Quantitative imaging of aggregated emulsions

Experimental observations of fractal landscape dynamics in a dense emulsion

Many soft and biological materials display so-called 'soft glassy' dynamics; their constituents undergo anomalous random motions and complex cooperative rearrangements. A recent simulation model of one soft glassy material, a coarsening foam, suggested that the random motions of its bubbles are due to the system configuration moving over a fractal energy landscape in high-dimensional space. Here we show that the salient geometrical features of such high-dimensional fractal landscapes can be explored and reliably quantified, using empirical trajectory data from many degrees of freedom, in a model-free manner. For a mayonnaise-like dense emulsion, analysis of the observed trajectories of oil droplets quantitatively reproduces the high-dimensional fractal geometry of the configuration path and its associated local energy minima generated using a computational model. That geometry in turn drives the droplets' complex random motion observed in real space. Our results indicate that experimental studies can elucidate whether the similar dynamics in different soft and biological materials may also be due to fractal landscape dynamics.

Affine and nonaffine motions in sheared polydisperse emulsions

We study dense and highly polydisperse emulsions at droplet volume fractions ϕ≥0.65. We apply oscillatory shear and observe droplet motion using confocal microscopy. The presence of droplets with sizes several times the mean size dramatically changes the motion of smaller droplets. Both affine and nonaffine droplet motions are observed, with the more nonaffine motion exhibited by the smaller droplets which are pushed around by the larger droplets. Droplet motions are correlated over length scales from one to four times the mean droplet diameter, with larger length scales corresponding to higher strain amplitudes (up to strains of about 6%).

Cortical microtubule arrays undergo rotary movements in Arabidopsis hypocotyl epidermal cells

Plant-cell expansion is controlled by cellulose microfibrils in the wall with microtubules providing tracks for cellulose synthesizing enzymes. Microtubules can be reoriented experimentally and are hypothesized to reorient cyclically in aerial organs, but the mechanism is unclear. Here, Arabidopsis hypocotyl microtubules were labelled with AtEB1a-GFP (Arabidopsis microtubule end-binding protein 1a) or GFP-TUA6 (Arabidopsis alpha-tubulin 6) to record long cycles of reorientation. This revealed microtubules undergoing previously unseen clockwise or counter-clockwise rotations. Existing models emphasize selective shrinkage and regrowth or the outcome of individual microtubule encounters to explain realignment. Our higher-order view emphasizes microtubule group behaviour over time. Successive microtubules move in the same direction along self-sustaining tracks. Significantly, the tracks themselves migrate, always in the direction of the individual fast-growing ends, but twentyfold slower. Spontaneous sorting of tracks into groups with common polarities generates a mosaic of domains. Domains slowly migrate around the cell in skewed paths, generating rotations whose progressive nature is interrupted when one domain is displaced by collision with another. Rotary movements could explain how the angle of cellulose microfibrils can change from layer to layer in the polylamellate cell wall.