Experimental soft-matter science

Soft materials consist of basic units that are significantly larger than an atom but much smaller than the overall dimensions of the sample. The label “soft condensed matter” emphasizes that the large basic building blocks of these materials produce low elastic moduli that govern a material’s ability to withstand deformations. Aside from softness, there are many other properties that are also caused by the large size of the constituent building blocks. Soft matter is dissipative, disordered, far from equilibrium, nonlinear, thermal and entropic, slow, observable, gravity affected, patterned, nonlocal, interfacially elastic, memory forming, and active. This is only a partial list of how matter created from large component particles is distinct from “hard matter” composed of constituents at an atomic scale. Issues inherent in soft matter raise problems that are broadly important in diverse areas of science and require multiple modes of attack. For example, far-from-equilibrium behavior is confronted in biology, chemistry, geophysics, astrophysics, and nuclear physics. Similarly, issues dealing with disorder appear broadly throughout many branches of inquiry wherever rugged landscapes are invoked. This article reviews the discussions that occurred during a workshop held on 30–31 January 2016 in which opportunities in soft-matter experiment were surveyed. Soft matter has had an exciting history of discovery and continues to be a fertile ground for future research.

Figure 1

The oscillatory flapping state of a filament in a flowing two-dimensional soap film. Flow structures, modulated by the moving filament, are advected downstream. The flow is visualized using interference imaging. From [107].

Figure 2

The fastest particles in (a) a supercooled and (b) a glassy colloid. (a) Lower central (red) and upper left (blue) clusters contain 69 and 50 particles, respectively. (b) The largest cluster (upper left corner of the box, red) contains 21 particles. (a), (b) The particles that are not moving as rapidly are shown with smaller size to aid visualization. From [98]. (c) Motion in a glass composed of ellipsoidal particles. Both rotational and translational motion can be determined. Shaded (colored) particles in the image show the spatial distributions of the fastest 8% of translational motions of the ellipsoids. From [110].

Figure 3

(Top) Branching patterns seen in the limbs of trees and (bottom) in dielectric breakdown of a Lichtenberg figure.

Figure 4

TEM image of gold colloid aggregate showing a DLA structure. From [100].

Figure 5

(a) Snapshot of nematic order assumed by rods vibrated perpendicular to the plane of the image showing giant number fluctuations. From [63]. (b) Active liquid crystals exhibiting buckling, folding, and internal fracture. The two arrows indicate the positions of two topological charges associated with the disclination defects in the order parameter. The scale bar is $20\mu \mathrm{m}$. From [81].

Figure 6

Force chains in a two-dimensional granular material that is compressed uniaxially. The material is placed between two crossed polarizers. When under stress, the granular particles become birefringent and rotate the polarization of the transmitted light. The particles that are under stress are thus made visible so that the stress paths, from one side of the material to the other, are made apparent. From [57].

Figure 7

Liquid drop impact craters from a 3.1 mm water drop. Images show ring-shaped, solid pellet-shaped, asymmetric residues as impact parameters are varied. From [109].

Figure 8

Channeling of water in dry hydrophilic sandy soils at different grain diameters. Left panel: the grain diameter is 0.5 mm; right panel: 1.0 mm. From [99].

Figure 9

Universal grippers based on jamming in granular material. A rubber membrane, containing a granular material, can be deformed so that its shape conforms to that of an object to be manipulated. The membrane can be evacuated so that the granular material becomes rigid due to the external air pressure. This produces a universal gripper that can lift a large variety of objects with different shapes. Here the gripper is shown picking up salt and pepper shakers. From [1].

Figure 10

(a) X-ray tomography image of a “granular polymer” consisting of many equal-length segments of a connected chain of spheres. (b) Close-up image of shaded (red) region in (a). The connecting links are visible from which it is possible to determine which spheres are attached together in each chain. From [112].

Figure 11

Two type-$1/2$ strings in a liquid crystal crossing each other and reconnecting. These defects have been taken as analogs of cosmic strings in the early Universe. From [25].

Figure 12

Millimeter-size water drop on a hydrophobic surface textured with regularly shaped micropillars. The regular spacing of the pillars creates colored stripes. From [14].

Figure 13

Tears of wine. The image shows the tear line is at the top, a thin film region in the middle, and four ridges at the bottom right. From [44].

Figure 14

Vortex loop in water created by an accelerated airfoil with the topology of a knot. From [52].

Figure 15

(a) Thick sheet with circular holes. (b) This geometry produces a negative Poisson ratio well into the nonlinear regime; as the material is compressed in one direction, it is compressed in the perpendicular direction as well. The material is birefringent under strain so that the stresses can be visualized. From [61].

Figure 16

Side and top views of a circular polystyrene sheet wrapping a water drop immersed in silicone oil. Sheet thickness is 241 nm and the scale bar is 1 mm. From [72].

Figure 17

Bird flocks over the Rome train station. From [17].

Figure 18

(a) Scanning electron microscope image of a bimaterial colloid with a protruding hematite cube. (b) Formation of a living crystal. When illuminated, the particles assemble into crystals. The inset shows initial homogeneous distribution. (c) When the light is turned off, the crystals melt. The main panel shows after 10 s; the inset shows after 100 s. From [70].

Figure 19

Labyrinthine patterns. (a) Turing pattern. Stationary chemical patterns formed in a gel layer with striped patterns with wavelength 0.52 mm. From [95]. (b) Modulated phase. A thin layer of ferrofluid in a magnetic field with a period $\approx 2\mathrm{mm}$. Adapted from Fig. 4 of [78]) as in [2].

Figure 20

Proportionate growth in miscible viscous fingering. (a) The darker (blue) fluid displaces a more viscous fluid with which it is miscible. The interface between the two fluids at an early time can be enlarged so that it lies on top of the interface at a later time. Even the small features are nearly identical after the images have been scaled. (b) This form of pattern is common in the biological world (e.g., mammalian growth), but few physical examples exist. From [10].

Figure 21

Confocal imaging of a suspension of droplets. The bright spots, where the lighter colored (green) droplets contact each other, allow contact forces between the suspension droplets to be measured. From [13].

Figure 22

Turbulent Orb by Ned Kahn displayed in Battery Park City, New York in 1990. The sculpture consists of a large, rotatable, spherical glass vessel, filled with a deep-blue fluid that reveals the currents within. From [49].