Rahul Chajwa

All Publications

• Inflation-induced motility for long-distance vertical migration. Current biology : CB Larson, A. G., Chajwa, R., Li, H., Prakash, M. 2024

  Abstract

  The vertical migrations of pelagic organisms play a crucial role in shaping marine ecosystems and influencing global biogeochemical cycles. They also form the foundation of what might be the largest daily biomass movement on Earth. Surprisingly, among this diverse group of organisms, some single-cell protists can transit depths exceeding 50m without employing flagella or cilia. How these non-motile cells perform large migrations remains unknown. It has been previously proposed that this capability might rely on the cell's ability to regulate its internal density relative to seawater. Here, using the dinoflagellate algae Pyrocystis noctiluca as a model system, we discover a rapid cell inflation event post cell division, during which a single plankton cell expands its volume 6-fold in less than 10min. We demonstrate this rapid cellular inflation is the primary mechanism of density control. This self-regulated cellular inflation selectively imports fluid less dense than surrounding seawater and can thus effectively sling-shot a cell and reverse sedimentation within minutes. To accommodate its dramatic cellular expansion, Pyrocystis noctiluca possesses a unique reticulated cytoplasmic architecture that enables a rapid increase in overall cell volume without diluting its cytoplasmic content. We further present a generalized mathematical framework that unifies cell-cycle-driven density regulation, stratified ecology, and associated cell behavior in the open ocean. Our study unveils an ingenious strategy employed by a non-motile plankton to evade the gravitational sedimentation trap, highlighting how precise control of cell size and cell density can enable long-distance migration in the open ocean.

  View details for DOI 10.1016/j.cub.2024.09.046

  View details for PubMedID 39423814

• Hidden comet tails of marine snow impede ocean-based carbon sequestration. Science (New York, N.Y.) Chajwa, R., Flaum, E., Bidle, K. D., Van Mooy, B., Prakash, M. 2024; 386 (6718): eadl5767

  Abstract

  Gravity-driven sinking of "marine snow" sequesters carbon in the ocean, constituting a key biological pump that regulates Earth's climate. A mechanistic understanding of this phenomenon is obscured by the biological richness of these aggregates and a lack of direct observation of their sedimentation physics. Utilizing a scale-free vertical tracking microscopy in a field setting, we present microhydrodynamic measurements of freshly collected marine snow aggregates from sediment traps. Our observations reveal hitherto-unknown comet-like morphology arising from fluid-structure interactions of transparent exopolymer halos around sinking aggregates. These invisible comet tails slow down individual particles, greatly increasing their residence time. Based on these findings, we constructed a reduced-order model for the Stokesian sedimentation of these mucus-embedded two-phase particles, paving the way toward a predictive understanding of marine snow.

  View details for DOI 10.1126/science.adl5767

  View details for PubMedID 39388567

• Waves, Algebraic Growth, and Clumping in Sedimenting Disk Arrays PHYSICAL REVIEW X Chajwa, R., Menon, N., Ramaswamy, S., Govindarajan, R. 2020; 10 (4)

  View details for DOI 10.1103/PhysRevX.10.041016

  View details for Web of Science ID 000580889500001

• Nonmutual torques and the unimportance of motility for long-range order in two-dimensional flocks. Physical review. E Dadhichi, L. P., Kethapelli, J., Chajwa, R., Ramaswamy, S., Maitra, A. 2020; 101 (5-1): 052601

  Abstract

  As the constituent particles of a flock are polar and in a driven state, their interactions must, in general, be fore-aft asymmetric and nonreciprocal. Within a model that explicitly retains the classical spin angular momentum field of the particles we show that the resulting asymmetric contribution to interparticle torques, if large enough, leads to a buckling instability of the flock. More precisely, this asymmetry also yields a natural mechanism for a difference between the speed of advection of polarization information along the flock and the speed of the flock itself, concretely establishing that the absence of detailed balance, and not merely the breaking of Galilean invariance, is crucial for this distinction. To highlight this we construct a model of asymmetrically interacting spins fixed to lattice points and demonstrate that the speed of advection of polarization remains nonzero. We delineate the conditions on parameters and wave number for the existence of the buckling instability. Our theory should be consequential for interpreting the behavior of real animal groups as well as experimental studies of artificial flocks composed of polar motile rods on substrates.

  View details for DOI 10.1103/PhysRevE.101.052601

  View details for PubMedID 32575192

• Kepler Orbits in Pairs of Disks Settling in a Viscous Fluid. Physical review letters Chajwa, R., Menon, N., Ramaswamy, S. 2019; 122 (22): 224501

  Abstract

  We show experimentally that a pair of disks settling at negligible Reynolds number (∼10^{-4}) displays two classes of bound periodic orbits, each with transitions to scattering states. We account for these dynamics, at leading far-field order, through an effective Hamiltonian in which gravitational driving endows orientation with the properties of momentum. This treatment is successfully compared against the measured properties of orbits and critical parameters of transitions between types of orbits. We demonstrate a precise correspondence with the Kepler problem of planetary motion for a wide range of initial conditions, find and account for a family of orbits with no Keplerian analog, and highlight the role of orientation as momentum in the many-disk problem.

  View details for DOI 10.1103/PhysRevLett.122.224501

  View details for PubMedID 31283274