Essentials of Soft Matter Science

Francoise Brochard-Wyart, Pierre Nassoy, Pierre-Henri Puech

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9781498773966
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Condensed Matter

Authored by world-leading physicists, this introductory textbook explores the basic principles of polymers, colloids, liquid crystals, wetting, and foams. It is a practical 'toolbox' for readers to acquire basic knowledge in the field and facilitate further reading and advanced courses. Undergraduate students in physics, biology, and the medical sciences will learn the basics of soft matter physics, in addition to scaling approaches in the spirit of the Nobel prize laureate in physics in 1991, Pierre-Gilles de Gennes, the inventor of soft matter physics and close collaborator to author Françoise Brochard-Wyart.

Features:

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• Accessible and compact approach

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• Contains exercises to enhance understanding

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• All chapters are followed by a short 1-2 page "insert chapter" which serve as illustrations with concrete examples from everyday life (e.g. the Paris Metro, a zebrafish, a gecko, duck feathers etc.)

CHAPTER 1

Introduction

1.1 The Birth of Soft Matter

P.-G. de Gennes (1932–2007) (Figure 1.1) is considered the inventor of the science called soft matter. After spectacular initial contributions in solid physics (magnetism, superconductivity), his career in the theoretical physics of condensed matter opened up to a very wide spectrum of subjects, namely liquid crystals, polymers, colloids, wetting and adhesion, and biophysics, which define soft matter. Although de Gennes’ record is very impressive, the importance of his work mainly relies on his style, in permanent contact with the experimentalists and the industrial world, and on the idea that all the physical phenomena can be explained in simple terms. Pierre-Gilles had the passion to transmit his knowledge and discoveries to a wide audience, from schoolchildren to researchers, with accurate and colorful words and an impetus that triggered scientific vocations. His work was rewarded by the Nobel Prize in Physics in 1991.

We shall start this book on the physics of soft matter with some excerpts from the Nobel lecture delivered by Pierre-Gilles de Gennes in Stockholm in December 1991.

FIGURE 1.1 Pierre-Gilles de Gennes.

1.1.1 What Do We Mean by Soft Matter?

Americans prefer to call it “complex fluids”. This is a rather ugly name, which tends to discourage the young students. But it does indeed bring in two of the major features:

Complexity. We may, in a certain primitive sense, say that modern biology has proceeded from studies on simple model systems (bacteria) to complex multicellular organisms (plants, invertebrates, vertebrates…). Similarly, from the explosion of atomic physics in the first half of this century, one of the outgrowths is soft matter, based on polymers, surfactants, liquid crystals, and also on colloidal grains.

Flexibility. I like to explain this through one early polymer experiment, which has been initiated by the Indians of the Amazon basin: they collected the sap from the hevea tree, put it on their foot, let it “dry” for a short time. And, behold, they have a boot. From a microscopic point of view, the starting point is a set of independent, flexible polymer chains. The oxygen from the air builds in a few bridges between the chains, and this brings in a spectacular change: we shift from a liquid to a network structure which can resist tension – what we now call a rubber (in French: caoutchouc, a direct transcription of the Indian word). What is striking in this experiment, is the fact that a very mild chemical action has induced a drastic change in mechanical properties: a typical feature of soft matter.

1.1.2 Style of Research in Soft Matter

1.1.2.1 Simple Experiments

I would like now to spend a few minutes thinking about the style of soft matter research. One first, major, feature, is the possibility of very simple experiments.(…) Let me take the example of surfactants, molecules with two parts: a polar head which likes water, and an aliphatic tail which hates water. Benjamin Franklin performed a beautiful experiment using surfactants; on a pond at Clapham Common, he poured a small amount of oleic acid, a natural surfactant which tends to form a dense film at the water-air interface. He measured the volume required to cover all the pond. Knowing the area, he then knew the height of the film, something like three nanometers in our current units. This was, to my knowledge, the first measurement of the size of molecules. In our days, when we are spoilt with exceedingly complex toys, such as nuclear reactors or synchrotron sources, I particularly like to describe experiments of this Franklin style to my students.

Let me quote two examples. The first concerns the wetting of fibers. Usually a fiber, after being dipped in a liquid, shows a string of droplets, and thus, for some time, people thought that most common fibers were non-wettable. F. Brochard analysed theoretically the equilibria on curved surfaces, and suggested that in many cases we should have a wetting film on the fiber, in between the droplets. J.M. di Meglio and D. Queré established the existence, and the thickness, of the film, in a very elegant way [1]. They created a pair of neighbouring droplets, one small and one large, and showed that the small one emptied slowly into the big one (as capillarity wants it to go). Measuring the speed of the process, they could go back to the thickness of the film which lies on the fiber and connects the two droplets: the Poiseuille flow rates in the film are very sensitive to thickness. Another elegant experiment in wetting concerns the collective modes of a contact line, the edge of a drop standing on a solid. If one distorts the line by some external means, it returns to its equilibrium shape with a relaxation rate dependent upon the wavelength of the distortion, which we wanted to study. But how could we distort the line? I thought of very complex tricks, using electric fields from an evaporated metal comb, or other, even worse, procedures. But Thierry Ondarcuhu came up with a simple method.

1. 1) He first prepared the unperturbed contact line L by putting a large droplet on a solid.
2. 2) He then dipped a fiber in the same liquid, pulled it out, and obtained, from the Rayleigh instability, a very periodic string of drops.
3. 3) He laid the fiber on the solid, parallel to L, and generated a line of droplets on the solid.
4. 4) He pushed the line L (by tilting the solid), up to the moment where L touched the droplets; then coalescence took place, and he had a single, wavy line on which he could measure relaxation rates [2].

1.1.2.2 Theory

1.1.2.2.1 Reducing a Complex Problem to the Essence

Working with experimentalists or engineers in industry, and later with biologists, when confronted with situations involving a large number of parameters, P.-G. de Gennes had the art of unveiling the central physical phenomenon. We are used to characterizing this approach by comparing it with the style of Picasso, in particular the famous series of “Bull” lithographs, in which Picasso sketches a bull with less and less detail to finish with a few lines. These drawings profoundly marked Pierre-Gilles de Gennes (Figure 1.2):

Everyone has his treasure of images of which we only had a glimpse but that we never forget. An example for me: Picasso painting with large white lines on a window and filmed by Clouzot. Everything I tried to painstakingly draw later was born from those moments.

(Pierre-Gilles de Gennes in L’émerveillement [3])

FIGURE 1.2 The Abduction of Sabines, drawn by Pierre-Gilles de Gennes in 1983 (left) during a stay in Florence (right: the sculpture of Giambologna in the Loggia dei Lanzi). Private collection and picture from FBW.

1.1.2.2.2 Formulating a Problem by Using Dimensional Arguments and Scaling Laws

P.‑G. de Gennes strives to give simple analytical expressions for his findings, even if they are most often the result of complex calculations. He always gives a physical interpretation and uses drawings to explain it, like the image of the blobs or the reptation model, which opened up the physics of polymers to a wide audience.

1.1.2.2.3 Having a Broad Scientific Culture to Make Analogies between Various Disciplines

I have e...