Droplet Absorption Research Articles

Numerical analysis of multistage ultrasonic atomization enhanced ammonia-water falling film absorption

The absorption refrigeration system is a widely used system that utilizes waste heat and is environmentally friendly. However, many current systems still suffer from challenges such as large volume, and high initial investment. Meanwhile, optimizing the absorber is crucial for enhancing system performance as it is one of the main components. In this study, a novel absorber model is proposed and its influencing factors of performance are comprehensively analyzed. In this model, the traditional falling film absorber is divided into multiple stages, with an ultrasonic atomizer (UA) installed in each stage to enhance absorption. In each stage, falling film and atomization absorption occur simultaneously, and their outputs are combined and weighted at the exit to maintain continuous droplet absorption, effectively utilizing the entire absorber volume. In this process, the vapor–liquid contact area is increased through atomization, and simultaneously, generated heat is removed by the cooling water during mixing. As a result, the absorber’s absorption capacity increases, and the outlet solution temperature decreases. The results demonstrate that compared to the traditional falling film absorber, the proposed absorber, when divided into two stages with a droplet diameter of 50 μm and an atomization rate x = 0.7, shows a 60.64 % increase in ammonia absorbed rate. Additionally, compared to double-stage absorber, triple-stage absorber increases the ammonia absorption by 9.52 %. As the number of stages increased, the strengthening effect persisted, albeit with a gradually diminishing rate of increase. Scaling up to ten stages results in a 152.63 % absorption rate increase and a size optimization of 60.43 %. In this paper, multi-stage ultrasonic atomization is innovatively introduced into the falling film absorber, which enhances ammonia absorption and reduces the volume of the absorber. Additionally, the multi-stage atomization offers an innovative method to improve the droplet absorption process.

Water interaction with MCC powder exposed to wetting–drying cycles: Comparison of capillary imbibition techniques

We investigate how wetting-and-drying (WD) cycles change the properties of microcrystalline cellulose (MCC) powder (Avicel PH101), due to hornification, by performing the following experiments: (i) drop absorption in a slightly compressed powder bed, (ii) capillary imbibition in a packed powder column, and (iii) drop absorption into tablets obtained by direct compression. We use the MCC powder as received, as well as the powder obtained after one or more WD cycles. In each case, the powder is sieved and fractionated into two samples, with different particle size distribution. First, we perform drop absorption experiments using both water and silicone oil (Polydimethylsiloxane - PDMS) drops. We observe that WD cycles increase the absorption time for water, a particularly marked effect after the first cycle, for both particle size distributions. Comparing the absorption times of water and oil drops we obtain a parameter β that characterizes the water-MCC interaction in lieu of the contact angle, and accounts for both capillary and swelling phenomena. We observed that the main effect is produced after the first WD cycle exhibited by a large (more than 50%) reduction in the β value. Column imbibition experiments performed using water and PDMS show that, initially, water penetrates at a larger rate than PDMS. This initialing faster water uptake is more significant for larger particles and fewer WD cycles. A power law regime is obtained in each case with a ζ exponent. In particular, a Washburn-like imbibition regime with ζ=0.5 is recovered at long times for both liquids. In the last experimental technique, we study the absorption of water drops deposited on tablets obtained by direct compression of the full distribution of powder particle sizes. We observe that absorption time increases with increasing number of WD cycles of the MCC powder. This is quantified by the 30% reduction of the ζ exponent that models the penetration rate of the drop.In summary, we use three kinds of experiments that cover a wide range of characteristic time scales to quantify the effect of WD cycles on the interaction of water with a swelling powder like MCC. The importance of comparing responses under different timescales is that the swelling phenomenon modifies the properties of the material in time, while imbibition happens. It is shown by this study that in some cases, the phenomena can be decoupled (droplet absorption in porous media) while in other cases it cannot be (droplet on tablets and intermediate times in capillary rise). Finally, at long times in capillary rise, even when the swelling and imbibition effects are not decoupled, it is possible to estimate the permeability ratio, χ, and its dependence with the WD cycles when the results are used in combination with the ones from the droplet absorption on powder test. The permeability decreased about 8 times for no WD cycles, and it decreased much less (2 and 3 for small and large particles respectively) for one and two WD cycles. We were able to quantitative characterize the effects of water-MCC interaction and this is important in order to predict how some pharmaceutical processes, like wet granulation, will be affected.

Effects of the soil moisture content and leaf memory effect on pesticide droplet absorption

Most pesticide spraying studies focus on increasing droplet deposition on target surfaces, with little consideration of post-deposition pesticide droplet absorption mechanisms. The pesticide droplet absorption efficiency of leaves is related to environmental factors and plant growth status during pesticide application. In field environments, irrigation-based soil moisture content regulation is a key plant growth status adjustment method. A theoretical pesticide droplet leaf absorption model was established to study the effects of soil moisture content on droplet absorption. Controlled environment experiments were carried out to investigate the absorption of pesticide droplets, and a chi-square test was used to verify the model reliability. The results showed that pesticide droplet leaf absorption flux first increased and then decreased with increasing soil water content. The diurnal leaf water potential and stomatal conductance variationsindicated a memory effect, resulting in diurnal variationsin the pesticide droplet leaf absorption flux. Under the same soil water conditions, the absorption flux first increased and then decreased from 8 am to 6 pm (Beijing Time). Effective application of pesticides can be significantly enhanced by controlling soil moisture content and selecting the appropriate spraying time. The best spraying combination improved the application effectiveness nearly threefold overthat of the worst combination. The results are crucial for rational pesticide use, crop spraying optimization, and water savings.

Analysis of wetability properties for kraft paper and corrugated cardboard

Corrugated cardboard and kraft paper are widely used in the packaging industry, providing versatility of use and protection for products. The analysis of the wettability properties of these materials is extremely important, as it allows us to understand how they react to humidity. This research aimed to study the wettability properties of corrugated paper (PO) and kraft paper in order to expand knowledge about the quality of packaging produced with them. The study was conducted in the laboratory using corrugated cardboard (PO) with waves of type "A", "B", "C" "E" and kraft paper obtained from coils with and without coating. The wettability tests performed were total immersion, absorption time and drop penetration method. The collected data was interpreted using descriptive and graphical analysis generated by the R program. The research concluded that for the total immersion test, both materials absorbed more water in the vertical position. In the droplet absorption and penetration time tests, cardboards with type “E” waves showed the highest results when compared to other materials, providing a superior barrier for products that will be transported in boxes of this type. Furthermore, the coated Kraft paper also demonstrated good performance for drop penetration testing, confirming the coating's effectiveness as a moisture barrier.

Enhancing colloid stability of polymer microspheres in water through SiO2 coating: Effects of coating cycles and surface coverage

In this study, we investigated the wettability and agglomeration characteristics of polymer microspheres coated with low-temperature deposited SiO2 in a fluidized bed atomic layer deposition (ALD) setup. Surface characterization revealed the presence of a significant amount of deposited Si-OH groups within the first cycles. A drastic decrease in agglomerate size, water contact angle (WCA), and droplet absorption time of the powder was observed when coating was applied. Furthermore, we observed an increase in the amount of Si-OH present on the particle surface with increasing coating cycles, while no significant improvement in water affinity was found after the first coating cycles. Our findings suggest that surface coverage is the primary factor in improving the colloid stability of particles, coated at low temperatures. The low temperature operation of our system introduced a chemical vapor deposition (CVD) component to our coating process, which allowed full surface coverage to be achieved within the first two coating cycles.

Coupled Eulerian Wall Film–Discrete Phase model for predicting respiratory droplet generation during a coughing event

Infectious respiratory diseases have long been a serious public health issue, with airborne transmission via close person-to-person contact being the main infection route. Coughing episodes are an eruptive source of virus-laden droplets that increase the infection risk of susceptible individuals. In this study, the droplet generation process during a coughing event was reproduced using the Eulerian wall film (EWF) model, and the absorption/expulsion of droplets was tracked using the discrete phase model (DPM). A realistic numerical model that included the oral cavity with teeth features and the respiratory system from the throat to the first bifurcation was developed. A coughing flow profile simulated the flow patterns of a single coughing episode. The EWF and DPM models were coupled to predict the droplet formation, generation, absorption, and exhalation processes. The results showed that a large droplet number concentration was generated at the beginning of the coughing event, with the peak concentration coinciding with the peak cough rate. Analysis of the droplet site of origin showed that large amounts of droplets were generated in the oral cavity and teeth surface, followed by the caudal region of the respiratory system. The size of the expelled droplets was 0.25–24 μm, with the peak concentration at 4–8 μm. This study significantly contributes to the realm on the site of origin and localized number concentration of droplets after a coughing episode. It can facilitate studies on infection risk assessment, droplet dispersion, and droplet generation mechanisms from other sneezing or phonation activities.

Droplet absorption and spreading into thin layers of polymer hydrogels

From biological tissues to microactuators and absorption of solvents into layers of paint, macroscopically non-porous materials with the capacity to swell when in contact with a solvent are ubiquitous. In these systems, owing to strong solid–fluid interactions, chemically driven flows can yield large geometric changes. We study experimentally and theoretically the canonical problem of the swelling of a thin hydrogel layer by a single water drop. Using a bespoke experimental set-up, we observe fast absorption leading to a radially spreading axisymmetric blister. We use a fully three-dimensional linear poroelastic framework with nonlinear kinematic equations to obtain governing equations, which we then reduce with thin-layer scalings to a one-dimensional nonlinear diffusion equation for the evolution of the blister geometry. In the limits of large and small deformations, the evolution of the blister characteristic height and radius are self-similar, following power laws in time. Our experimental measurements show that the evolution of the blister is broadly within the theoretical predictions in the large and small deformation regimes. In the general intermediate deformation regime, the measurements are well captured by our reduced one-dimensional diffusion model, which does not require the sophisticated and computationally expensive numerical approaches necessary for the original two-dimensional nonlinear coupled transport problem. By adapting modelling techniques from the fluid dynamics of thin porous elastic layers to a polymer swelling problem, our modelling framework extends the range of polymer swelling problems that can be treated with semi-analytical methods. Moreover, our detailed experimental data can serve as a test case for future nonlinear poroelastic frameworks of swelling polymer materials.

Enhancing the Surface Hydrophilicity of an Aluminum Alloy Using Two-Step Anodizing and the Effect on Inkjet Printing Characteristics

Aluminum alloy anodizing is widely used in the surface treatment industry to provide surface protection and decoration. The resulting anodic aluminum oxide film enables durable printing and dyeing of metals owing to its nanoporous structure, which easily absorbs ink. Conventional one-step anodizing of Al 1050 using sulfuric acid was observed to form a surface with small pore diameters less than 10 nm and lead to an average contact angle of 30°, whereas two-step anodizing yielded a regular pore pattern with significantly larger pores, reducing the contact angle to less than 20°. This change in pore structure and the corresponding enhanced hydrophilicity directly impacted inkjet printing characteristics; inkjet printing of 13 pL droplets on the one-step and two-step anodized surfaces showed that the average dot diameter varied from 72.2 μm to 48.0 μm according to applied voltage and anodizing time. The ink dot diameters on the two-step film were smaller than those on the one-step film produced under the same conditions, and the dot diameters decreased as the average pore diameter increased under an increasing anodizing voltage up to 20 V, indicating improved hydrophilicity. The pore volumes produced by two-step anodizing were larger, facilitating ink droplet absorption during spreading, which was examined by elemental analysis of cross-sections of the ink-filled porous specimen.

Influence of Surface Roughness on Droplet Evaporation and Absorption: Insights into Experiments from Lubrication-Theory-Based Models

While solid substrates are often idealized as being perfectly smooth, all real surfaces possess some level of topographical and chemical heterogeneity. This heterogeneity can greatly influence droplet dynamics. Mathematical models based on lubrication theory that account for surface roughness reveal how topographical defects induce contact-line pinning and affect the deposition patterns of colloidal particles suspended in the droplet. Contact-line pinning profoundly changes the behavior of droplet evaporation on horizontal and inclined impermeable substrates and droplet absorption on horizontal permeable substrates. Models accounting for surface roughness yield predictions that are qualitatively consistent with experimental observations and also provide insight into the underlying physical mechanisms. These models are a foundation for the exploration of a rich array of problems concerning droplet dynamics which are of both fundamental and practical interest.

Capillary adsorption of droplets into a funnel-like structure

The capability of droplet absorption into a funnel-like capillary tube is enormously affected by the combined influence of the droplet size, the opening angle, and the intrinsic wettability. Through a quantitative investigation of the penetration criteria, we demonstrate that a finite-volume droplet can be totally absorbed by a hydrophobic funnel-like structure depending on the opening angle. This result is sharply contrary to the previous conclusion for infinite-volume droplets in straight capillary tubes and our predictions extend the limit of the maximum contact angle for the complete penetration of droplets into capillary tubes in the literature.

Time-Resolved Ultrafast Interatomic Coulombic Decay in Superexcited Sodium-Doped Helium Nanodroplets.

The autoionization dynamics of superexcited superfluid He nanodroplets doped with Na atoms is studied by extreme-ultraviolet (XUV) time-resolved electron spectroscopy. Following excitation into the higher-lying droplet absorption band, the droplet relaxes into the lowest metastable atomic 1s2s 1,3S states from which interatomic Coulombic decay (ICD) takes place either between two excited He atoms or between an excited He atom and a Na atom attached to the droplet surface. Four main ICD channels are identified, and their decay times are determined by varying the delay between the XUV pulse and a UV pulse that ionizes the initial excited state and thereby quenches ICD. The decay times for the different channels all fall in the range of ∼1 ps, indicating that the ICD dynamics are mainly determined by the droplet environment. A periodic modulation of the transient ICD signals is tentatively attributed to the oscillation of the bubble forming around the localized He excitation.

Biotreatments Using Microbial Mixed Cultures with Crude Glycerol and Waste Pinewood as Carbon Sources: Influence of Application on the Durability of Recycled Concrete.

Two eco-friendly healing bioproducts generated from microbial mixed cultures (MMC) for the production of polyhydroxyalkanoates (PHA) were used as surface treatments, with two residual materials used as the substrates, namely crude glycerol and pinewood bio-oil. Their ability to improve the durability of concrete samples containing recycled aggregates was assessed. To determine this protective capacity, 180 samples were analyzed using different tests, such as water penetration under pressure, capillary absorption, freeze–thaw and water droplet absorption test. Three types of conditions were used: outdoor–indoor exposure, re-application of biopolymers and application in vertical exposure conditions. The results showed reductions of up to 50% in the water penetration test and a delay in the water droplet absorption test of up to 150 times relative to the reference. The surface application of these bioproducts significantly reduced the degree of water penetration in recycled concrete, increasing its useful lifespan and proving to be a promising treatment for protecting concrete surfaces.

Foliar water uptake in the needles of Pinus torreyana

The natural habitat of Pinus torreyana, the Torrey pine, is restricted to two locales in coastal Southern California that experience substantial fog and low clouds during the dry months of a Mediterranean climate. In similar semi-arid climate systems that encounter fog or low clouds, many plants can capture atmospheric moisture and are capable of direct foliar water uptake to reduce water stress. In this study, we investigated if the needles of P. torreyana are also capable of direct water uptake. In addition to water immersion, we measured the surface properties along a needle using microdroplets. The droplet contact angle is a measure of surface wettability, and the droplet absorption is a measure of localized foliar water uptake. The results showed that the entire length of the P. torreyana needle, including under the base sheath, is hydrophilic and capable of direct water uptake. The spatial gradients of the wettability and the water uptake along the needle are relatively small. Moreover, the wettability and water uptake increase from young shoots to one-year and two-year old needles. Our results also showed that the decrease in water uptake at higher contact angles can be described by a linear regression. Compared with the leaves of four shrubs in the same habitat, Heteromeles arbutifolia, Malosma laurina, Rhus integrifolia, and Eriodictyon crassifolium, the P. torreyana needles have lower contact angles and higher water uptake rates.

A reference-component coordinate system approach to model the mass transfer of a droplet with binary volatiles

A theoretical framework based on the reference-component centered coordinates was modified to enable the prediction study of the simultaneous absorption and evaporation of droplets consisting of two volatiles. A new equation of Robin boundary condition was imposed at the droplet-ambience interface, coupling with a new numerical scheme for solution. Experimental validation was performed with the following situations: evaporation of single pure droplet and bicomponent droplet, and simultaneous absorption and evaporation of droplet. The model predicted the mass profiles reasonably well for droplet evaporation while over-prediction was found for the case of simultaneous absorption and evaporation of droplet. Further preliminary evaluation has found the necessity to encounter the phenomenon of mass flux depression when predicting the simultaneous absorption and evaporation of droplet. This will provide a potential predictive tool for the processes which involves droplet absorption, such as antisolvent-vapor precipitation and gas scrubbing.

Use of Polycaprolactone Electrospun Nanofiber Mesh in a Face Mask.

Electrospun nanofiber mesh has previously been used as an air filtration device. However, the qualification of polycaprolactone (PCL) nanofiber mesh cloth in face masks to protect individuals against airborne particles carrying microorganisms has yet to be investigated. The long-term goal of this study is to develop methods to use PCL nanofiber mesh to provide better protection against microorganisms. To achieve this goal, we observed the morphology, water droplet absorption, thermal (differential scanning calorimetry), mechanical, and airborne particle filtering capabilities, and also the microbial activities of a PCL cloth, to evaluate whether it is suitable to act as a filter in a face mask. We have produced a polycaprolactone (PCL) nanofiber cloth after electrospinning it onto a drum for 3 and 10 min, referred to hereafter as PCL-3 and PCL-10, respectively. Our study found that the middle protection layer (control) of the Henry Schein Earloop Procedure Mask contains pores (average diameter = 5.72 ± 0.62 µm) which are 48 times larger than the diameter of a microorganism an average diameter of ~120 nanometers. However, PCL-10 nanofiber membranes show pores with an average diameter of 1.42 ± 0.34 µm. Our contact angle measurement tests found that all the samples were very hydrophobic (contact angle values varied between 120 and 150 degrees). However, both PCL cloths’ contact angle values were lower compared to the control. The produced PCL cloths showed a lower water droplet absorption compared to the control. Thermal studies found that PCL is stable in extreme conditions and no plasticizing effect occurs due to the presence of a solvent. Mechanical tests showed that PCL-10 cloth had higher strength and modulus compared to the control and PCL-3 under tension loading conditions. A vacuum experiment found that the PCL-10 fiber cloth could withstand a negative pressure of 18 Psi without any signs of breakage, and the mask was able to capture airborne particles and microorganisms. The feasibility of immobilizing anti-bacterial nanoparticles with PCL during electrospinning creates the future potential of producing an anti-bacterial face mask using PCL.

Spectral radiative properties of seawater-in-oil emulsions in visible-infrared region

Optical identification, classification and quantification of different oil slicks play important roles in oil-slick monitoring and detection. Under the actions of wind, waves and other marine forces, oil slicks on the sea surface mix with seawater to form oil-seawater emulsions. Herein, we focus on the spectral radiative properties of seawater-in-oil emulsions in the visible-infrared region. The complex refractive indices of crude oil are precisely measured using the combined ellipsometry-transmission method, after which the radiative properties of seawater droplets embedded in crude oil and seawater-in-oil emulsions obtained using the traditional and improved Lorenz-Mie theory are compared and analyzed to illustrate the effects of the absorption of host medium. The directional-hemispherical emittance and reflectance for seawater-in-oil emulsions are calculated using the Monte Carlo method. Results show that even though the traditional Mie-calculated radiative properties deviate obviously from the improved Lorenz-Mie theory results, directional-hemispherical reflectance and emittance of seawater-in-oil emulsions show relatively small difference. Besides, increasing the droplet volume fraction and film thickness may generally lead to enhanced scattering of seawater droplets, further leading to enhanced light reflection and reduced light emission. With the increase in both droplet size and absorption index of seawater and crude oil, the effects of seawater droplet absorption and oil absorption on overall attenuation of emulsions increase considerably; they may weaken the reflection and strengthen the emission. Obtaining the key radiative parameters of different oil species and oil emulsions will be helpful for promoting the practical application of quantitative optical remote sensing of oil slicks on sea surface.

Sustainable cement mortar bioformulated with a bioproduct obtained from fermentation of biodiesel’ crude glycerol

This study analyses the potential of a new eco-friendly bioproduct to construction sustainability and its contribution for improving the fresh and hardened properties of cement mortar. The bioproduct was obtained from biomass grown using crude glycerol (biodiesel production waste). With similar functionality than petrochemical-based additions, these bioproducts encourage the decrease on the use of fossil-based raw materials with the concomitant reduction in their carbon footprint. The effect of bioproduct's sonication and storage for 3 day at 4 °C were assessed. Properties such as consistence, porosity, density, compressive and flexural strength, water droplet absorption, capillary absorption, drying rate, thermal conductivity and ultrasonic pulse velocity of the bioformulated mortars were compared with a control cement mortar. The findings show that the bioproducts can be used as renewable and eco-friendly alternative to petrochemical-based polymer admixtures to lengthen mortar service life, even after storage. Sonicated bioproduct improved the water related mortar properties, while non-sonicated bioproduct developed higher mechanical properties. Contributing for a cleaner production of cementitious products, an original waste from chemical industry was transformed into a valuable raw material for use in the construction industry.

SARS-CoV-2 droplet deposition path and its effects on the human upper airway in the oral inhalation

Background and Objective: It is crucial to study the uptake of viral droplets in the human respiratory system to control, prevent, and treat diseases.Methods: In this study, a well-verified real anatomical model was used; the passage of air in the human upper respiratory system computed using high-quality Computer Tomography (CT) images. Then, the airflow field, along with the coronavirus micro-droplets injection, was examined in this realistic model using the Fluid-Structure Interaction (FSI) method. The Discrete Phase Model (DPM) was used to solve the field, and with the help of it, the accurate assessment of the temporal and spatial motion of the deposition in the virus-impregnated droplets was obtained in vitro in the upper respiratory system.Results: The results show that the amount of deposited micro-droplets in the nasal cavity area is meager at the inhalation only through the oral. However, it has the most residence time in this area. The most and least droplet absorption occurred in the oral cavity and larynx-trachea, respectively. Deposition efficiency is about 100% in 30 L/min flow rate and 10 μm diameter; in other words, no droplet enters the lungs. This study's other achievements include the relatively inverse relationship between droplets deposition efficiency in some parts of the upper airway, which have the most deformation in the tract.Conclusions: Utilization of a realistic model with accurate and precise computational analysis can end speculation about the deposition zone, accumulation, and the effects of the COVID-19 virus on the upper respiratory tract. On the other hand, recognizing the virus-containing droplet location can ease understanding the areas where the virus can first infect in the upper respiratory tract.

Numerical investigation of quasi-sessile droplet absorption into wound dressing capillaries

The key concept in wound dressing design and development is the fact that keeping a wound moist accelerates healing. Therefore, the selection of the appropriate wound dressing type is vital. The absorption of wound exudate by wound dressings can be considered as a microfluidic phenomenon that can be investigated either by performing high resolution laboratory experiments or by utilizing high resolution Computational Fluid Dynamics numerical simulations. As an initial step, in the present paper, the effects of the pore size (wound dressing porosity), the liquid (wound exudate) viscosity, and the initial droplet diameter are numerically investigated using a simplified analog of the phenomenon that consists of a quasi-sessile droplet being absorbed by a single cylindrical pore. For this purpose, an enhanced Volume Of Fluid model, developed in the general context of OpenFOAM, is validated and applied. It is found that distinct droplet absorption rates exist with specific relationships derived using best-fit lines that can predict the absorption rates for particular values of pore size and liquid viscosity. For the examined Eo and Oh number ranges (0.0015 < Eo < 0.15 and 0.0035 < Oh < 0.095), these distinct droplet absorption rates are directly linked with four different droplet evolution regimes that are grouped in a well-defined flow map. Finally, it is shown that the resulting liquid absorption rates are not significantly affected by the initial droplet diameter and that an appropriate wound dressing porosity can be selected by an estimation of the wound exudate physical properties.

Variation in carotenoid-containing retinal oil droplets correlates with variation in perception of carotenoid coloration

In the context of mate choice, males may vary continuously in their expression of assessment signals, typically reflecting information about variation in mate quality. Similarly, females may exhibit variation in mate preference, which could be due to differences in how individual females perceive signals. The extent to which perception varies across individuals, however, and whether differences in sensory physiology underlie perceptual differences is poorly understood. Carotenoid pigments create the orange-red coloration of many assessment signals, and they also play a role in color discrimination in many vertebrates via their presence in retinal oil droplets. Here, we link variation in oil droplet carotenoid concentration with the ability of female zebra finches (Taeniopygia guttata) to discriminate an orange-red color continuum that parallels variation in male beak color, a mate assessment signal. We have shown previously that zebra finch females perceive this color range categorically, meaning they label color stimuli from this continuum as belonging to two categories and exhibit better discrimination between colors from different categories as compared with equally different colors from within a category. We quantified behavioral color discrimination and R-type (red) cone oil droplet spectral absorption, a proxy for carotenoid concentration. Oil droplet absorption was strongly predictive of variation in behavioral color discrimination ability. In particular, higher carotenoid concentration in oil droplets correlated with increased discrimination of colors from different sides of the previously identified category boundary. These data show that differences in the sensory periphery can correlate with individual variation in perception of a signal-relevant color range.Significance statementSignal receivers vary in their preferences for signaling traits, but whether this is due to variation in how different receivers perceive signals is not well-understood. We show that variation between individual zebra finch females in perception of an orange-red continuum range correlates with the carotenoid concentration of retinal oil droplets. These data provide the first direct evidence that individual variation in oil droplet carotenoid concentration can lead to variation in color discrimination ability. Linking variation in signal-relevant color discrimination ability with variation in retinal physiology suggests a potential mechanism contributing to individual variation in signal assessment.