Binary Mixture Droplet Evaporation on Microstructured Decorated Surfaces and the Mixed Stick-Slip Modes.

The interactions between liquid droplets and solid surfaces during wetting and phase change are important to many applications and are related to the physicochemical properties of the substrate and the fluid. In this work, we investigate experimentally the evaporation of pure water, pure ethanol, and their binary mixture droplets, accessing a wide range of surface tensions, on hydrophobic micro-pillared surfaces varying the spacing between the pillars. Results show that on structured surfaces, droplets evaporate following three classical evaporative behaviors: constant contact radius/pinning, stick-slip, or mixed mode. In addition, we report two further droplet evaporation modes, which are a mixed stick-slip mode where the contact angle increases while the contact radius decreases in a stick-slip fashion and a mixed stick-slip mode where both the contact angle and the contact radius decrease in a stick-slip fashion. We name these evaporation modes not yet reported in the literature as the increasing and decreasing contact angle mixed stick-slip modes, respectively. The former ensues because the fluid surface tension increases as the most volatile fluid evaporates coupled to the presence of structures, whereas the latter is due to the presence of structures for either fluid. The duration of each evaporation mode is dissimilar and depends on the surface tension and on the spacing between structures. Pure water yields longer initial pinning times on all surfaces before stick-slip ensues, whereas for binary mixtures and pure ethanol, initial pinning ensues mainly on short spacing structures due to the different wetting regimes displayed. Meanwhile, mixed stick-slip modes ensue mainly for high ethanol concentrations and/or pure ethanol independent of the solid fraction and for low ethanol concentrations on large spacing. Contact line jumps, changes in contact angle and pinning forces are also presented and discussed. This investigation provides guidelines for tailoring the evaporation of a wide range of surface tension fluids on structured surfaces for inkjet printing, DNA patterning, or microfluidics applications.

Semicrystalline Polymer Micro/Nanostructures Formed by Droplet Evaporation of Aqueous Poly(ethylene oxide) Solutions: Effect of Solution Concentration.

Deposits formed after evaporation of sessile droplets, containing aqueous solutions of poly(ethylene oxide), on hydrophilic glass substrates were studied experimentally and mathematically as a function of the initial solution concentration. The macrostructure and micro/nanostructures of deposits were studied using stereo microscopy and atomic force microscopy. A model, based on thin-film lubrication theory, was developed to evaluate the deposit macrostructure by estimating the droplet final height. Moreover, the model was extended to evaluate the micro/nanostructure of deposits by estimating the rate of supersaturation development in connection with the driving force of crystallization. Previous studies had only described the macrostructure of poly(ethylene oxide) deposits formed after droplet evaporation, whereas the focus of our study was the deposit micro/nanostructures. Our atomic force microscopy study showed that regions close to the deposit periphery were composed of predominantly semicrystalline micro/nanostructures in the form of out-of-plane lamellae, which require a high driving force of crystallization. However, deposit central areas presented semicrystalline micro/nanostructures in the form of in-plane terraces and spirals, which require a lower driving force of crystallization. Increasing the initial concentration of solutions led to an increase in the lengths and thicknesses of the out-of-plane lamellae at the deposits' periphery and enhanced the tendency to form spirals in the central areas. Our numerical study suggested that the rate of supersaturation development and thus the driving force of crystallization increased from the center toward the periphery of droplets, and the supersaturation rate was lower for solutions with higher initial concentrations at each radius. Therefore, periphery areas of droplets with lower initial concentrations were suitable for the formation of micro/nanostructures which require higher driving forces, whereas central areas of droplets with higher initial concentration were desirable for the formation of micro/nanostructures which require lower driving forces. These numerical results were in good qualitative agreement with the experimental findings.

On the Effect of Substrate Viscoelasticity on the Evaporation Kinetics and Deposition Patterns of Nanosuspension Drops.

This study investigates the evaporation of sessile pure water and nanosuspension drops on viscoelastic polydimethylsiloxane (PDMS) films. We varied the viscoelasticity of the PDMS films by controlling the curing ratio and categorized them into three types: stiff (10:1, 20:1, 40:1), soft (60:1, 80:1), and very soft (100:1, 120:1, 140:1, 160:1). On stiff surfaces, pure water drops initially evaporate in a constant contact radius (CCR) mode, followed by a constant contact angle mode, and finally in a mixed mode of evaporation. Nanosuspension drops follow the same trend as water drops but with a difference toward the end of their lifetimes, when a short second CCR mode is observed. Complete evaporation of nanosuspension drops on stiff substrates leads to particle deposition patterns similar to a coffee ring with cracks and deposition tails. On soft surfaces, the initial spreading is followed by a pseudo-CCR mode. Complete evaporation of nanosuspension drops on soft substrates leads to deposits in the form of a uniform ring with a sharp ox-horn profile. Unexpectedly, the initial spreading is followed by a mixed mode on very soft substrates, on which wetting ridges (WRs) pulled up by the vertical component of surface tension are clearly observed in the vicinity of the contact line (CL). As the evaporation proceeds, the decreasing contact angle breaks the force balance in the horizontal direction at the CL and gives rise to a net horizontal force, which causes the CL to recede, transferring the horizontal force to the WR. Because of the viscoelastic nature of the very soft substrate, this horizontal force acting on the WR cannot be completely countered by the bulk of the substrate underneath. As a result, the WR moves horizontally in a viscous-flow way, which also enables the CL to be continuously anchored to the ridge and to recede relative to the bulk of the substrate. Consequently, a mixed mode of evaporation occurs. Complete evaporation of nanosuspension drops on very soft substrates leads to finger-like deposits.

How does blood regulate cerebral temperatures during hypothermia?

Macro-modeling of cerebral blood flow can help determine the impact of thermal intervention during instances of head trauma to mitigate tissue damage. This work presents a bioheat model using a 3D fluid-porous domain coupled with intersecting 1D arterial and venous vessel trees. This combined vascular porous (VaPor) model resolves both cerebral blood flow and energy equations, including heat generated by metabolism, using vasculature extracted from MRI data and is extended using a tree generation algorithm. Counter-current flows are expected to increase thermal transfer within the brain and are enforced using either the vascular structure or flow reversal, represented by a flow reversal constant, C R . These methods exhibit larger average brain cooling (from 0.56 °C ± <0.01 °C to 0.58 °C ± <0.01 °C) compared with previous models (0.39 °C) when scalp temperature is reduced. An greater reduction in core brain temperature is observed (from 0.29 °C ± <0.01 °C to 0.45 °C ± <0.01 °C) compared to previous models (0.11 °C) due to the inclusion of counter-current cooling effects. The VaPor model also predicts that a hypothermic average temperature (<36 °C) can be reached in core regions of neonatal models using scalp cooling alone.

Manufacturing of microcirculation phantoms using rapid prototyping technologies.

In this paper, we describe a method for the manufacturing of a microcirculation phantom that may be used to investigate hemodynamics using optics based methods. We made an Acrylonitrile Butadiene Styrene (ABS) negative mold, manufactured in a Fused Deposition Modelling (FDM) printer, embedded it in Polydimethysilioxane (PDMS) and dissolved it from within using acetone. We successfully made an enlarged three-dimensional (3D) network of microcirculation, and tested it using red blood cell (RBC) analogues. This phantom may be used for testing medical imaging technology.

Analysis of DNA binding and nucleotide flipping kinetics using two-color two-photon fluorescence lifetime imaging microscopy.

Uracil DNA glycosylase plays a key role in DNA maintenance via base excision repair. Its role is to bind to DNA, locate unwanted uracil, and remove it using a base flipping mechanism. To date, kinetic analysis of this complex process has been achieved using stopped-flow analysis but, due to limitations in instrumental dead-times, discrimination of the "binding" and "base flipping" steps is compromised. Herein we present a novel approach for analyzing base flipping using a microfluidic mixer and two-color two-photon (2c2p) fluorescence lifetime imaging microscopy (FLIM). We demonstrate that 2c2p FLIM can simultaneously monitor binding and base flipping kinetics within the continuous flow microfluidic mixer, with results showing good agreement with computational fluid dynamics simulations.

Convective rolls and hydrothermal waves in evaporating sessile drops.

Recent experiments on the evaporation of sessile droplets have revealed the spontaneous formation of various patterns including the presence of hydrothermal waves. These waves had previously been observed, in the absence of evaporation, in thin liquid layers subjected to an imposed, uniform temperature gradient. This is in contrast to the evaporating droplet case wherein these gradients arise naturally due to evaporation and are spatially and temporally varying. In the present paper, we present a theory of evaporating sessile droplets deposited on a heated surface and propose a candidate mechanism for the observed pattern formation using a linear stability analysis in the quasi-steady-state approximation. A qualitative agreement with experimental trends is observed.

Three-dimensional molecular mapping in a microfluidic mixing device using fluorescence lifetime imaging.

Fluorescence lifetime imaging (FLIM) is used to quantitatively map the concentration of a small molecule in three dimensions in a microfluidic mixing device. The resulting experimental data are compared with computational fluid-dynamics (CFD) simulations. A line-scanning semiconfocal FLIM microscope allows the full mixing profile to be imaged in a single scan with submicrometer resolution over an arbitrary channel length from the point of confluence. Following experimental and CFD optimization, mixing times down to 1.3+/-0.4 ms were achieved with the single-layer microfluidic device.