Catching Bubble-Driven Micromotors in Action

LS13437 - Catalytic micromotors are extremely small moving particles, about the size of bacterial organisms, that convert chemical energy through a catalytic reaction involving bubble generation, to propel the particle through the fluid.  Until now direct observation of the bubble propulsion for spherical micromotors has eluded researchers since bubble formation is very difficult on convex surfaces.  By using larger micromotors with a reduced surface curvature and an ultra-fast camera, the bubble growth and burst processes that drive spherical micromotors have been resolved.  Interestingly, it was found that bubble growth tends to propel the micromotor in one direction, while the bursting of the bubble that leaves a zone of depression pulls the micromotor back. The competition between the two processes creates an almost back and forth motion of the micromotor.  However, the growth process induces a greater displacement than that in the burst process, giving the micromotor a net displacement in one direction.  A theory is developed that quantitatively describes this behavior.  These results provide further insight into the behavior of micromotors, which are expected to have various biomedical applications in the future, such as advanced drug delivery.

Weighing Cells with Pictures

Physicists determine the weight of cells with a standard optical microscope.

LK12971 - Researchers have now developed a technique to determine the mass of cells and microorganisms with a standard light microscope. Beginning in the 1950s, laboratories around the world have concocted ingenious methods to weigh the tiny organic samples. Until now, however, these methods have all relied on custom-built instruments, involved mathematical analysis, and sample preparations that are often not compatible with living tissue. A new image-processing algorithm now enables the reconstruction of the three-dimensional mass distribution inside cells from microscopic images. The method paves the way for general microscope users to perform sensitive optical measurements of organisms and individual cells at the picogram (trillionth of a gram) level.

In Search of Rogue Wave Sources

LU13570 - New experiments presented in this paper reveal that extreme wave events, or “rogue waves” on the surface of a vertically vibrated liquid are caused by the interaction between oscillating solitons. Such solitons, also known as oscillons, have been found in a variety of physical systems such as granular medium, non-Newtonian fluids, optical media and plasmon oscillons in nanoparticle arrays. New observations made in the water surface ripple and reported here suggest that oscillons interact within a lattice and that such interaction causes their horizontal mobility and merger which leads to the formation of closed regions, or craters. Rogue waves, or strong vertical jets originate from the centers of such craters. The probability of large wave events greatly increases when oscillons move faster on the surface of water. These results will be useful in understanding physics of the rogue wave generation in other nonlinear systems.

Testing Stressed Viruses

LK13179 - One of the big questions about materials at the nanoscale is whether macroscopic theories can be applied to nano-sized systems. To answer this question, we focus on self-assembling, viral nanoparticles to test the predictions of elasticity theory. Of all known viruses those with icosahedral symmetry are the most common. Their structure can be described by folding a hexagonal lattice into a sphere-like configuration. By doing so defects with 5-fold symmetry are created at the icosahedral vertices.  According to elasticity theory these vertices should be under a permanent pre-stress, a prediction which never has been verified experimentally. Here we compare naturally occurring viruses with those that have missing proteins at their vertices, and test the hypothesis that the latter particles are stress-free. Deforming the viruses by Atomic Force Microscopy and comparing our results to detailed simulations has verified our hypothesis about pre-stress. This is the first time that the predictions about pre-stressed vertices in viral particles has been demonstrated experimentally and is a huge support for the use of continuum elasticity to describe nanometer sized objects.

Experimental Violation of Heisenberg's Uncertainty Principle

LU13293 - While there is a rigorously proven relationship about uncertainties intrinsic to any quantum system, often referred to as "Heisenberg's Uncertainty Principle," Heisenberg originally formulated his ideas in terms of a relationship between the precision of a measurement and the disturbance it must create. Although this latter relationship is not rigorously proven, it is commonly believed
(and taught) as an aspect of the broader uncertainty principle. Here, we experimentally observe
a violation of Heisenberg's \measurement-disturbance relationship", using weak measurements to
characterize a quantum system before and after it interacts with a measurement apparatus. Our
experiment implements a 2010 proposal of Lund and Wiseman to con rm a revised measurement-
disturbance relationship derived by Ozawa in 2003. Its results have broad implications for the
foundations of quantum mechanics and for practical issues in quantum mechanics.

Leading the Way to Lead-Free Piezoelectrics

LQ13398 - Energy and environment are two pressing global challenges.  Piezoelectric materials enable the conversion between electrical and mechanical energies, and have been used in many important applications, such as devices for ultrasound medical imaging and for energy harvesting from vibrations.  In the past six decades, lead-containing ceramics (greater than 60% weight of lead) have dominated the piezoelectric technology even though they pose a serious threat to the environment and human health.  For best piezoelectric performances, the compositions are fine tuned to be at the so-called morphotropic phase boundaries and the ceramics go through a poling processing where they are exposed to a highest possible electric field.  A recent discovery (to be published in the September 6 issue of Physical Review Letters) breaks the ice for new lead-free piezoelectrics.  The original approach of poling to the highest possible field is incorrect and the composition is not necessary to be at the morphotropic phase boundary.  Such a fundamental alteration to the long-standing paradigm adds a new dimension to the development of high-performance lead-free piezoelectrics.  

Learning from an Eggshell

LU13685  - If you squeeze an eggshell along its major axis, the shell is strikingly rigid and it is extremely challenging to break it with our bare hands. Conversely, if the eggshell is compressed along its equator, the resulting deflections are larger and, past a critical load one is typically able to fracture it. In our paper, we have rationalized this difference in the rigidity of an eggshell depending on the shell-load orientation to be due to the local geometry near the points of indentation. We have introduced a predictive framework for the rigidity of thin elastic shells which can also account for the situation when the shell is over-pressurized. Our concept of Geometry-Induced Rigidity can be used in reverse, as a precision non-destructive tool, to measure parameters of a shell (e.g. thickness) upon knowing the geometry of the underlying surface and the local mechanical response. The scale-invariance of Geometry-Induced Rigidity suggests that our framework should find uses across length scales: from the mechanical testing of viral capsids through Atomic Force Microscopy, to ocular tonometry procedures or in the design of architectural shells. All this work was inspired by the remarkable physics of an elegant eggshell!