Plethora of transitions during breakup of liquid filaments

Thinning and breakup of liquid filaments are central to dripping of leaky faucets, inkjet drop formation, and raindrop fragmentation. As the filament radius decreases, curvature and capillary pressure, both inversely proportional to radius, increase and fluid is expelled with increasing velocity from the neck. As the neck radius vanishes, the governing equations become singular and the filament breaks. In slightly viscous liquids, thinning initially occurs in an inertial regime where inertial and capillary forces balance. By contrast, in highly viscous liquids, initial thinning occurs in a viscous regime where viscous and capillary forces balance. As the filament thins, viscous forces in the former case and inertial forces in the latter become important, and theory shows that the filament approaches breakup in the final inertial–viscous regime where all three forces balance. However, previous simulations and experiments reveal that transition from an initial to the final regime either occurs at a value of filament radius well below that predicted by theory or is not observed. Here, we perform new simulations and experiments, and show that a thinning filament unexpectedly passes through a number of intermediate transient regimes, thereby delaying onset of the inertial–viscous regime. The new findings have practical implications regarding formation of undesirable satellite droplets and also raise the question as to whether similar dynamical transitions arise in other free-surface flows such as coalescence that also exhibit singularities.Drop formation is ubiquitous in daily life, industry, and nature (1–3). The phenomenon is central to inkjet printing (4, 5), dripping from leaky faucets (6, 7), measurement of equilibrium and dynamic surface tension (8, 9), DNA arraying and printing of cells (10, 11), chemical separations and analysis (12, 13), production of particles and capsules (14, 15), printing of wires and transistors (16, 17), and mist formation in waterfalls and fragmentation of raindrops (18, 19). Fig. 1A shows an experimental setup for studying the dynamics of a drop of an incompressible Newtonian fluid of density ρ, viscosity μ, and surface tension σ forming from a tube of radius R (Fig. 1B and Drop Formation from a Tube and Filament Thinning). A salient feature of, and key to understanding, drop formation is the occurrence of a thin filament that connects an about-to-form primary drop to the rest of the fluid that is attached to the tube (Fig. 1 B and C). Thus, it often proves convenient to study filament thinning in the idealized setup depicted in Fig. 1D (Drop Formation from a Tube and Filament Thinning). As time t advances and the filament radius decreases, curvature and capillary pressure, both of which are at leading order inversely proportional to radius, increase and fluid is expelled with increasing velocity from the neck. At the instant t = t_b when the neck radius vanishes, a finite time singularity occurs and the filament breaks. When the filament breaks, one or more satellite droplets may also form. These satellites are typically much smaller than the primary drop (20) and almost always undesirable in applications (2).Open in a separate windowFig. 1.Methods used for studying and new phase diagram for capillary thinning and pinch-off. (A) Experimental setup used to capture images of the thinning neck of a drop forming from a nozzle. The images are then postprocessed to obtain the minimum neck radius as a function of the time remaining until breakup. (B) Snapshot of a drop forming from a tube that highlights the pinching zone in the vicinity of the pinch point. (C) Two series of images that focus on the pinching zones and depict the evolution in time of thinning filaments for drops of two fluids with different viscosities (i.e., different Oh). (D) Setup for simulating filament thinning and pinch-off: periodically perturbed jet (Left) and domain of axial length λ/2 used in simulations (Right). (E) Phase space showing trajectories taken by filaments of a slightly viscous (Oh < 1) and a highly viscous (Oh > 1) fluid. The large squares indicate the starting states at t = 0. The arrows along each trajectory show the direction of evolution.For Newtonian filaments, three theories have been developed to describe the dynamics in the vicinity of the pinch-off singularity (Scaling Theories of Pinch-Off). When viscous effects are weak, thinning and pinching occur in an inertial (I) regime (21–23) where inertial and capillary forces balance, and the minimum filament radius h_min (Fig. 1B) and the instantaneous Reynolds number Re(t) vary with dimensionless time τ to breakup ashminR∼τ2/3, Re(t)∼1Ohτ1/3,[1]where τ ≡ (t_b ? t)/t_I and tI≡ρR3/σ (Scaling Theories of Pinch-Off). For real liquids, the Ohnesorge number Oh=μ/ρRσ is not identically zero no matter how small the viscosity. Thus, for low-viscosity liquids, Oh ? 1 and Eq. 1 shows that regardless of how large the Reynolds number is initially, as τ → 0 and breakup is approached, Re(t) → 0. Therefore, the inertial regime cannot persist all of the way to breakup and can only describe the initial dynamics for low-viscosity fluids. Similarly, when viscous effects are dominant, thinning and pinching occur in a viscous (V) regime (24) where viscous and capillary forces balance, and h_min and Re(t) vary with τ ashminR∼τ, Re(t)∼1Oh2τ2β−1,[2]where τ ≡ (t_b ? t)/t_V, t_V ≡ μR/σ, and β = 0.175 (Scaling Theories of Pinch-Off). For real liquids, Oh cannot be infinite no matter how large the viscosity. Thus, for high-viscosity liquids, Oh ? 1 and Eq. 2 shows that regardless of how small the Reynolds number is initially, as τ → 0 and breakup is approached, Re(t) → ∞. Therefore, the V regime cannot persist all of the way to breakup and hence can only describe the initial dynamics even for high-viscosity fluids. Therefore, as the filament radius tends to zero, a transition occurs from either the I or the V regime to a final inertial–viscous (IV) regime in which all three forces, i.e., inertial, viscous, and capillary, balance and the instantaneous Reynolds number Re(t) ～ 1 (25). From Eqs. 1 and 2, the transition from the I to the IV regime and that from the V to IV regime can be calculated by setting Re(t) to be order one. Thus, transition from the I to the IV regime should occur when (2, 18)h_min/R ～ Oh², [3]and that from the V to the IV regime should occur when (2, 18)h_min/R ～ Oh^2/(2β?1).[4]However, whereas careful simulations and experiments have shown that the transition from the I to the IV regime does indeed occur, it has been found to take place for values of h_min that are about an order of magnitude smaller than that predicted from theory (Eq. 3) (26). Furthermore, the transition from the V to the IV regime has not yet been demonstrated to occur from simulation and an attempt for an experimental demonstration of the transition (27) was perhaps at too small a value of Oh (Oh = 0.49) to be conclusively in the V regime. In this paper, we demonstrate that in contrast with the conventional wisdom that the dynamics of capillary pinching should exhibit a transition from either the I to the IV regime or the V to the IV regime, the transition from either of the two initial regimes to the final IV regime is in fact more complex and, unexpectedly, can be delayed by the occurrence of a number of intermediate transient regimes as shown in Fig. 1E. The possibility of such complexity has been anticipated in part by Eggers (18) but no study has yet been carried out to explore the existence of these intermediate regimes or contemplate its implications in other free-surface flows exhibiting finite time singularities.In this work, the dynamics of filament thinning is studied both numerically and experimentally. Simulations are performed to track how sinusoidal perturbations on a liquid cylinder cause it to break, which have been successfully used in the past to study pinch-off and scaling for Newtonian (26, 28) as well as non-Newtonian fluids (29, 30) (Fig. 1D and Simulations). In the experiments, high-speed imaging and image analysis are used to measure the evolution in time of the minimum filament radius for liquids dripping from a tubular nozzle. Glycerol–water mixtures are used as working fluids to explore systems with different values of Oh.Fig. 2A shows the computed variation of h_min with τ for a slightly viscous liquid of Oh = 0.23. The simulations make plain that after sufficient time has passed so that the initial transients have decayed, the filament first thins in the I regime, where h_min ～ τ^2/3, as expected. According to conventional wisdom, the thinning dynamics is expected to transition from the I regime to the IV regime when h_min ～ Oh² = 0.053; thenceforward, the thinning is to follow IV scaling where h_min = 0.0304τ/Oh. However, the simulation results show that this transition is delayed and does not take place until h_min has fallen below a value that is about an order of magnitude smaller than that predicted by the theoretical estimate. Moreover, the simulations show unexpectedly that the dynamics switches over from the I to the IV regime only after passing through an intermediate V regime, where h_min = 0.0709τ/Oh. The existence of these regimes can be verified by plotting the local Reynolds number Re_local (Simulations) in the thinning filament in the vicinity of the pinch point as a function of h_min (Fig. 2B). Fig. 2B clearly shows that at early times when h_min ≈ 0.2, Re_local ? 1, confirming the existence of the I regime. However, when h_min has fallen to  ≈ 0.03, Re_local ? 1, which clearly demonstrates that the dynamics has entered the newly discovered intermediate V regime. Finally, as the filament asymptotically approaches breakup, i.e., for values of h_min ≈ 10^?3 or smaller, Re_local ～ 1, demonstrating that near the singularity, all three forces (viz., inertial, viscous, and capillary) balance each other and the dynamics lies in the IV regime. To confirm the correctness of these computationally made predictions, dripping experiments have been carried out with two liquids of Oh = 0.23 and Oh = 0.55. For the former, Fig. 2C shows the transition from the initial I regime to the intermediate V regime, with the latter regime lasting nearly over two decades in h_min. When Oh = 0.23, it is not possible to observe the transition from the V regime to the final IV regime because that transition occurs for neck radii smaller than a micrometer, which is the lower limit of length scales that can be imaged using visible light. The experimental results for Oh = 0.55 depicted in Fig. 2D, on the other hand, do show the transition to the final IV regime, albeit with an intermediate V regime of much shorter duration.Open in a separate windowFig. 2.Simulations and experiments demonstrating the existence of an intermediate viscous regime between the initial inertial regime and the final IV regime for slightly viscous fluids (Oh < 1). (A) Variation of minimum neck radius with time until breakup when Oh = 0.23 obtained from simulations. (B) Computed variation with minimum neck radius of the local Reynolds number in the neighborhood of the pinch point verifies the existence of all three regimes: Re_local ? 1 in the I regime, Re_local ? 1 in the V regime, and Re_local ～ 1 in the IV regime. (C) Experimental confirmation of the existence of an intermediate V regime when Oh = 0.23. The IV regime is not attained here because of optical limitations. (D) At a slightly higher value of Oh than that in C (Oh = 0.55), the V to IV transition is observed experimentally. (C and D, Insets) Same data as in the main figures are presented but use linear rather than logarithmic axes.Having demonstrated the existence of the intermediate V regime, we now turn our attention to understanding the reason for its occurrence, which is facilitated by examining flow fields within thinning filaments. To do so, we turn our attention to a filament of Oh = 0.07 which, as shown in Fig. 3A, clearly depicts the existence of all three scaling regimes. The instantaneous streamlines and pressure contours at three different times when the dynamics lies in each of these three regimes are shown in Fig. 3 B–D over 0 ≤ z ≤ λ/2 ≡ 4. At early times, the minimum in the filament radius is located at z = λ/2 ≡ 4, i.e., halfway between two swells, one located at z = 0 and the other at z = λ ≡ 8 (Fig. 3B). As the filament continues to thin, the fluid accelerates as it flows from the neck, where pressure is highest, toward the two swells, where pressure is lowest. On account of this effect, which is attributable to finite fluid inertia (20, 22), the filament begins to thin fastest at two locations that are located on either side of z = λ/2. Within the computational domain, this leads to a shift in the minimum radius from the end of the domain (z = 4) to its interior, i.e., z ≈ 1.95. As shown in Fig. 3C, the occurrence of this new minimum gives rise to a new stagnation zone in the interior of the domain in the vicinity of which the flow has slowed down considerably and even reversed. This shift in the location of h_min and the accompanying slowing down of the flow then takes the dynamics into the V regime. Although the new stagnation zone persists for some time, the filament does not break while in the V regime. The capillary pressure which continues to rise as the filament continues to thin accelerates fluid out of the thinning neck and causes inertia to become significant once again, thereby taking the filament into the IV regime. Hence, with the simulation and experimental results shown in Figs. 2 and ​and3,3, the thinning and breakup dynamics of slightly viscous filaments for which Oh < 1 are seen to exhibit I to V to IV scaling as τ → 0. Furthermore, these results at long last shed light on the reason for the delay in the transition to the final IV regime that had remained perplexing and unexplained for over a decade.Open in a separate windowFig. 3.Simulation results when Oh = 0.07 highlight the formation of a stagnation zone within the filament and help explain why the intermediate viscous regime exists. (A) Variation of minimum neck radius with time until breakup that shows occurrence of all three regimes and transition from I to IV regime through an intermediate V regime. (See below for the explanation of the arrow.) (B) Instantaneous streamlines and pressure contours within the thinning filament when the dynamics lies in the I regime. As shown in the figure, this I regime has a slender geometry (21) rather than a fully developed double-cone structure (23). The legend on the top right identifies contour values of the pressure. At this instant in time, the minimum neck radius is located at z = λ/2 = 4 and the fluid accelerates as it flows from the neck to the swell. (C) Instantaneous streamlines and pressure contours in the filament at a later time than in B where the acceleration of the fluid has resulted in shifting of the neck from the top end of the domain (z = 4) to a location between the two ends (0 < z < 4). The time and minimum filament radius when this shift commences is identified by the arrow in A. (Inset) A new stagnation zone has formed away from the two ends, resulting in a region of reversed flow and the slowing down of the flow in the vicinity of the new minimum in filament radius. (D) The stagnation zone persists but because of the large capillary pressures that develop as the neck continues to thin, fluid is once more accelerated as it flows away from the neck. Thus, inertial forces come into play again and compete with viscous and capillary forces in setting the final fate of the filament.Having clarified the heretofore inadequately understood thinning dynamics of slightly viscous fluids of Oh < 1, we next show that highly viscous fluids of Oh > 1 exhibit even more subtle behavior during capillary thinning. Fig. 4 shows results of simulations and experiments for a fluid of Oh = 1.81. As expected, both simulations (Fig. 4A) and experiments (Fig. 4B) reveal that the initial and final scaling regimes are the V and IV regimes. Conventional wisdom dictates that the transition from the V to the IV regime should occur when h_min ～ Oh^2/(2β?1) = 0.162, which is contradicted by both simulations and experiments. The simulations show (Fig. 4A), and experiments confirm (Fig. 4B), that there exists an intermediate I regime that follows the initial V regime. Local Reynolds number calculations near the pinch point from the simulations are yet even more revealing (Fig. 4C): they show the existence of an intermediate V regime that lies between the intermediate I and the final IV regime. Therefore, according to Fig. 4, the capillary thinning of highly viscous filaments for which Oh > 1 is seen to transition from V to I to V to IV regimes as τ → 0. Furthermore, it is worth noting that Fig. 4A depicts, to our knowledge, the first demonstration by simulation of the transition from an initial V regime to the final IV regime.Open in a separate windowFig. 4.Simulations and experiments demonstrating the existence of several intermediate regimes between the initial viscous regime and the final IV regime for a highly viscous fluid of Oh = 1.81. (A) Computations show that as the filament thins, the dynamics transitions from an initial V regime to the final IV regime through an intermediate I regime (but see C). (B) Experiments accord with the predictions from simulations and exhibit the same transition dynamics. (C) However, local Reynolds number calculations from the simulations reveal more information about the transitions. Whereas the initial V, intermediate I, and final IV regimes are confirmed from the computed variation of Re_local with h_min, this analysis also indicates the existence for a very short time of an intermediate V regime after the I regime.In conclusion, our analysis provides, to our knowledge, the first correct trajectories in the phase space of (h_min, Re_local) that are taken by filaments as they undergo capillary pinching. A particularly interesting finding is that the dynamics cannot reach the asymptotic universal IV regime directly from the I regime without passing through an intermediate transient V regime even though this latter regime may be very short-lived. The presence of the intermediate V regime indicates that even for a low-viscosity fluid, at some stage viscous force (along with capillary force) will dominate the dynamics during filament thinning and breakup. The existence of intermediate regimes has several practical implications as occurrence of slender threads that pinch symmetrically at their midpoints is associated with breakup of highly viscous filaments undergoing creeping flow, whereas occurrence of satellites is associated with inviscid fluids (20, 23, 24, 30). Therefore, the presence of the intermediate I regime makes plain that a visible satellite drop may form even during breakup of highly viscous filaments that reach the IV regime for values of h_min below the limit set by visible light. Additionally, the existence of multiple regime transitions before a filament enters the final IV regime helps explain why it has heretofore proven difficult to observe this regime during pinch-off of highly viscous filaments.The unexpected findings of this work raise a number of questions. Two issues that have not been addressed here are that the amount of time spent by filaments in each regime remains unclear and that similar transitions that may take place during capillary pinching of complex fluids (29, 30) remain unexplored. Moreover, it is well known that there are a number of other free-surface flows that exhibit finite time singularities. Chief among these is the coalescence singularity that arises when two drops are just allowed to touch and then merge into one (31). Whether transitions of the sort uncovered in this work exist in problems like coalescence are worthy topics for future study and may help explain why it took over a decade to uncover the true asymptotic regime of coalescence (31, 32).     

Angiography Alone Versus Angiography Plus Optical Coherence Tomography to Guide Percutaneous Coronary Intervention: Outcomes From the Pan-London PCI Cohort

Objectives

This study aimed to determine the effect on long-term survival of using optical coherence tomography (OCT) during percutaneous coronary intervention (PCI).

SIK2 orchestrates actin-dependent host response upon Salmonella infection

Salmonella is an intracellular pathogen of a substantial global health concern. In order to identify key players involved in Salmonella infection, we performed a global host phosphoproteome analysis subsequent to bacterial infection. Thereby, we identified the kinase SIK2 as a central component of the host defense machinery upon Salmonella infection. SIK2 depletion favors the escape of bacteria from the Salmonella-containing vacuole (SCV) and impairs Xenophagy, resulting in a hyperproliferative phenotype. Mechanistically, SIK2 associates with actin filaments under basal conditions; however, during bacterial infection, SIK2 is recruited to the SCV together with the elements of the actin polymerization machinery (Arp2/3 complex and Formins). Notably, SIK2 depletion results in a severe pathological cellular actin nucleation and polymerization defect upon Salmonella infection. We propose that SIK2 controls the formation of a protective SCV actin shield shortly after invasion and orchestrates the actin cytoskeleton architecture in its entirety to control an acute Salmonella infection after bacterial invasion.

Salmonella enterica is a gram-negative, facultative intracellular human pathogen, annually causing more than 100 million food- and waterborne infections worldwide. Salmonella Typhimurium causes severe gastroenteritis, which could turn into a systemic infection in children, immune-compromised, or elderly people (1, 2). Concurrently, multidrug resistant bacteria are globally emerging and threatening our health systems, calling for a better understanding of the underlying virulence mechanism and host response.Pathogenic bacteria have evolved the inherent ability to infect and to establish their niche within host cells. For colonizing nonphagocytic cells such as epithelial cells, Salmonella uses a trigger mechanism–based entry mode. Bacterial virulence factors are then injected via a Type III-secretion system (T3SS) into the host cell to induce cytoskeletal rearrangements leading to membrane ruffling and macropinocytosis-driven internalization into a sealed phagosome (3, 4). This specialized compartment is referred to as the Salmonella-containing vacuole (SCV) and serves as the intracellular replicative niche by hiding the bacteria from the humoral and cell-autonomous immune response (5). Salmonella invasion requires a cooperative action of several bacterial effector proteins hijacking multiple host targets. One of the main targets forcing Salmonella´s uptake is the actin cytoskeleton by subverting the host Rho GTPases system. Bacterial effector proteins such as SopE/SopE2 mimic host nucleotide exchange factors (GEFs) to stimulate Rac1 and CDC42 activity (6, 7). Once GTP-activated, Rho GTPases stimulate downstream pathways to drive actin filament (F-actin) assembly and rearrangement.The actin cytoskeleton network is regulated by actin-binding proteins (ABPs), which orchestrate assembly and disassembly of actin in higher networks (8). Monomeric, globular actin (G-actin) is nucleated into new actin filaments, or the existing F-actin is elongated, stabilized, or disassembled by ABPs. The major actin nucleation factor is the multimeric Arp2/3 complex, which generates branched actin filament networks. Formins generate long unbranched actin filaments and represent another actin nucleation family. Together with actin nucleation-promoting factors, small Rho GTPases control ABPs in a spatiotemporal manner. Actin polymerization and membrane ruffling are necessary for Salmonella invasion. Following Salmonella internalization, the SCV undergoes SPI-1–dependent biogenesis and is transported to a juxtanuclear position at 1 to 2 h postinfection (pi). At later time-points (4 to 6 h pi), SPI-2–dependent effector proteins are expressed to further mature the SCV, allowing bacterial proliferation. Pioneering work described that, at later stages of the infection (≥6 h pi), an actin meshwork around the SCV stabilizes and protects the vacuolar niche (9–13).Here, we report SIK2 as a Salmonella resistance factor and a regulator of the actin cytoskeleton. SIK2 belongs to the AMPK kinase family and was named after its homolog SIK1, found to be expressed upon high-salt diet-induced stress in rats (14, 15). SIK2 has been implicated into multiple biological roles including melanogenesis, cancer progression, and gluconeogenesis (16–18). SIK2 depletion results in a loss of SCV integrity and bacterial escape into the host cytosol, causing intracellular Salmonella hyperproliferation. Notably, SIK2 depletion results in a severe pathological cellular actin nucleation and polymerization defect upon Salmonella infection. Hence, SIK2 may represent a cellular safeguard, which controls the actin cytoskeleton and SCV integrity, thereby serving as a prime regulator of Salmonella proliferation subsequent to cellular internalization.     

Universal scaling laws for the disintegration of electrified drops

Drops subjected to strong electric fields emit charged jets from their pointed tips. The disintegration of such jets into a spray consisting of charged droplets is common to electrospray ionization mass spectrometry, printing and coating processes, and raindrops in thunderclouds. Currently, there exist conflicting theories and measurements on the size and charge of these small electrospray droplets. We use theory and simulation to show that conductivity can be tuned to yield three scaling regimes for droplet radius and charge, a finding missed by previous studies. The amount of charge that electrospray droplets carry determines whether they are coulombically stable and charged below the Rayleigh limit of stability or are unstable and hence prone to further explosions once they are formed. Previous experiments reported droplet charge values ranging from 10% to in excess of . Simulations unequivocally show that electrospray droplets are coulombically stable at the instant they are created and that there exists a universal scaling law for droplet charge, .     

Epithelial Cells Infected with Chlamydophila pneumoniae (Chlamydia pneumoniae) Are Resistant to Apoptosis

The obligate intracellular pathogen Chlamydophila pneumoniae (Chlamydia pneumoniae) initiates infections in humans via the mucosal epithelia of the respiratory tract. Here, we report that epithelial cells infected with C. pneumoniae are resistant to apoptosis induced by treatment with drugs or by death receptor ligation. The induction of protection from apoptosis depended on the infection conditions since only cells containing large inclusions were protected. The underlying mechanism of infection-induced apoptosis resistance probably involves mitochondria, the major integrators of apoptotic signaling. In the infected cells, mitochondria did not respond to apoptotic stimuli by the release of apoptogenic factors required for the activation of caspases. Consequently, active caspase-3 was absent in infected cells. Our data suggest a direct modulation of apoptotic pathways in epithelial cells by C. pneumoniae.     

Immunomodulation by 9-amino-1,2,3,4-tetrahydroacridine (THA): 1. Down-regulation of natural cell-mediated cytotoxicity in vitro.

THA (Tacrine), a drug used in the experimental therapy of dementia of Alzheimer's disease type, and whose biochemical site of action is believed to be the neural cholinesterase, is shown, for the first time, to be an immunosuppressant in vitro on normal human peripheral blood lymphocytes in microgram quantities. THA down-regulates non-MHC restricted natural killer (NK) cell activity without affecting the general viability of cells. This down-regulation can be demonstrated at all effector and target (K562) concentrations, in purified resting NK cells as well as in lymphokine (interleukin 2) activated killer cells in 3- or 16-h NK assays and in all the blood samples tested. Kinetic analysis shows that the Vmax (maximal cytotoxic potential) and Km of NK cell-mediated cytolysis are also attenuated. Single cell assays using agarose matrix reveal that THA moderately interferes with tumor target binding/recognition events and strongly abrogates the delivery of lethal hit, thus lowering the frequency of active killer cells among THA-treated lymphocytes. THA down-regulates NK cells upon direct interaction and does not require the help of non-NK cells. The THA sensitive site(s) on NK cells does not appear to be perturbed significantly either by their proliferative status or by membrane modulations that may be normally induced by interleukin 2. The in vitro immunomodulatory pharmacological properties of THA reveal that the biological site of action of THA extends to non-neural cells also. Such non-neural models may be helpful in exploring the pathophysiological neuroimmunomodulatory properties of THA at cellular and molecular levels.     

Age-associated alterations in human natural killer cells. 1. Increased activity as per conventional and kinetic analysis

We report a positive association between the human peripheral blood natural killer (NK) cell activity and the age (20-94 years) of 137 healthy volunteers. Irrespective of the methods of data representation, the elderly (greater than 80 years) express a statistically significant (at 0.001 level) higher (35-80.7%) mean NK activity when compared to younger adults (less than 40 years). Results of repeat assays and paired assays support a similar conclusion. This difference can be demonstrated at a wide range of effector or target cell concentrations or times of assay and is not influenced by in vivo lymphocyte count. Single-cell assay results suggest that an increase in the frequency of NK cells may be responsible for the higher NK activity in the elderly. These findings were confirmed by an enzyme-like kinetic analysis. Vmax, the maximum cytotoxic potential of the lymphocytes from the elderly, is nearly four times higher than that of younger adults. It is concluded that unlike the age-related general decline in T- and B-cell reactivity (as demonstrated here with concanavalin A and pokeweed mitogen), the NK cell system is highly active in a majority of the healthy elderly.