Shape-gradient composite surfaces: water droplets move uphill

The approach of water droplets self-running horizontally and uphill without any other forces was proposed by patterning the shape-gradient hydrophilic material (i.e., mica) to the hydrophobic matrix (i.e., wax or low-density polyethylene (LDPE)). The shape-gradient composite surface is the best one to drive water droplet self-running both at the high velocity and the maximal distance among four different geometrical mica/wax composite surfaces. The driving force for the water droplets self-running includes: (1) the great difference in wettability of surface materials, (2) the low contact angle hysteresis of surface materials, and (3) the space limitation of the shape-gradient transportation area. Furthermore, the average velocity and the maximal distance of the self-running were mainly determined by the gradient angle (alpha), the droplet volume, and the difference of the contact angle hysteresis. Theoretical analysis is in agreement with the experimental results.     

Colloid probe AFM investigation of the influence of cross-linking on the interaction behavior and nano-rheology of colloidal droplets

The repulsive forces between a glass sphere and immobilized colloidal droplets of poly(dimethylsiloxane) (PDMS) (with various levels of internal cross-linking) have been determined in aqueous solution using colloid probe atomic force microscopy. On initial surface approach, droplet deformation is negligible and interaction forces resemble those expected for electrical double layer interaction of rigid spheres. Upon further approach, droplet flattening results in forces that deviate below rigid body electrical double layer interaction. The extent of droplet deformation has been determined in terms of the deviation from hard-sphere interaction. Droplet deformability is strongly dependent on the droplet cross-linking level and hence controlled by some combination of the bulk rheological and interfacial properties of the droplets. Droplet nano-rheology has been determined from the extent of force curve hysteresis. For liquidlike droplets, with low levels of cross-linking, no force curve hysteresis is observed and the elastic deformation may be described by a single spring constant, which is controlled by the interfacial properties. For highly cross-linked droplets, the extent of deformation is controlled by the droplet's bulk rheology rather than the interfacial properties. Upon retraction of the surfaces, force curve hysteresis is observed and is due to the viscoelastic response of the PDMS. The extent of hysteresis is dependent on the rate of approach/retraction and the loading force and has been theoretically analyzed to determine nano-rheological parameters that describe droplet relaxation processes. Elastic moduli and relaxation times of the PDMS droplets vary over several orders of magnitude as a function of cross-linking.     

Two-dimensional fluidics based on differential lyophobicity and gravity

We have prepared planar fluidics devices using binary chemical patterns consisting of hydrophobic "roads" on which water droplets slide easily and more hydrophobic "curbs" that direct droplet motion. Contact angle and contact angle hysteresis both control the motion of liquid droplets on surfaces. The difference between the advancing contact angles of the two regions prevents the liquid from crossing the interface between them. The low hysteresis of the roads allows facile movement. Gravity (slight tilting of samples) forces droplets to move effortlessly in defined pathways even though the difference in contact angles is not large and both regions are hydrophobic.     

Methodology for calculating the volume of condensate droplets on topographically modified, microgrooved surfaces

Liquid droplets on micropatterned surfaces consisting of parallel grooves tens of micrometers in width and depth are considered, and a method for calculating the droplet volume on these surfaces is presented. This model, which utilizes the elongated and parallel-sided nature of droplets condensed on these microgrooved surfaces, requires inputs from two droplet images at ? = 0° and ? = 90°--namely, the droplet major axis, minor axis, height, and two contact angles. In this method, a circular cross-sectional area is extruded the length of the droplet where the chord of the extruded circle is fixed by the width of the droplet. The maximum apparent contact angle is assumed to occur along the side of the droplet because of the surface energy barrier to wetting imposed by the grooves--a behavior that was observed experimentally. When applied to water droplets condensed onto a microgrooved aluminum surface, this method was shown to calculate the actual droplet volume to within 10% for 88% of the droplets analyzed. This method is useful for estimating the volume of retained droplets on topographically modified, anisotropic surfaces where both heat and mass transfer occur and the surface microchannels are aligned parallel to gravity to assist in condensate drainage.     

Contact-angle hysteresis on super-hydrophobic surfaces

The relationship between perturbations to contact angles on a rough or textured surface and the super-hydrophobic enhancement of the equilibrium contact angle is discussed theoretically. Two models are considered. In the first (Wenzel) case, the super-hydrophobic surface has a very high contact angle and the droplet completely contacts the surface upon which it rests. In the second (Cassie-Baxter) case, the super-hydrophobic surface has a very high contact angle, but the droplet bridges across surface protrusions. The theoretical treatment emphasizes the concept of contact-angle amplification or attenuation and distinguishes between the increases in contact angles due to roughening or texturing surfaces and perturbations to the resulting contact angles. The theory is applied to predicting contact-angle hysteresis on rough surfaces from the hysteresis observable on smooth surfaces and is therefore relevant to predicting roll-off angles for droplets on tilted surfaces. The theory quantitatively predicts a "sticky" surface for Wenzel-type surfaces and a "slippy" surface for Cassie-Baxter-type surfaces.     

Sliding variability of droplets on a hydrophobic incline due to surface entrained air bubbles

The ability of a liquid droplet to move on an incline has important ramifications in discrete volume fluidic devices. By taking advantage of the spontaneous and copious formation of visible air bubbles within water droplets on a polytetrafluoroethylene (PTFE) surface, we uncovered a direct correlation between their presence and the ability of droplets to slide down an incline. We forward two possible mechanisms to account for this behavior. The first is attributed to the air bubbles creating regions where additional solid-liquid-vapor phase interfaces are present; wherein due to the buoyancy force acting upwards, the orientation of the contact angles of each bubble (which should also be in hysteresis but in the opposite direction of the hysteresis at the droplet rim contact lines) dictate that the net force of the bubbles in the droplet act down an incline. We show here that this mechanism cannot fully account for the bubble enhanced sliding behavior. The second mechanism is based on the occurrence of the droplet front advancing first, causing the droplet to elongate and thus allowing the receding contact line to partially sweep inwards over the bubbles. This causes a series of point-wise disruptions on the contact line that permits the droplet to slide down more readily. The relatively short time of ～180s during which these micron sized bubbles decrease in size indicates a possibility of this mechanism contributing to a transient means to reduce the retention force of droplets that reside on hydrophobic surfaces.     

Analysis of small droplets with a new detector for liquid chromatography based on laser-induced breakdown spectroscopy

The miniaturization of analytical techniques is a general trend in speciation analytics. We have developed a new analytical technique combining high pressure liquid chromatography (HPLC) with laser-induced breakdown spectroscopy (LIBS). This enables a molecule-specific separation followed by an element-specific analysis of smallest amounts of complex samples. The liquid flow coming from a HPLC pump is transformed into a continuous stream of small droplets (diameter 50–100 μm, volume 65–500 pl) using a piezoelectric pulsed nozzle. After the detection of single droplets with a droplet detector, a Q-switched Nd:YAG Laser is triggered to emit a synchronized laser pulse that irradiates a single droplet. The droplets are evaporated and transformed to the plasma state. The spectrum emitted from the plasma is collected by a spherical mirror and directed through the entrance slit of a Paschen–Runge spectrometer equipped with channel photomultipliers. The spectrometer detects 31 elements simultaneously covering a spectral range from 120 to 589 nm. Purging the measurement chamber with argon enables the detection of vacuum–UV lines. Since the sample is transferred to the plasma state without dilution, very low flow rates in the sub-μl/min range can be realised.     

High-speed droplet generation on demand driven by pulse laser-induced cavitation

We report on a pulse laser-driven droplet generation (PLDG) mechanism that enables on-demand droplet generation at rates up to 10,000 droplets per second in a single-layer PDMS-based microfluidic device. Injected droplet volumes can be continuously tuned between 1 pL and 150 pL with less than 1% volume variation.     

Deformation of water droplets on solid surface in electric field

The purpose of this paper is to analyze the deformation of water droplets on a solid surface under electric stress. A mathematical model making it possible to simulate the axisymmetric as well as non-axisymmetric deformations of droplets is developed. According to this model, the droplet deformation depends on several parameters such as the volume and the number of droplets, the conductivity and the permittivity of droplets, their proximity to one another, the surface of the solid material, and the location of each droplet on the dielectric surface. The results of the simulation show the disturbance of the background field through the presence of a single or multiple droplets. An experimental study is also achieved by considering one to three droplets aligned simultaneously on a dielectric smooth surface between two electrodes subjected to AC voltages. The influence of the background field and the droplet location regarding the electrodes on the deformation of water droplets are evidenced.     

Sliding of water droplets on hydrophobic surfaces with various hydrophilic region sizes

Four patterned surfaces with hydrophilic areas of different sizes were prepared using photolithography with a smooth octadecyltrimethoxysilane (ODS) hydrophobic coating. The hydrophilic area in the surfaces was aligned hexagonally with a constant area fraction. The sliding angle and contact angle hysteresis of the water droplets increased concomitantly with increasing pattern size. The increase of the contact line distortion between defects at the receding side plays an important role in this trend. The droplet sliding velocity also increased concomitantly with increasing pattern size. This trend was simulated by a simple flow model. The contribution of the interface between the ODS region and the hydrophilic area was deduced from this trend. This study demonstrated the different size dependency of the chemical surface defects for sliding behavior between the critical moment at which a droplet slides down and the period when a droplet is sliding.     

Slip-stick wetting and large contact angle hysteresis on wrinkled surfaces

Wetting on a corrugated surface that is formed via wrinkling of a hard skin layer formed by UV oxidation (UVO) of a poly(dimethylsiloxane) (PDMS) slab is studied using advancing and receding water contact angle measurements. The amplitude of the wrinkled pattern can be tuned through the pre-strain of the PDMS prior to surface oxidation. These valleys and peaks in the surface topography lead to anisotropic wetting by water droplets. As the droplet advances, the fluid is free to move along the direction parallel to the wrinkles, but the droplet moving orthogonal to the wrinkles encounters energy barriers due to the topography and slip-stick behavior is observed. As the wrinkle amplitude increases, anisotropy in the sessile droplet increases between parallel and perpendicular directions. For the drops receding perpendicular to the wrinkles formed at high strains, the contact angle tends to decrease steadily towards zero as the drop volume decreases, which can result in apparent hysteresis in the contact angle of over 100°. The wrinkled surfaces can exhibit high sessile and advancing contact angles (>115°), but the receding angle in these cases is generally vanishing as the drop is removed. This effect results in micrometer sized drops remaining in the grooves for these highly wrinkled surfaces, while the flat analogous UVO-treated PDMS shows complete removal of all macroscopic water drops under similar conditions. These wetting characteristics should be considered if these wrinkled surfaces are to be utilized in or as microfluidic devices.     

Dynamic behavior of water droplets on solid surfaces with pillar-type nanostructures

In the present study, we investigated the static and dynamic behavior of water droplets on solid surfaces featuring pillar-type nanostructures by using molecular dynamics simulations. We carried out the computation in two stages. As a result of the first computational stage, an initial water cube reached an equilibrium state at which the water droplet showed different shapes depending on the height and the lateral and gap dimensions of the pillars. In the second computational stage, we applied a constant body force to the static water droplet obtained from the first computational stage and evaluated the dynamic behavior of the water droplet as it slid along the pillar-type surface. The dynamic behavior of the water droplet, which could be classified into three different groups, depended on the static state of the water droplet, the pillar characteristics (e.g., height and the lateral and gap dimensions of the pillars), and the magnitude of the applied body force. We obtained the advancing and receding contact angles and the corresponding contact angle hysteresis of the water droplets, which helped classify the water droplets into the three different groups.     

微流控芯片液滴生成与检测技术研究进展

微流控芯片液滴技术是一种操控微小体积液体的新技术,既可实现高通量微观样本的生成及控制,也可进行独立液滴的操作.分散的微液滴单元可作为理想的微反应器,在生物医药中的药物筛选、材料筛选和高附加值微颗粒材料合成领域展现出巨大的应用潜力.液滴微流控芯片是利用流体剪切力的改变,使互不相溶的两相流体在其界面处生成稳定、有序的液滴,...     

Experimental investigation of droplet acceleration and collision in the gas phase in a microchannel

We developed a novel microfluidic system, termed a micro-droplet collider, by utilizing the spatial-temporal localized liquid energy to realize chemical processes, which achieved rapid mixing between droplets having a large volume ratio by collision. In this paper, in order to clarify the characteristics of the micro-droplet collider, dynamics of droplet acceleration, stationary motion and collision in the gas phase in a microchannel were experimentally investigated with visualized images using a microscope equipped with a high-speed camera. The maximum velocity of 450 mm s(-1) and acceleration of 1500 m s(-2) of a 1.6 nL water droplet were achieved at an air pressure of 100 kPa. Measurement results of dynamic contact angles of droplets indicated that wettability of the surface played an important role in the stability of droplet acceleration and collision. We found that the bullet droplet penetrated into the target droplet at collision, which differed from bulk scale. The deformation of the droplet was strongly suppressed by the channel structure, thus stable collision and efficient utilization of the droplet energy were possible. These results are useful for estimating the localized energy, for improving the system in order to realize extreme performance, and for extending the applications of microfluidic devices.     

Partitioning invertase between a dilute water solution and generated droplets

Water droplets or mist occur naturally in the air at seashores. These water droplets carry inorganic and organic substances from the sea to the land via the air, creating fertile land in sandy coastal areas (1). The same phenomenon occurs in an air-fluidized bed bioreactor (2). In an air-fluidized bed reactor, proteins can be transferred from the bioreactor semisolid bulk phase to an enriched droplet phase. This protein transfer process (droplet fractionation) can be experimentally simulated by shaking a separatory funnel containing a dilute solution of a given protein, which can be an enzyme like invertase. The created droplets become richer in invertase (protein) than that of the original dilute solution. The droplets can then be coalesced by tranpping them and recovering the concentrated protein in the new liquid phase. Typically, in such a droplet fractionation process a collected enzyme can be degraded in its ability tocatalyze a chemical reaction. In this article, we explore whether the initial solution pH control variable can be adjusted to minimize the decrease of enzyme activity in this process. The protein droplet recovery problem is one in which the recovered amount of desired protein (enzyme) in the droplet is maximized, subject to the minimization of the enzyme activity loss. The partition coefficient, which is the ratio between the protein concentration in the droplets and the residual solution, is maximized at approx 4.8 and occurs at pH 3.0. Here, the partition coefficient for invertase decreases as the initial solution pH increases, between pH 3.0 and 8.0. Interestingly, the initial solution surface tension seems to be inversely proportional to the partition coefficient. The partition coefficien treachesa maximum value at a surface tension value of approx 63 mN/m at pH 3.0. The enzymatic activity of the initial, the residual, and the droplet solutions all decrease as the bulksolution pH increases. Adecrease of enzymatic activity was observed in the residual bulk solution when compared with that in the initial bulk solution at all pH levels. Also, up to 90% of the invertase activity was lost in the droplets when compared to the initial bulk solution.     

Induced detachment of coalescing droplets on superhydrophobic surfaces

Coalescence of a falling droplet with a stationary sessile droplet on a superhydrophobic surface is investigated by a combined experimental and numerical study. In the experiments, the droplet diameter, the impact velocity, and the distance between the impacting droplets were controlled. The evolution of surface shape during the coalescence of two droplets on the superhydrophobic surface is captured using high speed imaging and compared with numerical results. A two-phase volume of fluid (VOF) method is used to determine the dynamics of droplet coalescence, shape evaluation, and contact line movement. The spread length of two coalesced droplets along their original center is also predicted by the model and compared well with the experimental results. The effect of different parameters such as impact velocity, center to center distance, and droplet size on contact time and restitution coefficient are studied and compared to the experimental results. Finally, the wetting and the self-cleaning properties of superhydrophobic surfaces have been investigated. It has been found that impinging water drops with very small amount of kinetic impact energy were able to thoroughly clean these surfaces.     

Condensation-induced wetting state and contact angle hysteresis on superhydrophobic lotus leaves

In this paper, we demonstrate how condensed moisture droplets wet classical superhydrophobic lotus leaf surfaces and analyze the mechanism that causes the increase of contact angle hysteresis. Superhydrophobic lotus leaves in nature show amazing self-cleaning property with high water contact angle (>150°) and low contact angle hysteresis (usually <10°), causing droplets to roll off at low inclination angles, in accordance with classical Cassie–Baxter wetting state. However, when superhydrophobic lotus leaves are wetted with condensation, the condensed water droplets are sticky and exhibit higher contact angle hysteresis (40–50°). Compared with a fully wetted sessile droplet (classical Wenzel state) on the lotus leaves, the condensed water droplet still has relatively large contact angle (>145°), suggesting that the wetting state deviates from a fully wetted Wenzel state. When the condensed water droplets are subjected to evaporation at room conditions, a thin water film is observed bridging over the micropillar structures of the lotus leaves. This causes the dew to stick to the surface. This result suggests that the condensed moisture does not uniformly wet the superhydrophobic lotus leaf surfaces. Instead, there occurs a mixed wetting state, between classical Cassie–Baxter and Wenzel states that causes a distinct increase of contact angle hysteresis. It is also observed that the mixed Cassie–Baxter/Wenzel state can be restored to the original Cassie–Baxter state by applying ultrasonic vibration which supplies energy to overcome the energy barrier for the wetting transition. In contrast, when the surface is fully wetted (classical Wenzel state), such restoration is not observed with ultrasonic vibration. The results reveal that although the superhydrophobic lotus leaves are susceptible to being wetted by condensing moisture, the configured wetting state is intermediate between the classical Cassie–Baxter and Wenzel states.     

Laser techniques in acoustically levitated micro droplets

Laser techniques were applied to an acoustically levitated droplet for remote investigation of the diameter, species concentration and temperature of the suspended droplet. To this end, the third and the fourth harmonic of a Nd:YAG laser were used for investigation of elastic, fluorescence and phosphorescence signals from the droplet. The droplet was seeded with thermographic phosphors and acetone for the phosphorescence and fluorescence measurements, respectively. The techniques were applied simultaneously using an imaging stereoscope. The imaging device allowed for an identical visualization of incoming signal through separate optical filters. Temperature measurements in droplets is important in the study of e.g. exothermic chemical reactions, spray processes, combustion, and in bioanalytical applications where the biological material is temperature sensitive or dependent on optimal temperature for function. Results from these investigations showed that temperature measurements in acoustically levitated droplets using laser-induced phosphorescence are feasible. The results also show the potential of simultaneous laser based measurements on levitated droplets. Diameter variation (surface area), mixture concentration and temperature were measured simultaneously.     

Reactions of Microsolvated Organic Compounds at Ambient Surfaces: Droplet Velocity,Charge State,and Solvent Effects

The exposure of charged microdroplets containing organic ions to solid-phase reagents at ambient surfaces results in heterogeneous ion/surface reactions. The electrosprayed droplets were driven pneumatically in ambient air and then electrically directed onto a surface coated with reagent. Using this reactive soft landing approach, acid-catalyzed Girard condensation was achieved at an ambient surface by directing droplets containing Girard T ions onto a dry keto-steroid. The charged droplet/surface reaction was much more efficient than the corresponding bulk solution-phase reaction performed on the same scale. The increase in product yield is ascribed to solvent evaporation, which causes moderate pH values in the starting droplet to reach extreme values and increases reagent concentrations. Comparisons are made with an experiment in which the droplets were pneumatically accelerated onto the ambient surface (reactive desorption electrospray ionization, DESI). The same reaction products were observed but differences in spatial distribution were seen associated with the “splash” of the high velocity DESI droplets. In a third type of experiment, the reactions of charged droplets with vapor phase reagents were examined by allowing electrosprayed droplets containing a reagent to intercept the headspace vapor of an analyte. Deposition onto a collector surface and mass analysis showed that samples in the vapor phase were captured by the electrospray droplets, and that instantaneous derivatization of the captured sample is possible in the open air. The systems examined under this condition included the derivatization of cortisone vapor with Girard T and that of 4-phenylpyridine N-oxide and 2-phenylacetophenone vapors with ethanolamine.