Navish Wadhwa

Flow disturbances generated by feeding and swimming zooplankton

Significance Plankton compromise their survival when they swim and feed because the fluid disturbances that they generate may be perceived by predators. Because the abundance and population dynamics of zooplankton in the ocean are governed by their access to food and exposure to predators, an important question is to what extent and how zooplankton may minimize the fluid disturbances that they generate. We show that when swimming and feeding are integrated processes, zooplankton generate fluid disturbances that extend much farther in the water than is the case for zooplankton that swim only to relocate. Quiet swimming is achieved through “breast swimming” or by swimming by jumping, whereas other propulsion modes are much noisier. This pattern applies independent of organism size and species. Interactions between planktonic organisms, such as detection of prey, predators, and mates, are often mediated by fluid signals. Consequently, many plankton predators perceive their prey from the fluid disturbances that it generates when it feeds and swims. Zooplankton should therefore seek to minimize the fluid disturbance that they produce. By means of particle image velocimetry, we describe the fluid disturbances produced by feeding and swimming in zooplankton with diverse propulsion mechanisms and ranging from 10-µm flagellates to greater than millimeter-sized copepods. We show that zooplankton, in which feeding and swimming are separate processes, produce flow disturbances during swimming with a much faster spatial attenuation (velocity u varies with distance r as u ∝ r−3 to r−4) than that produced by zooplankton for which feeding and propulsion are the same process (u ∝ r−1 to r−2). As a result, the spatial extension of the fluid disturbance produced by swimmers is an order of magnitude smaller than that produced by feeders at similar Reynolds numbers. The “quiet” propulsion of swimmers is achieved either through swimming erratically by short-lasting power strokes, generating viscous vortex rings, or by “breast-stroke swimming.” Both produce rapidly attenuating flows. The more “noisy” swimming of those that are constrained by a need to simultaneously feed is due to constantly beating flagella or appendages that are positioned either anteriorly or posteriorly on the (cell) body. These patterns transcend differences in size and taxonomy and have thus evolved multiple times, suggesting a strong selective pressure to minimize predation risk.

Characteristic Sizes of Life in the Oceans, from Bacteria to Whales

The size of an individual organism is a key trait to characterize its physiology and feeding ecology. Size-based scaling laws may have a limited size range of validity or undergo a transition from one scaling exponent to another at some characteristic size. We collate and review data on size-based scaling laws for resource acquisition, mobility, sensory range, and progeny size for all pelagic marine life, from bacteria to whales. Further, we review and develop simple theoretical arguments for observed scaling laws and the characteristic sizes of a change or breakdown of power laws. We divide life in the ocean into seven major realms based on trophic strategy, physiology, and life history strategy. Such a categorization represents a move away from a taxonomically oriented description toward a trait-based description of life in the oceans. Finally, we discuss life forms that transgress the simple size-based rules and identify unanswered questions.

Size structures sensory hierarchy in ocean life.

Survival in aquatic environments requires organisms to have effective means of collecting information from their surroundings through various sensing strategies. In this study, we explore how sensing mode and range depend on body size. We find a hierarchy of sensing modes determined by body size. With increasing body size, a larger battery of modes becomes available (chemosensing, mechanosensing, vision, hearing and echolocation, in that order) while the sensing range also increases. This size-dependent hierarchy and the transitions between primary sensory modes are explained on the grounds of limiting factors set by physiology and the physical laws governing signal generation, transmission and reception. We theoretically predict the body size limits for various sensory modes, which align well with size ranges found in literature. The treatise of all ocean life, from unicellular organisms to whales, demonstrates how body size determines available sensing modes, and thereby acts as a major structuring factor of aquatic life.

Hydrodynamics and energetics of jumping copepod nauplii and copepodids

Within its life cycle, a copepod goes through drastic changes in size, shape and swimming mode. In particular, there is a stark difference between the early (nauplius) and later (copepodid) stages. Copepods inhabit an intermediate Reynolds number regime (between ~1 and 100) where both viscosity and inertia are potentially important, and the Reynolds number changes by an order of magnitude during growth. Thus we expect the life stage related changes experienced by a copepod to result in hydrodynamic and energetic differences, ultimately affecting the fitness. To quantify these differences, we measured the swimming kinematics and fluid flow around jumping Acartia tonsa at different stages of its life cycle, using particle image velocimetry and particle tracking velocimetry. We found that the flow structures around nauplii and copepodids are topologically different, with one and two vortex rings, respectively. Our measurements suggest that copepodids cover a larger distance compared to their body size in each jump and are also hydrodynamically quieter, as the flow disturbance they create attenuates faster with distance. Also, copepodids are energetically more efficient than nauplii, presumably due to the change in hydrodynamic regime accompanied with a well-adapted body form and swimming stroke.

Non-coalescence of jets

The phenomenon of non-coalescence between fluid jets was first reported by Lord Rayleigh, more than a century ago. Rayleigh described the observation in words without any experimental data or pictures. To the best of our knowledge, this curious phenomenon received no attention from the scientific community since then. We present the first experimental demonstration of the non-coalescence of two and three jets of the same fluid, and of non-coalescence between a jet and droplets of the same fluid. Figure 1 shows two jets of silicone oil (viscosity 10 cSt at 25 !C) with diameter 500 lm, impinging obliquely onto a vertical jet of the same fluid and diameter. Instead of coalescing, the jets from the sides rebound off the middle jet due to lubrication by a thin film of air separating the jets. The layer of air is continuously replenished by the motion of the jets, resulting in indefinitely sustained noncoalescence between the jets. As the jet velocity (v) increases, the air film is drained and the two jets coalesce. Figure 2 shows the transition from coalescence to non-coalescence between two jets when the experiment is carried out below a critical jet velocity. We also observed non-coalescence between a jet and drops of the same fluid, an example of which is shown in Figure 3. The drops plunged into the jet from two sides without coalescing into it, bending it at two locations.

A boundary element model of multiple microcirculatory bubbles in cardiovasculature

In order to investigate the role of gravity in a novel cancer treatment strategy called Gas Embolotherapy, we have computationally studied the evolution dynamics of two bubbles sticking to and sliding on the opposite walls of a 2D channel, under gravity-driven flow. We have modeled the moving three-phase contact lines using Tanner laws including contact angle hysteresis and have accounted for the gas-liquid interfacial dynamics in our model. Our model uses a Boundary Element Method (BEM) based moving-interface, multi-domain, iterative method to compute the flows and stresses on the domain boundaries at various instants of time. Since the normal and shear stresses acting on the endothelial layer of blood vessels are a major concern in the development of gas embolotherapy, we have examined the effect of bubble evolution and induced flows on the wall stresses. For a range of initial bubble pressures, we have studied the role of gravity by varying the Bond number and by using two different inclinations of the...