Better understanding of particle-particle and particle-fluid interactions requires accurate 3D measurements of particle distributions and motions. We introduce the application of in-line digital holographic microscopy as a viable tool for measuring distributions of dense micrometer (3.2 microm) and submicrometer (0.75 microm) particles in a liquid solution with large depths of 1-10 mm. By recording a magnified hologram, we obtain a depth of field of approximately 1000 times the object diameter and a reduced depth of focus of approximately 10 particle diameters, both representing substantial improvements compared to a conventional microscope and in-line holography. Quantitative information on depth of field, depth of focus, and axial resolution is provided. We demonstrate that digital holographic microscopy can resolve the locations of several thousand particles and can measure their motions and trajectories using cinematographic holography. A sample trajectory and detailed morphological information of a free-swimming copepod nauplius are presented.
Understanding how bacteria move close to surfaces is crucial for a broad range of microbial processes including biofilm formation, bacterial dispersion, and pathogenic infections. We used digital holographic microscopy to capture a large number (> 10 3 ) of three-dimensional Escherichia coli trajectories near and far from a surface. We found that within 20 μm from a surface tumbles are suppressed by 50% and reorientations are largely confined to surface-parallel directions, preventing escape of bacteria from the near-surface region. A hydrodynamic model indicates that the tumble suppression is likely due to a surfaceinduced reduction in the hydrodynamic force responsible for the flagellar unbundling that causes tumbling. These findings imply that tumbling does not provide an effective means to escape trapping near surfaces. The motility of bacteria near surfaces is relevant in a broad range of applications, from biofilm formation on medical instruments and wounds [1], to biofouling of engineered surfaces [2], and bioremediation of pollutants in the environment [3,4]. The presence of surfaces is known to alter bacterial motility by inducing circular swimming trajectories [5,6] and trapping cells in the near-surface region [7][8][9][10][11][12]. These near-surface behaviors have been attributed to longrange hydrodynamic interactions between swimming bacteria and the nearby surface [5,[7][8][9][10][11][12][13]: the surface modifies velocity and pressure fields around a swimming cell, and consequently forces and torques on the cell. Surfaces can also interfere with motility through steric interactions. For phytoplankton and spermatozoa, direct interaction of flagella with the surface is an important driver of surface scattering [14]. For smaller bacterial cells, measurements of the flow field around individual swimmers [15] indicate that hydrodynamic interactions are weak, suggesting that physical contact is critical in determining cell-surface interactions.Efforts to understand the trapping of bacteria by surfaces have largely neglected the effect of tumbles, the reorientations exhibited by wild-type peritrichous bacteria in their swimming trajectories. Studies have focused instead on smooth-swimming mutants for a range of species, including Escherichia coli [6,7,9,15,16], Caulobacter crescentus [17], and Bacillus subtilis [18]. When tumbling has been considered, in the context of surface interactions in E. coli, it was suggested to act as a mechanism that favors the cells' escape from the near-surface region [7,19]. However, subsequent observations have shown that wild-type (i.e., tumbling) E. coli attach to surfaces as effectively as a smooth-swimming mutant [20]. This inconsistency highlights the current limitations in our understanding of the surface interactions of bacteria. Here, we describe the effect of a surface on wild-type E. coli, and specifically on its ability to tumble, by capturing three-dimensional swimming trajectories of thousands of individual cells in a microfluidic device. We discovered that...
The quantification of three-dimensional (3D) flow structures and particle dynamics is crucial for unveiling complex interactions in turbulent flows. This review summarizes recent advances in volumetric particle detection and 3D flow velocimetry involving holography. We introduce the fundamental principle of holography and discuss the debilitating depth-of-focus problem, along with methods that have been implemented to circumvent it. The focus of this review is on recent advances in the development of in-line digital holography in general, and digital holographic microscopy in particular. A mathematical background for the numerical reconstruction of digital holograms is followed by a summary of recently introduced 3D particle tracking and velocity measurement techniques. The review concludes with sample applications, including 3D velocity measurements that fully resolve the flow in the inner part of a turbulent boundary layer, the diffusion of oil droplets in high-Reynolds number turbulence, and predator-prey interactions among swimming microorganisms in dense suspensions, as well as oceanic and atmospheric field experiments.
The shallow depth of field of conventional microscopy hampers analyses of 3D swimming behavior of fast dinoflagellates, whose motility influences macroassemblages of these cells into oftenobserved dense ''blooms.'' The present analysis of cinematic digital holographic microscopy data enables simultaneous tracking and characterization of swimming of thousands of cells within dense suspensions. We focus on Karlodinium veneficum and Pfiesteria piscicida, mixotrophic and heterotrophic dinoflagellates, respectively, and their preys. Nearest-neighbor distance analysis shows that predator and prey cells are randomly distributed relative to themselves, but, in mixed culture, each predator clusters around its respective prey. Both dinoflagellate species exhibit complex highly variable swimming behavior as characterized by radius and pitch of helical swimming trajectories and by translational and angular velocity. K. veneficum moves in both left-and right-hand helices, whereas P. piscicida swims only in right-hand helices. When presented with its prey (Storeatula major), the slower K. veneficum reduces its velocity, radius, and pitch but increases its angular velocity, changes that reduce its hydrodynamic signature while still scanning its environment as ''a spinning antenna.'' Conversely, the faster P. piscicida increases its speed, radius, and angular velocity but slightly reduces its pitch when exposed to prey (Rhodomonas sp.), suggesting the preferred predation tactics of an ''active hunter.'' T he swimming behavior of dinoflagellates, biflagellated planktonic protists that are sometimes associated with harmful algal blooms or ''red tides'' (1), is vital to their success in aquatic ecosystems (2). Predation, which involves complex microbial interactions, is an important facet of the behavior of heterotrophic and mixotrophic (combining phototrophic and heterotrophic nutrition) dinoflagellates (3). Dinoflagellates typically move in helical trajectories (4, 5), which may help them in detecting nutrient gradients (6), although little is known about how differences in species or environment (i.e., resource availability) affect their swimming characteristics. However, evidence suggests that certain dinoflagellates adapt swimming strategy that increases their encounter rate with prey as the quarry concentration decreases (7).Being limited by the shallow depth of field of conventional microscopy, most studies of dinoflagellates' swimming have been performed in thin containers, where ''wall effects'' are likely to affect behavior. Triggering of imaging systems as subjects cross in-focus planes or 3D traversing systems that follow organisms provide only limited solutions to this problem. The tendency of dinoflagellates to cluster together in dense suspensions further complicates measurements of behavior of individuals in their natural setting. In this study, we use high-speed cinematic digital holographic microscopy, as described in Materials and Methods, to overcome these challenges. Ensuing analysis provides simultaneous data...
Toxins produced by the harmful algal bloom (HAB) forming, mixotrophic dinoflagellate Karlodinium veneficum have long been associated with fish kills. To date, the perceived ecological role for toxins has been relief from grazing pressures. Here, we demonstrate that karlotoxins also serve as a predation instrument. Using high-speed holographic microscopy, we measure the swimming behavior of several toxic and nontoxic strains of K. veneficum and their prey, Storeatula major, within dense suspensions. The selected strains produce toxins with varying potency and dosages, including a nontoxic one. Results clearly show that mixing the prey with the predatory, toxic strains causes prey immobilization at rates that are consistent with the karlotoxins' potency and dosage. Even prey cells that continue swimming slow down after exposure to toxic predators. The swimming characteristics of predators vary substantially in pure suspensions, as quantified by their velocity, radii of helical trajectories, and direction of helical rotation. When mixed with prey, all toxic strains that are involved in predation slow down. Furthermore, they substantially reduced their predominantly vertical migration, presumably to remain in the vicinity of their prey. Conversely, the nontoxic control strain does not alter its swimming and does not affect prey behavior. In separate experiments, we show that exposing prey to exogenous toxins also causes prey immobilization at rates consistent with potency. Clearly, the toxic predatory strains use karlotoxins as a means of stunning their prey, before ingesting it. These findings add a substantiated critical understanding for why some HAB species produce such complex toxin molecules.digital holographic microscopy | karlotoxin | predator-prey interactions | harmful algal blooms
Three-dimensional velocity distributions and corresponding wall stresses are measured concurrently in the inner part of a turbulent boundary layer over a smooth wall using digital holographic microscopy. The measurements are performed in a square duct channel flow atReδ= 50000 andReτ= 1470. A spatial resolution of 3–8 wall units (δυ= μm) in streamwise and spanwise directions and 1 wall unit in the wall-normal direction are sufficient for resolving buffer layer structures and for measuring the instantaneous wall shear stresses from velocity gradients in the viscous sublayer. Mean velocity and Reynolds stress profiles agree well with previous publications. Rudimentary observations classify the buffer layer three-dimensional flow into (i) a pair of counter-rotating inclined vortices, (ii) multiple streamwise vortices, some of them powerful, and (iii) no apparent buffer layer structures. Each appears in about one third of the realizations. Conditional sampling based on local wall shear stress maxima and minima reveals two types of three-dimensional buffer layer structures that generate extreme stress events. The first structure develops as spanwise vorticity lifts from the wall abruptly and within a short distance of about 10 wall units, creating initially a vertical arch. Its only precursors are a slight velocity deficit that does not involve an inflection point and low levels of vertical vorticity. This arch is subsequently stretched vertically and in the streamwise direction, culminating in formation of a pair of inclined, counter-rotating vortices with similar strength and inclination angle exceeding 45°. A wall stress minimum exists under the point of initial lifting. A pair of stress maxima develops 35δυdownstream, on the outer (downflow) sides of the vortex pair and is displaced laterally by 35–40δυfrom the minimum. This flow structure exists not only in the conditionally averaged field but in the instantaneous measurement as well and appears in 16.4% of the realizations. Most of the streamwise velocity deficit generated by this phenomenon develops during this initial lifting, but it persists between the pair of vortices. Distribution of velocity fluctuations shows that spanwise transport of streamwise momentum plays a dominant role and that vertical transport is small under the vortices. In other regions, e.g. during initial lifting, and between the vortices, vertical transport dominates. The characteristics of this structure are compared to early experimental findings, highlighting similarities and differences. Abundance of pairs of streamwise vortices with similar strength is inconsistent with conclusions of several studies based on analysis of direct numerical simulation (DNS) data. The second buffer layer structure generating high wall stresses is a single, predominantly streamwise vortex, with characteristic diameter of 20–40δυand inclination angle of 12°. It generates an elongated, strong stress maximum on one side and a weak minimum on the other and has been observed in 20.4% of the realizations. Except for a limited region of sweep above the high-stress region, this low-lying vortex mostly induces spanwise momentum transport. This structure appears to be similar to those observed in several numerical studies.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.