Acoustic manipulation of small droplets

Wixforth, Achim; Strobl, Christoph; Gauer, C.; Toegl, A.; Scriba, J.; Guttenberg, Zeno

doi:10.1007/s00216-004-2693-z

Cited by 309 publications

(262 citation statements)

References 6 publications

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“…Droplet movement arises due to the asymmetric release of [P 6,6,6,14 ] + , a very efficient cationic surfactant, which is a constituent of the IL droplet. When [P 6,6,6,14 ] + diffuses from the droplet into the aqueous solution, it causes a sudden drop of the surface tension of the solution (see ESI, Table S1).…”

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“…The rate of [P 6,6,6,14 ] + release depends on the solubility of the closely associated Cl -anion (ESI , Table S1), as the formation of free [P 6,6,6,14 ] + (the active surfactant at the air-water boundary) in the aqueous phase depends on the local Cl -concentration at the IL-aqueous boundary. 20 In [P 6,6,6,14 ][Cl], the Cl -anion is strongly associated with the [P 6,6,6,14 ] + cation, rendering the IL strongly lipophilic, which enables droplets to be formed on the water-air interface. The [P 6,6,6,14 ] + cation is only sparingly soluble in water, and its partition into the aqueous phase from the IL droplet phase is strongly modulated by the local Cl -concentration.…”

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“…20 In [P 6,6,6,14 ][Cl], the Cl -anion is strongly associated with the [P 6,6,6,14 ] + cation, rendering the IL strongly lipophilic, which enables droplets to be formed on the water-air interface. The [P 6,6,6,14 ] + cation is only sparingly soluble in water, and its partition into the aqueous phase from the IL droplet phase is strongly modulated by the local Cl -concentration. Hence, in the presence of an aqueous phase Cl -gradient, differential release of [P 6,6,6,14 ] + occurs and an asymmetric surface tension gradient is created, leading to a Marangoni like flow, which causes the droplet to move from areas of low surface tension towards areas of high surface tension (Figure 2).…”

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“…The [P 6,6,6,14 ] + cation is only sparingly soluble in water, and its partition into the aqueous phase from the IL droplet phase is strongly modulated by the local Cl -concentration. Hence, in the presence of an aqueous phase Cl -gradient, differential release of [P 6,6,6,14 ] + occurs and an asymmetric surface tension gradient is created, leading to a Marangoni like flow, which causes the droplet to move from areas of low surface tension towards areas of high surface tension (Figure 2). The channel was initially filled with NaOH 10 -2 M solution.…”

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“…In the third approach the channels were initially filled with a solution of 10 -5 M NaCl followed by the addition of some NaCl crystals (~ 10 mg) at the desired destination (see ESI video S3). In each case, 10 -30 s after the addition of the chemoattractant, small droplets (~ 10 µl) of [P 6,6,6,14 ][Cl] were placed at specific locations and these spontaneously moved to the desired destination ( Figure 2). As soon as a droplet is placed in the channel it begins moving along the channel towards the source of the chemoattractant traversing corners on its way.…”

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Self-propelled chemotactic ionic liquid droplets

et al. 2015

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Herein we report the chemotactic behaviour of self-propelled droplets composed solely of the ionic liquid trihexyl(tetradecyl)phosphonium chloride ([P 6,6,6,14 ][Cl]). These droplets spontaneously move along an aqueousair boundary in the direction of chloride gradients to specific destinations due to asymmetric release of [P 6,6,6,14 ] + cationic surfactant from the droplet into the aqueous phase.The ability to move in response to an external stimulus is essential for many life forms. Certain cells such as bacteria, somatic cells, and other single cell or multicellular organisms move in response to chemical stimuli present within their surrounding environment. 1, 2 This phenomenon is known as chemotaxis and it is crucial for many biological processes such as feeding or fleeing toxins, migration and action of somatic cells such as those involved in the immune system, 3, 4 reproductive cells 5 and enzymes. 6 Notably there are only few equivalents of similar chemotactic-driven "micro-vehicles" in the synthetic world. Inspired by chemotactic organisms we developed synthetic biomimetic droplets which are self-propelled and capable of navigating a microfluidic network by introducing chemoattractants at the target destination within the fluidic channel.Various mechanisms to initiate and direct the movement of micro droplets have been reported, including switchable wettability of a substrate surface via chemical 7, 8 or electro-chemical stimuli, 9, 10 or using temperature gradients, 11, 12 magnetic 13 or acoustic forces 14 and even photo-stimulation to actuate droplets. 15 However, all of these methods involve relatively complex experimental arrangements and/or multi-component droplets, and require applied external energy to create droplet movement.Surfactant release has been employed to control the surface tension of aqueous systems in order to generate spontaneous movement of droplets at the aqueous-air interface in a contactless manner. When placed into an aqueous system the surfactant will interact with the water molecules and lower the surface tension of the soluti on. When the surface tension of a liquid is altered, liquid flows from areas of low surface tension to areas of high surface tension; a phenomenon known as the Marangoni effect. 16 Control over the droplet direction can be achieved by creating conditions under which asymmetric release of pre-loaded surfactant from the droplet occurs. Using stimuli-responsive surfactants, smart droplets have been designed which can solve complex mazes, 17 or move towards/away from a light source. 18, 19 We have investigated a number of strategies for generating spontaneous movement in droplet microvehicles based on the generation of concentration gradients of chemoattractants diffusing from a target destination within a fluidic system. Chemoattractants are chemical agents which induce positive chemotaxis in living organisms, in the same manner that chemorepellents induce negative chemotaxis. In contrast to previous studies, in which the degree of protonation of surfactant...

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Self-propelled chemotactic ionic liquid droplets

et al. 2015

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Miniaturization of the Entire Analytical Process II: Micro Total Analysis Systems (µTAS)

Ríos

Escarpa

Simonet

2009

Miniaturization of Analytical Systems

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One of the oldest and most important challenges in analytical chemistry is the accurate measurement of a specific compound in a complex matrix. Generally, this can be carried out by following two main approaches: improving selectivity toward the detection system by using selective (bio)sensors or improving selectivity toward separation systems with nonspecific detectors coupled after the separation of sample mixture in distinct zones of single analytes. Miniaturization of the first approach was studied in Chapter 7, and the miniaturization of the second approach will be studied here.The micro total analysis system (mTAS) concept was developed from a modification of the total analysis system (TAS) approach by downsizing and integrating its multiple steps (injection, reaction, separation and detection) on to a single device, yielding a sensor-like system with a fast response time, low sample consumption, onsite operation and high stability [1,2]. The fact that miniaturized analysis systems contain all the elements needed to perform the required analysis is clearly reflected in the term 'mTAS'. The recent strong interest in this approach is stimulated by the fact that the attempt to find solutions for chemical measurement problems through development of individual sensors for each of the desired parameters has not been very successful up to now [3]. The main reason for this probably lies in the wide

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Rapid and Controllable Digital Microfluidic Heating by Surface Acoustic Waves

Shilton

Mattoli

Travagliati

et al. 2015

Adv Funct Materials

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5895wileyonlinelibrary.com substrate, owing to the sound-velocity mismatch between substrate and fl uid, SAWs will radiate acoustic energy into the fl uid. This will induce a pressure wave and drive a steady-state fl ow known as acoustic streaming, the basis for the gamut of possible SAW microfl uidic operations. At the same time, however, the viscous dissipation of the acoustic energy into the fl uid can generate a heating effect. [ 31 ] This heating effect is a much-discussed issue in SAW-driven microfl uidics and questions surrounding the problem of fl uid heating are continually arising. How much will the dissipation of the acoustic wave into a droplet raise its temperature? Will any temperature rise be enough to negatively affect biological samples or even evaporation rates? This is especially of concern in the case of free-droplets where a systematic study of fl uid-heating effects would be desirable over the typical SAW-microfl uidic operational frequency and power ranges. At the same time, it is interesting to understand to which extent any microfl uidic heating effects can actually be positively exploited to enhance biological or chemical reactions in lab-on-a-chip devices when desired.To date, only few studies investigated SAW thermal effect in microfl uidics. The fi rst reported characterization of SAWdriven liquid heating effects was carried out by Kondoh et al. in 2005 and further expanded upon in 2009. [ 31-33 ] In these experiments, the authors demonstrated that the primary source of fl uid heating is, in fact, the radiated acoustic wave, with temperature changes being strongly dependent on input power and fl uid viscosity. It was not clear, however, if droplet heating was isolated from heating effects occurring at the interdigital transducer (IDT). The latter can signifi cantly heat up the substrate-and through it the droplet on its surface-if not properly controlled. In 2006, Beyssen et al. reported similar results using comparable experimental conditions, further quantifying the effect of fl uid viscosity on temperature changes and uniformity. [ 34 ] In 2009, Kulkarni et al. showed the possibility to use this heating mechanism as an energy source for synthetic chemistry in digital microfl uidic systems. [ 35 ] More recently, Roux-Marchand et al. further investigated the temperature uniformity within viscous glycerol droplets and found decreased uniformity at high SAW powers but did not discuss absolute temperature changes. [ 36 ] As a recent application of acoustic microdroplet heating, Reboud et al. demonstrated polymerase chain reactions (PCRs) in an oil-covered droplet. [ 37 ] Fast and controllable surface acoustic wave (SAW) driven digital microfl uidic temperature changes are demonstrated. Within typical operating conditions, the direct acoustic heating effect is shown to lead to a maximum temperature increase of about 10 °C in microliter water droplets. The importance of decoupling droplets from other on-chip heating sources is demonstrated. Acousticheating-driven temperature changes reac...

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Acoustic manipulation of small droplets

Cited by 309 publications

References 6 publications

Self-propelled chemotactic ionic liquid droplets

Self-propelled chemotactic ionic liquid droplets

Miniaturization of the Entire Analytical Process II: Micro Total Analysis Systems (µTAS)

Rapid and Controllable Digital Microfluidic Heating by Surface Acoustic Waves

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