometry. The use of Lissajous figure was first proposed as early as 1944.  K. Weissenberg first proposed a harmonic analysis of LAOS data in 1964. In fact it was not easy for early experimentalists to maintain or measure small deformation esp. for complex fluids before necessary technology break through in actuators and transducers (e.g. linear voltage differential transformer, LVDT). LAOS was therefore more frequently reached in the past than in today whenever an oscillatory shearing was performed.

Harmonic decomposition of the LAOS data was done in various way before the age of computer. One way was fitting the data with a Volterra intergral equation of a limited number of orders to find the corresponding high order kernels (which was not a real decomposition).

Solartron advertisement

Solartron transfer function analyser (TFA) advertised on Sci. Am. (1959)

The commercial success of Weissenberg rheogoniometer series evoke the desire to extract higher harmonics from the electric signals. One noticeable idea by Harris and Bogie was using the once-called ‘dynamic analysis’ system, product of Solartron Electronic Groups Ltd. It was in essence a cross-correlation method to obtain higher harmonic information on an analog circuit design. The system consisted of a resolved component indicator (or transfer function analyser, as it was more frequently called) excited by a pair of in-phase / quadratic signals from a mechanical reference generator†.

The transfer function analyser technique lasted for as long as one decade on Weissenberg rheogoniometer as its model upgraded from R16 to R18, but bolder try on a computer using FFT was performed in as early as 1971. The analog signal was sampled and input in a signal averager to raise the S/N ratio. From then on, the later development was not hard to imagine: smaller and faster computers coupled with more sensitive and accurate measurement systems.

A major transition occurred, though, from Weissenberg rheogoniometer to the Rheometrics mechanic spectrometer (RMS) series during the 1970s, which also shift the LAOS and harmonic analysis to the new device. The device had a few of new feature compared to the Weissenberg counterpart. First, it directly gave the result of G’ and G”, otherwise the earlier rheologists had to manually calculated from the Lissajous curves. Second, it provided an oven chamber to perform high temperature experiment, a feature not easily achievable with old rheogoniometers. And third, it upgrade the driving system from gears-based to electronic motor††. These features gained a wide acceptance very fast, and ARES soon became a standard of rheometer as well as the platform for LAOS, as the Weissenberg rheogoniometer once had been.

† Interested readers may refer to a technical report of the former Royal Aircraft Establishment in 1964 about a measurement system for amplitude and phase available here, which may give a brief description of the technical status at that time.

†† Interested readers are referred to Rev. Sci. Instrum. 1984, 55, 1675.

Book review: Principles of measurement systems

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Principles of Measurement Systems (4th Edition) (Paperback)

By (author) John Bentley
List Price: $115.20 USD
New From: $84.89 In Stock
Used from: $74.50 In Stock


The principles of measurement should have been much more frequently involved than only when we need to customize an instrument. At present, the measuring techniques have completely digitized. The principles we learnt in undergraduate physics experiment class may not applicable now. Before the electronic age, most measuring instruments convert the physical quantity into a pointer-scale indicator where human eyes are required to determine the quantity. For example the thermometer converts the temperature into the height of the liquid in a tube. Even the reflective index, if measured by an classic Abbe reflectometer, was converted into the sharpness of the edge between two blocks of color, also requiring human eyes. Students are trained to read as accurately as possible by eyes in the labs during undergraduate study, but after they enter their PhD projects they practically encounter only modern instruments which display digits on a screen or export ascii text files in a computer. Therefore they easily lost the caution of measurement non-ideality, or they lack the knowledge to care about this†.

The modern measuring system starts almost inevitably from sensors, a large class of electronic components, rather than the human eyes. Digits on the screen can of course be consistently read by all people including the physically challenged provided additional techniques. This is the triumph of the electronic age indeed. But the problems do not disappear; they are instead hidden inside the instrument between lines of the electronic circuits. What was once understood in an undergraduate class becomes too technical to explain now. Therefore although we have long entered the electronic age we still skip this part in the undergraduate education. And most graduate students trust the instrument companies very much in their PhD project until the problem becomes explicit. Problems become explicit when we need to design a measurement system customized for a research purpose.

In China there is such a class called like “Principles of Electronic Measurement”. But it is limited in the departments of electronics, electrics, industrial control, automation, etc., out of the sight of the students of physical sciences. But my feeling becomes stronger and stronger that experimental physical research must based on full knowledge about the general measurement principles as well as emerging measuring techniques. Otherwise, the perspectives of scientific research is limited by instrument company instead of pioneered by scientists. I am interested in how widely students in other countries can access this course.

That’s why I, different from the reviewers on its item page on Amazon.com, think this book useful. Indeed as one of the reviewer commented:

If you hope to use this book to get a general overview of measurement systems, then you may be alright, but if you want to be able to perform actual calculations, you should look elsewhere.

This may be insufficient for a text book for professional fields e.g. applied electronics or industrial control, but the practical case for physical sciences is that students need informed in the first place what they need to care about, in a systematic way rather than tips and tricks. I also compared this book with several Chinese textbooks of electronic measurement, and I found the selection of content is the best in Bentley’s book for informing electronic non-experts the necessity to care about what. It becomes rather easy task for one to find more literature for special cases after informed, whereas being completely uninformed or knowing only random, incomplete tips will mislead a researcher quite far.

† I am, of course, talking about the situation in China only.

Visiting KIT

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I am visiting Prof. M. Whilhelm’s lab here in Karlsruhe Institute of Technology since May 15 until May 22 to be trained of FT-rheology technique. The information of the first two days was rich and I just had time to write a summary in Chinese.

I feel quite at home in Germany. The food here is very similar to what I can eat in a typical western restaurant in Guangzhou. Nowadays the food culture is quite globalized among big cities I guess. And I get on well with the group members.

In the first two days several students there introduced what they did to me whenever they had time. And the most helpful discussion was with Kathrin who is researching rheology. I also had a much deeper discussion with another student Deepak who is currently focus on the rheology of polymer/carbon nanotubes composite. We found common interest in the relationship of percolation threshold with the LAOS phenomena. And I also learned useful way of thinking when it comes to developing new measuring parameters. It is somewhat unexpected that I even found common interest with students doing synthesis. Alicia is now working with PS-PLLA copolymers, and I worked on PEO-PLLA copolymer during master years too (see my introduction page). So we went into very detailed discussion in the experimental tricks and problems.

I hope I did not expose too much above… I just want to thank them for sharing. But this is exactly the problem I want to talk about here: where should we draw the line between the secret and the share-able?

After a submission of a manuscript to a journal but before it’s accepted, it is a common practice that the manuscript be classified. This mean there is indeed a stage when the finding is secret to others. But what about cooperation? If I am asked about cooperation in future project, it of course means that we have to share ideas that is not published. What is the guideline here? And if there is not a clear deal whether we are going to cooperate or not, as a visiting student should I still share as much as possible? Otherwise why am I visiting?

Maybe the answer is not straight forward and delicate but I would like to know your experience, if any.

Some remarks on the too-many-PhD topic

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Too many science

As Nature started a special issue of it, the too-many-PhD topic reached a new heated status. However I feel that the problem people really worry with is not about PhD, but the whole scientific infrastructure, although this case the problem becomes even nowhere to start with.

I really feel that we do not need so many scientists and research projects. I keep following the latest research by RSS subscription of major journals of my interested fields. It turns out that I skip 90% of the titles because these papers give no new knowledge, being only the publish-or-perish asignment of the authors. A huge amount of papers published, hence experiments done, hence money granted and resources used, is only for the positive feedback of the publish-or-perish fear nowadays ruling the whole scientific infrastructure. “Too many PhD” is only the side effect.

Many scientists feel the grave pressure of peer-reviewing burden. They find themselves reviewing mediocre papers and applications nonstop, meanwhile fabricating mediocre papers and applications nonstop. Publishers expand their flagships of “high impact journals” at a scary speed — just have a look at the new journals of ACS these years. What’s essentially different between Nano Lett. and ACS Nano? And this works just because the price is on the university libraries, who have to pay the subscription however high the cost it may be in order not to fall behind.

The free market fails to react against the overproduction of PhD because the publishing-or-perishing professors need more PhD students constantly to complete more research projects. If the young people, seeing the bad market feedback of the PhD degrees, refuse to go for PhD study, the professors will blame the government and the society, for not doing enough to encourage young people’s interest in science in high schools, or to create the positive and fun side of science among the public. So even more money have to spent on this, too, and really successfully, in attracting more young people into the in fact not-so-promising PhD career.

Too many PhD, too many papers, too many journals, and too much money spent on science… these are all heated topics in recent years but just the side effects of too much science.

To select or to culture?

Education in China have long suffering and been criticized of the wrong emphasis on selecting the talented rather than culturing them. Our high schools and universities teach weird but complex things which have no effect on establishing good personality and thoughts but quite effective in selecting those with higher IQ. And the society understands this, so in the labor market the employers care nothing about your field of training but only your level of academic degree. There is no hope that the professional training system really teaches the students things. The only hope lies in the IQ represented in how many levels of tests the candidate has overcome. Candidates with higher IQ, even though knowing nothing about the profession, at least learns faster. The result is the lack of professionalism in today’s China. This is the effect of a system paying too much emphasis on selecting talents.

Things seem quite similar in our scientific infrastructure globally. Unfortunately, we cannot tell in advance who is more talented in doing really high impact research (twofolds: not knowing who can, and not knowing which is high impact), so the only, though not quite intelligent, way is to first let all people do something before selecting those that really can do science. We select these excellent candidates out by evaluating many things such as citations, the h-index, etc., which is also a topic in constant debate. Then we reward them with research funding, which defines their survival in the whole field, and creates the publish-and-perish rush. As said, this is unfortunate but necessary.

So similar to China’s ill education system which selects rather than cultures youth, the selecting rather than culturing scientific infrastructure also generates many weird but complex research projects. Money has been spent on these projects most of which being just brushed off, and even more money has to spent on the rest excellent candidates because now they are going to do really high impact research. Who cares the actual quality of the research? The peers? Oftentimes the answer is that even the peers know limitedly about how good is the research. They as well rely on the selecting feature of the whole infrastructure, i.e. citation, h-index, etc. After all, who knows which research project will lead to the final cure of cancer, although all of them claims so?

It costs to know more

And people tend to ignore this bad news as if they don’t know this simple fact at all.

Our botanists have discovered the cell, and in the cell protoplasm, and in that protoplasm still something more, and in that atom yet another thing. It is evident that these occupations will not end for a long time to come, because it is obvious that there can be no end to them, and therefore the scientist has no time to devote to those things which are necessary to the people.

– Leo Tolstoy, On the Significance of Science and Art, Chapter IV

The huge input of money allows scientists today to push toward the finest ends in every finest discipline. The need to know and control the nature is unlimited, so is the cost. It costs exponentially more to see the atoms than to see the cells, now we have to even set up the LHC to see even more. When it comes to control, an example is the EU’s REACH, which will cost €9.5 billion and 54 million animals on toxicity testing over the next decade, said some research. However, REACH’s “white list’ policy ensures the most control on the unknown nature of chemical safety. We seem too eager and generous in knowing more about the nature than what the nature can support us to do so. And no one can give the most authorized selection to reduce this “to-know” list so we have no choice but go on.

Gap dependent rheology on rotational rheometers

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ResearchBlogging.orgGrenard, V., Taberlet, N., & Manneville, S. (2011). Shear-induced structuration of confined carbon black gels: steady-state features of vorticity-aligned flocs Soft Matter DOI: 10.1039/C0SM01515F

Conventional rotational rheometers allow test procedures of varying shear strain/stress, temperature and time, corresponding to multiple “sweep” tests. Modern rheometers are indeed designed to accurately vary these parameter according to command. Besides these conventional sweepable variants, comparison among different geometries (in bulk materials and surface) was also done by many researchers, though such experiments can only be done discretely; you cannot do a “geometry sweep”.

There are also cases when the gap confining the sample plays an significant role in its rheological propertes.

One of such cases is merely a form of experimental error. When we interpret the experimental result from a plate-plate geometry we assume that the two plates are perfectly concentric and parallel. Practically the two plates must have an extent of imperfection, and a tilted height between the two parallel always exists, although it is often negligible compared to the most used gap values. When the gap gets smaller until a value comparable to the tilted height of the plate-plate geometry, the inevitable effect is a gap dependency of the result. Procedures have been proposed by many authors to model the effect of this nonparallel imperfection on the result of viscometry. A common experimental technique to probe this effect is to run a series of tests at different gaps with a Newtonian fluid. In this case the gap dependency gives information of equipment rather than the tested samples.

There should be interesting gap dependence rheology that are not resulted from instrument imperfection but the confinement effect of the tested sample. Clasen and McKinley developed a device specifically for measuring complex fluids (which are often heterogeneous with micron-scaled structrures) between very small gaps, and indeed observed gap-dependent viscosity and yielding phenomena at a shear rate much lower than conventional rheometers can reach. The gap-dependent results are specifically from a very small gap limit.

Is there gap-dependent phenomena in the gap range of conventional rheometry? Recently I noticed the reported observation of shear alignment of carbon black suspensions, which show a gap-dependence topology. Carbon black particles suspended in a light mineral oil tend to align into parallel stripe of flocs under shear field. Counterintuitively, the stripe’s dimension is independent of shear rate, particle fraction, and location of observation. The only significant factor that found to affect the stripe width is the gap of shear geometry. Particularly, a power law dependence was observed. The simplicity of physics here is in itself beautiful. And the finding is also indicative in manufacture technique of width control stripes of anything. However, the author did not measure the effect of such shear induced structuring on the viscoelasticity of the suspension. It is equally interesting to know whether or not the stripe structures affect the viscosity of the bulk suspension.