Visualization of porosity and pore size gradients in electrospun scaffolds using laser metrology.

We applied a recently developed method, laser metrology, to characterize the influence of collector rotation on porosity gradients of electrospun polycaprolactone (PCL) widely investigated for use in tissue engineering. The prior- and post-sintering dimensions of PCL scaffolds were compared to derive quantitative, spatially-resolved porosity 'maps' from net shrinkage. Deposited on a rotating mandrel (200 RPM), the central region of deposition reaches the highest porosity, ~92%, surrounded by approximately symmetrical decreases to ~89% at the edges. At 1100 RPM, a uniform porosity of ~88-89% is observed. At 2000 RPM, the lowest porosity, ~87%, is found in the middle of the deposition, rebounding to ~89% at the edges. Using a statistical model of random fiber network, we demonstrated that these relatively small changes in porosity values produce disproportionately large variations in pore size. The model predicts an exponential dependence of pore size on porosity when the scaffold is highly porous (e.g., >80%) and, accordingly, the observed porosity variation is associated with dramatic changes in pore size and ability to accommodate cell infiltration. Within the thickest regions most likely to 'bottleneck' cell infiltration, pore size decreases from ~37 to 23 µm (38%) when rotational speeds increased from 200 to 2000 RPM. This trend is corroborated by electron microscopy. While faster rotational speeds ultimately overcome axial alignment induced by cylindrical electric fields associated with the collector geometry, it does so at the cost of eliminating larger pores favoring cell infiltration. This puts the bio-mechanical advantages associated with collector rotation-induced alignment at odds with biological goals. A more significant decrease in pore size from ~54 to ~19 µm (65%), well below the minimum associated with cellular infiltration, is observed from enhanced collector biases. Finally, similar predictions show that sacrificial fiber approaches are inefficient in achieving cell-permissive pore sizes.

Measuring Force of Droplet Detachment from Hydrophobic Surfaces via Partial Cloaking with Ferrofluids.

This paper presents a new approach to measure the force required to detach a water (a polar liquid) droplet from a hydrophobic surface. This is done by partially cloaking the droplet with a high-surface-tension oil-based ferrofluid and using a magnet to apply a controllable body force to the resulting compound droplet. Placing the assembly on a sensitive scale, the magnet can then be brought closer to the droplet to detach it from the surface while recording the forces applied to the droplet. The work presented here is novel as it uses the concept of partial cloaking in which the solid-droplet contact area is not contaminated by the ferrofluid (and the measured forces do not need postprocessing). Our study is accompanied by numerical simulations aimed at improving our understanding of the interplay between the interfacial forces in a two-phase droplet under the influence of a strong (detaching) body force and at providing additional data for in-depth analyses of these forces. In particular, the minimum ferrofluid volume required for successful water droplet detachment from hydrophobic surfaces is computed for ferrofluids of different surface tensions, and they are compared to experimental data obtained from detaching water droplets from electrospun polystyrene coatings. It is also shown that the detachment force measured via partial cloaking is independent of the volume of the ferrofluid used for the experiment.

Competing forces on a liquid bridge between parallel and orthogonal dissimilar fibers.

This paper presents a detailed investigation on the mechanical forces acting on a liquid bridge between dissimilar fibers in parallel and orthogonal configurations. These forces were measured experimentally, using a sensitive scale, and were also predicted computationally, via numerical simulation. Special attention was paid to the fiber-fiber spacing at which the liquid bridge detached from the fibers, and to how a transition from an equilibrium liquid bridge to a spontaneously (time-dependent) detaching bridge took place. It was found that, while varying the spacing between the fibers affects a liquid bridge differently for fibers with different relative angles with respect to one another, the spacing at which the bridge detaches from the fibers is independent of the fibers' relative angle. This paper also formulates the contribution of the geometrical and wetting properties of the fibers competing for the droplet that results from a liquid bridge detachment, and presents a mathematical expression to predict the fate of that droplet.

Dynamical insights into the mechanism of a droplet detachment from a fiber.

Quantifying the detachment behavior of a droplet from a fiber is important in many applications such as fog harvesting, oil-water separation, or water management in fuel cells. When the droplets are forcibly removed from hydrophilic fibers, the ease of detachment strongly depends on droplet volume and the rate of the process controlled by the applied force. Experiments, conducted on a ferrofluid under magnetic force, as well as continuum level calculations from fluid mechanics have so far been unable to resolve the time-dependent dynamics of droplet detachment and, most importantly, to assess the role of the applied force as the key determinant of the volume of the droplet residue remaining on the fiber after detachment. In the present work, we study the mechanism of water droplet detachment and retention of residual water on smooth hydrophilic fibers using nonequilibrium molecular dynamics simulations. We investigate how the applied force affects the breakup of a droplet and how the minimal detaching force per unit mass decreases with droplet size. We extract scaling relations that allow extrapolation of our findings to larger length scales that are not directly accessible by molecular models. We find that the volume of the residue on a fiber varies nonmonotonically with the detaching force, reaching the maximal size at an intermediate force and associated detachment time. The strength of this force decreases with the size of the drop, while the maximal residue increases with the droplet volume, V, sub-linearly, in proportion to the V2/3.

Novel method to characterize superhydrophobic coatings.

Superhydrophobic coatings possess a strong water-repellent characteristic, which, among several other potential applications, enhances the mobility of water droplets over the surface. The coating traps air within its micropores, such that a submerged moving body experiences shear-free and no-slip regions over, respectively, the air pockets and the solid surface. This, in turn, may lead to significant skin-friction reduction. The coating maintains its superhydrophobicity as long as the air remains entrapped. It is therefore of great interest to precisely measure the amount of trapped air, which is particularly difficult to estimate for coatings with disordered microstructures. A novel method to measure the effective thickness and gas volume fraction of superhydrophobic coatings with either ordered or random microroughness is advanced. The technique is applied to both aerogel and electrospun fibrous coatings. The experiments utilize a sensitive weighing scale (down to 10(-4) gm) and height gauge (down to 10 µm) to determine the buoyancy force on an immersed, coated glass-slide substrate. The measured force is used to calculate the volume fraction of entrapped air. The coating's effective thickness also follows from the same calculations. The sensitivity of our particular scale enables the measuring of thicknesses down to 3 µm, which is not readily possible with conventional thickness gauges. Smaller thicknesses could be measured using more sensitive scales.

Influence of flow on longevity of superhydrophobic coatings.

Previous studies have demonstrated the capability of superhydrophobic surfaces to produce slip flow and drag reduction, which properties hold considerable promise for a broad range of applications. However, in order to implement such surfaces for practical utilizations, environmental factors such as water movement over the surface must be observed and understood. In this work, experiments were carried out to present a proof-of-concept study on the impact of flow on longevity of polystyrene fibrous coatings. The time-dependent hydrophobicity of a submerged coating in a pressure vessel was determined while exposing the coating to a rudimentary wall-jet flow. Rheological studies were also performed to determine the effect of the flow on drag reduction. The results show that the longevity of the surface deteriorates by increasing the flow rate. The flow appears to enhance the dissolution of air into water, which leads to a loss of drag reduction.

A methodology for determining optimal thermal damage in magnetic nanoparticle hyperthermia cancer treatment.

Hyperthermia treatment of tumors uses localized heating to damage cancer cells and can also be utilized to increase the efficacy of other treatment methods such as chemotherapy. Magnetic nanoparticle hyperthermia is one of the least invasive techniques of delivering heat. It is based on injecting magnetic nanoparticles into the tumor and subjecting them to an alternating magnetic field. The technique is aimed at damaging the tumor without affecting the surrounding healthy tissue. In this preliminary study, we consider a simplified model (two concentric spheres that represent the tumor and its surrounding tissues) that employs a numerical solution of the Pennes bioheat equation. The model assumes a Gaussian distribution for the spatial variation of the applied thermal energy and an exponential decay function for the time variation. The objective of the study is to optimize the parameters that control the spatial and the time variation of the thermal energy. The optimization process is performed by formulating a fitness function that rewards damage in the region representing the tumor but penalizes damage in the surrounding tissues. Because of the flatness of this fitness function near the optimum, a genetic algorithm is used as the optimization method for its robust non-gradient-based approach. The overall aim of this work is to propose a methodology that can be used for hyperthermia treatment in a clinical scenario.

In situ, noninvasive characterization of superhydrophobic coatings.

Light scattering was used to measure the time-dependent loss of air entrapped within a submerged microporous hydrophobic surface subjected to different environmental conditions. The loss of trapped air resulted in a measurable decrease in surface reflectivity and the kinetics of the process was determined in real time and compared to surface properties, such as porosity and morphology. The light-scattering results were compared with measurements of skin-friction drag, static contact angle, and contact-angle hysteresis. The in situ, noninvasive optical technique was shown to correlate well with the more conventional methods for quantifying surface hydrophobicity, such as flow slip and contact angle.