Controlled assembly of heterotypic cells in a core–shell scaffold: organ in a droplet

Organ in a drop: we use droplet-based microfluidics to fabricate large-scale, monodisperse, portable micro organs, each in an individual drop. We spatially assemble hepatocytes and fibroblasts in a biocompatible core–shell scaffold, forming an artificial liver in each drop, and expressing enhanced liver-specific functions.

Statistical Quality of Spectral Polarimetric Variables for Weather Radar

Spectral polarimetry for weather radar capitalizes on both Doppler and polarimetric measurements to reveal polarimetric variables as a function of radial velocity through spectral analysis. For example, spectral differential reflectivity at a velocity represents the differential reflectivity from all the scatterers that have the same radial velocity of interest within the radar resolution volume. Spectral polarimetry has been applied to suppress both ground and biological clutter, retrieve individual drop size distributions from a mixture of different types of hydrometeors, and estimate turbulence intensity, for example. Although spectral polarimetry has gained increasing attention, statistical quality of the estimation of spectral polarimetric variables has not been investigated. In this work, the bias and standard deviation (SD) of spectral differential reflectivity and spectral copolar correlation coefficient estimated from averaged spectra were derived using perturbation method. The results show that the bias and SD of the two estimators depend on the spectral signal-to-noise ratio, spectral copolar correlation coefficient, the number of spectrum average, and spectral differential reflectivity. A simulation to generate time series signals for spectral polarimetry was developed and used to verify the theoretical bias and SD of the two estimators.

Rain microstructure retrievals using 2-D video disdrometer and C-band polarimetric radar

Measurements using the 2-D video disdrometer (2DVD) taken during a heavy rainfall event in Huntsville, Alabama, are analysed. The 2DVD images were processed to derive the rain microstructure parameters for each individual drop, which in turn were used as input to the T-matrix method to compute the forward and back scatter amplitudes of each drop at C-band. The polarimetric radar variables were then calculated from the individual drop contribution over a finite time period, e.g., 1 min. The calculated co-polar reflectivity, differential reflectivity, specific differential propagation phase and the co-polar correlation coefficient were compared with measurements from a C-band polarimetric radar located 15 km away. An attenuation-correction method based on the specific differential propagation phase was applied to the co-polar and differential reflectivity data from the C-band radar, after ensuring accurate radar calibration. Time series comparisons of the parameters derived from the 2DVD and C-band radar data show very good agreement for all four quantities, the agreement being sometimes better than the computations using the 1-min drop size distribution and bulk assumptions on rain microstructure (such as mean shapes and model-based assumptions for drop orientation). The agreement is particularly improved in the case of co-polar correlation coefficient since this parameter is very sensitive to variation of shapes as well as orientation angles. The calculations mark the first attempt at utilizing experimentally derived "drop- by-drop" rain microstructure information to compute the radar polarimetric parameters and to demonstrate the value of utilizing the 2-D video disdrometer for studying rain microstructure under various precipitation conditions. Histograms of drop orientation angles as well as the most probable drop shapes and the corresponding variations were also derived and compared with prior results from the 80 m fall "artificial rain" experiment.

Small-Scale Drop-Size Variability: Empirical Models for Drop-Size-Dependent Clustering in Clouds

By analyzing aircraft measurements of individual drop sizes in clouds, it has been shown in a companion paper that the probability of finding a drop of radius r at a linear scale l decreases as lD(r), where 0 ≤ D(r) ≤ 1. This paper shows striking examples of the spatial distribution of large cloud drops using models that simulate the observed power laws. In contrast to currently used models that assume homogeneity and a Poisson distribution of cloud drops, these models illustrate strong drop clustering, especially with larger drops. The degree of clustering is determined by the observed exponents D(r). The strong clustering of large drops arises naturally from the observed power-law statistics. This clustering has vital consequences for rain physics, including how fast rain can form. For radiative transfer theory, clustering of large drops enhances their impact on the cloud optical path. The clustering phenomenon also helps explain why remotely sensed cloud drop size is generally larger than that measured in situ.

Near-contact thermocapillary motion of two non-conducting drops

The axisymmetric, thermocapillary-driven motion of a pair of non-conducting, spherical drops in near contact is analysed for conditions of small Reynolds and Marangoni numbers. The pairwise motion and an associated contact force are computed by considering touching drops in point contact. Relative motion for nearly touching drops results from the contact force balanced by a lubrication resistance. A new, analytical solution is obtained for the axisymmetric temperature field around an unequal pair of non-conducting, tangent spheres embedded in an ambient temperature gradient. Numerical results for the pairwise migration velocity, contact force, and the relative and individual drop velocities are presented for all size ratios and a wide range of viscosity ratios, and asymptotic formulae are derived for small size ratios. For nearly equisized drops, the ratio of the relative velocity for two drops in near contact to that for widely separated drops is similar for thermocapillary-driven and gravity-driven motion. For small and moderate size ratios, however, this ratio is much larger for thermocapillary-driven relative motion than for gravity-driven relative motion, indicating that the former represents a more efficient coalescence mechanism. An explanation for this finding is provided in terms of the thermocapillary motion of the interface of the larger drop aiding the withdrawal of continuous phase from between the two drops.

Effects of flow velocity variation on rain driven transportation and the role of rain impact in the movement of solids

Rain-flow transportation in, and airsplash from, shallow sheet flows of varying velocity were investigated experimentally by using four drop sizes at constant intensity (64 mm h-1). Whereas airsplash yields were negligible, rain-flow transportation reached rates of up to 3 g m-1s-1 and exhibited two distinct modes of operation, or regimes, depending on whether the unimpacted flows were laminar or transitional to turbulent. In the first of these regimes, clouds of particles, suspended by individual drop impacts, were moved downstream before settling back to the bed. In the second regime, general turbulence, imposed by drop bombardment, made the flow everywhere able to entrain and transport particles without direct dependence on individual drop impacts. These findings are combined with those of previous studies to make an assessment of the roles of rain and flow in water erosion.

Effects of Variations in Drop Makeup on the Phytotoxicity of Glyphosate

Field and greenhouse trials have shown that ultra-low-volume applications (ULV) of glyphosate (N-(phosphonomethyl)glycine] exhibit a greater phytotoxic effect on barley (Hordeum vulgareL. ‘Bonanza′) at a given rate than do high-diluent-volume applications with a conventional hydraulic sprayer. Three greenhouse studies and one field trial were conducted to evaluate the influence of individual drop makeup on the effectiveness of ULV applications. Barley plants were treated at the four-leaf stage with one, three, or nine drops (1μl each) of glyphosate and a non-ionic surfactant with the concentration of the drops adjusted so that the total amount of active ingredient applied to each plant was constant. Application of a single, concentrated drop was significantly more effective in reducing total shoot growth than were applications of more dilute drops in greater number. When the dilute drop was supplemented with additional surfactant, phytotoxicity was restored, provided there was a sufficiently high concentration of glyphosate in the drop. With the substitution of a commercial formulation of glyphosate, the response of barley to variations in drop number and drop concentrations followed a similar trend. The response of field-grown plants did not differ from plants grown under controlled conditions.

The instability of evaporating charged drops

These studies constitute an extension of the work of Doyle, Moffett & Vonnegut (1964).Experiments showed that the evaporation of a charged droplet of water, aniline or toluene supported in a vertical electric field is accompanied by no discernible loss of charge and a consequent increase in electrical pressure in the drop surface owing to the decrease in radius. When the relationship between drop charge Q and radius R becomes consistent with the Rayleigh criterion the drop disintegrates at its uppermost point, where the electrical pressure is a maximum, to eject about 25% of its mass in the form of highly charged droplets. The measured relationship between the quantity of ejected charge, ΔQ, the mass loss, ΔM, and R was close to that derived theoretically from Rayleigh's equation, indicating that the value of ΔQ for a particular ΔM and R is approximately the theoretical minimum and that an individual drop may undergo a sequence of disintegrations, as was observed. The range of droplet radii studied by means of this technique was 30–200 μ.The absence of disintegrations during the evaporation of much larger charged drops suspended from insulating fibres was attributed to corona discharge.