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1.
ACS Omega ; 3(8): 9310-9317, 2018 Aug 31.
Artigo em Inglês | MEDLINE | ID: mdl-31459064

RESUMO

Facile creation of multiple drops at appropriate volumes on surfaces without the use of sophisticated instrumentation facilitates downstream evaporative preconcentration of liquid samples for analytical purposes. In this work, a superhydrophobic (SH) substrate comprising wells with a perforated mesh base was developed for simultaneous drop creation in a quick and convenient manner. In contrast to the method of pouring liquid directly over the SH wells, consistent liquid filling was readily achieved by a simple immersion approach. This method works well even for challenging situations where well diameters are smaller than 3.4 mm. Despite the poor liquid-retention properties of SH surfaces, inverting the wells did not result in liquid detachment under gravitational force, indicating strong pinning effects afforded by the well architecture. The perforated base of the well allowed the liquid to be completely removed from the well by compressed air. High-speed camera image processing was used to study the evolution of drop contact angle and displacement with time. It was found that the liquid body was able to undergo strong oscillations. Optical spectroscopy was used to confirm the ability of evaporative preconcentration of silver nanoparticles.

2.
Langmuir ; 33(48): 13892-13898, 2017 12 05.
Artigo em Inglês | MEDLINE | ID: mdl-29116806

RESUMO

Valves used to control liquid filling and draining processes from storage typically need to be actuated. Here, we show that similar flow enabling and restricting operations can be achieved through millimeter scale holes that function according to the amount of hydrostatic pressure applied without any other intervention. This phenomena is exhibited using receptacles where the base is made of either a hydrophilic or superhydrophobic substrate with hole sizes ranging from 1.0-2.0 mm. The construction is such that the drainage flow velocities are of the same order in both substrates and follow Torricelli's law trends. Nevertheless, the primary mechanisms responsible for resisting the onset of flow in each substrate are different; nonbreaching of the advancing contact angle threshold in the former, and stable maintenance of an elastic-like deformation of the liquid-gas interface that is connected to the surrounding plastron in the latter. These differences are demonstrated using an upward jet of water delivered to the orifice, where a discharging flow from the hydrophilic base occurred before the threshold hydrostatic pressure condition was attained, while liquid from the jet is subsumed into the liquid body of the receptacle with the superhydrophobic base without any leakage. These findings portend advantages in simplicity and robustness for a myriad of liquid-related processes.

3.
ACS Omega ; 2(9): 6211-6222, 2017 Sep 30.
Artigo em Inglês | MEDLINE | ID: mdl-31457866

RESUMO

Drops with larger volumes placed over a superhydrophobic (SH) surface with a hole do not fall through unless they are evaporated to a size that is small enough. This feature offers the ability to preconcentrate samples for biochemical analysis. In this work, the influence of pinning on the behavior of drops placed on a 0.1 mm thick SH substrate with a 2 mm diameter hole as they evaporated was investigated. With 16 µL of water dispensed, the sessile drop component volume was initially higher than that of the overhanging drop component and maintained this until the later stages where almost identical shapes were attained and full evaporation was achieved without falling off the hole. With 15 µL of water dispensed, the volume of the sessile drop was initially higher than that of the overhanging drop component but the liquid body was able to squeeze through the hole after 180 s due to the contact line not having sufficient pinning strength when it encountered the edge of the hole. This resulted in the liquid body either falling through the hole or remaining pinned with an oval-like shape. When it did not fall-off, the liquid body had volume and contact angle characteristics for the sessile drop and overhanging drop components that were reversed. In the later stages, however, nearly identical shapes were again attained and full evaporation was achieved without falling off the hole. The effects of pinning, despite the substrate being SH, offer another path toward achieving practical outcomes with liquid bodies without the need for chemical surface functionalization. Similarities and differences could be seen in the behavior of a sessile drop on a SH plate that was inclined at 30° to the horizontal and evaporated.

4.
Langmuir ; 32(33): 8550-6, 2016 08 23.
Artigo em Inglês | MEDLINE | ID: mdl-27479030

RESUMO

In this work, 2 µL water drops are placed on substrates that are created to have a circular hydrophilic region bounded by superhydrophobicity so that they exhibit high contact angles. When the substrate is translated by a linear stepper actuator, the random force components present in the actuator are shown to cause the drop to rock resonantly. When the substrate is translated downward at inclination angles of up to 6° with respect to the horizontal, the contact angle hysteresis increases progressively to a limiting condition. When the substrate is moved up at inclined angles, alternatively, the contact angle hysteresis increases initially to the limiting condition before it is progressively restored to its static state. These behaviors are accounted for by the reversible micro-Cassie to Wenzel wetting state transformations that are made possible by the hierarchical microscale and nanoscale structures present in the superhydrophobic regions.

5.
Soft Matter ; 11(38): 7474-7, 2015 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-26305149

RESUMO

Captive bubbles that reside on superhydrophobic surfaces with plastrons move uncontrollably when tilted. A system based on creating moveable local apexes on flexible superhydrophobic foils is shown to allow controlled transport. Simulations done reveal that specific bubble transport speeds are needed to form concentration gradients suited for aerotaxis study and sensing.


Assuntos
Gases/química , Técnicas Analíticas Microfluídicas/instrumentação , Água/química , Simulação por Computador , Desenho de Equipamento , Interações Hidrofóbicas e Hidrofílicas , Modelos Químicos , Movimento (Física) , Propriedades de Superfície
6.
Langmuir ; 31(24): 6695-703, 2015 Jun 23.
Artigo em Inglês | MEDLINE | ID: mdl-25986160

RESUMO

Captive bubbles on a superhydrophobic (SH) surface have been shown to increase in volume via injection of air through the surrounding plastron. The experimental contact diameter against volume trends were found to follow that predicted by the Surface Evolver simulation generally but corresponded with the simulated data at contact angle (CA) = 158° when the volume was 20 µL but that at CA = 170° when the volume was increased to 180 µL. In this regime, there was a simultaneous outward movement of the contact line as well as a small reduction in the slope that the liquid-air interface makes with the horizontal as air was injected. At volumes higher than 180 µL, air injection caused the diameter to reduce progressively until detachment. The inward movement of the contact line in this regime allowed the bubble body to undergo shape deformations to stay attached onto the substrate with larger volumes (300 µL) than predicted (220 µL at CA = 170°) using simulation. In experiments to investigate the effect of translating the SH surface, movement of captive bubbles was possible with 280 µL volume but not with 80 µL volume. This pointed to the possibility of transporting gas-phase samples on SH surfaces using larger captive bubble volumes.

7.
Lab Chip ; 15(4): 991-5, 2015 Feb 21.
Artigo em Inglês | MEDLINE | ID: mdl-25511936

RESUMO

Superhydrophobic surfaces aid biochemical analysis by limiting sample loss. A system based on wells here tolerated tilting up to 20° and allowed air logic transfer with evidence of mixing. Conditions for intact transfer on 15 to 60 µL drops using compressed air pressure operation were also mapped.


Assuntos
Ar , Técnicas Analíticas Microfluídicas , Pressão do Ar , Interações Hidrofóbicas e Hidrofílicas , Técnicas Analíticas Microfluídicas/instrumentação , Tamanho da Partícula , Propriedades de Superfície , Temperatura Ambiente
8.
Langmuir ; 30(21): 6095-103, 2014 Jun 03.
Artigo em Inglês | MEDLINE | ID: mdl-24810460

RESUMO

The effect of hydrostatic pressure on 6 µL air bubbles formed on micropillar structured PDMS and silicone surfaces using a 2 mm diameter stainless steel tip retreated at 1 mm/s was investigated. Dimensional analysis of the tip retraction process showed the experiments to be conducted in the condition where fluid inertial forces are comparable in magnitude with surface tension forces, while viscous forces were lower. Larger bubbles could be left behind on the structured PDMS surface. For hydrostatic pressures in excess of 20 mm H2O (196 Pa), the volume of bubble deposited was found to decrease progressively with pressure increase. The differences in width of the deposited bubbles (in contact with the substrate) were significant at any particular pressure but marginal in height. The attainable height before rupture reduced with pressure increase, thereby accounting for the reducing dispensed volume characteristic. On structured PDMS, the gaseous bridge width (in contact with the substrate) was invariant with tip retraction, while on silicone it was initially reducing before becoming invariant in the lead up to rupture. With silicone, hence, reductions in the contact width and height were both responsible for reduced volumes with pressure increase. Increased hydrostatic pressure was also found to restrict the growth in contact width on silicone during the stage when air was injected in through the tip. The ability to effect bubble size in such a simple manner may already be harnessed in nature and suggests possibilities in technological applications.

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