Surfactant-loaded capsules as Marangoni microswimmers at the air–water interface: Symmetry breaking and spontaneous propulsion by surfactant diffusion and advection

Ender, Hendrik; Froin, Ann-Kathrin; Rehage, Heinz; Kierfeld, Jan

doi:10.1140/epje/s10189-021-00035-8

Cited by 16 publications

(24 citation statements)

References 69 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Furthermore, in our systems (excluding our complex droplet), such changes in the convective flows modulate the ST such as the direction of the droplet is in the direction of higher ST. This is in line with the recent work of Ender et al [42] that have used a floating droplet system as in our case. We can therefore explain all of our results in terms of the interpretation of Marangoni effect (Figure 7).…”

Section: The Role Of St In Droplet Self-propulsion Mechanismsupporting

confidence: 93%

“…Such flows are created when the surface stress needs to be balanced by the viscous stress, resulting in the formation of convection flows. In previous studies, several attempts have been made to show that convective flows are toward lower ST. [18,41] At the same time, several other studies offered contradictory findings, wherein the droplet was propelled toward higher ST. [9,17,26,42] So far, however, there has been little discussion about the role of the surfactant concentration profile in the propulsion direction.…”

Section: The Role Of St In Droplet Self-propulsion Mechanismmentioning

confidence: 99%

See 1 more Smart Citation

Self‐Propulsion of Droplets via Light‐Stimuli Rapid Control of Their Surface Tension

Yucknovsky

Rich

Westfried

et al. 2021

Adv Materials Inter

View full text Add to dashboard Cite

Such systems can mimic the motion of living cells as similar stimuli are present in nature, and can also result in our fundamental understanding of early origin of life events.The movement of droplets is linked with nonuniform surface tension (ST), resulting in fluid flow, known as the Marangoni effect. [18][19][20] This effect is based on an asymmetric exposure of the droplet surface to a chemical cue, i.e., chemotaxis, whereas ion or pH gradients are most common. [21][22][23] The asymmetry in the droplet/solution interface creates a nonuniform change in the ST. Light is a convenient tool to control the asymmetry and ST of droplets, and accordingly the motion of droplets. To use light as stimuli, there is a need for photochromic molecules next to the droplets, undergoing a photochemical process. To date, the commonly used families of photochromic molecules for mediating dynamic processes, e.g., droplet motion are azobenzenes that undergo trans-cis photoisomerization, [24,25] and spiropyran and its derivatives that overgo photocleavage primary to the proton release, resulting in minute-length reaction timescales. [26][27][28][29][30][31][32] Therefore, a yet-to-beresolved challenge in the use of such photochromic molecules is the associated timescales and especially the reversibility of the dynamic process.Herein, we propose a new approach to chemophototaxis for gaining fast subsecond responses not only to turning on the light, but also for the reversible process of turning the light off, which is based on the use of Brønsted-Lowry photoacids and photobases. This class of organic molecules undergoes a dramatic change in their dissociation equilibrium constant (pKa) upon light excitation. [33] Thus, only in their electronically excited state do photoacids and photobases behave as strong acids and bases. The above-mentioned spiropyrans are also referred to as photoacids since their photocleavage process from spiropyran to merocyanine involves the release of a proton, however, there is a large difference in terms of mechanism and timescales relative to Brønsted-Lowry photoacids. In this context, we hypothesize that the fast excited-state proton transfer (ESPT) of Brønsted-Lowry photoacids and photobases can induce rapid chemical changes on the surface of pH-sensitive droplets and initiate their motion. We show here a variety of different droplet systems involving the use of photoacids and photobases either within the droplet, on its surface, or in solution, whereas we use light to self-propel and to guide the movement of the droplet. Nature demonstrates many examples of response and adaptation to external stimuli. Here, this study focuses on self-propulsion (motion) while presenting several self-propelling droplet systems responsive to pH gradients. Light is used as the gating source to gain reversibility, avoid the formation of chemical wastes, and control the self-propulsion remotely. To achieve light-stimuli ultrafast response, photoacids and photobases are used, capable of donating or capturing a proton, respect...

show abstract

Section: The Role Of St In Droplet Self-propulsion Mechanismsupporting

confidence: 93%

Section: The Role Of St In Droplet Self-propulsion Mechanismmentioning

confidence: 99%

Self‐Propulsion of Droplets via Light‐Stimuli Rapid Control of Their Surface Tension

Yucknovsky

Rich

Westfried

et al. 2021

Adv Materials Inter

View full text Add to dashboard Cite

show abstract

“…While the writing of this work was being finalized, two studies by Ender and coworkers appeared that address a very similar problem [62,63]. As will be discussed below, our results are essentially consistent with their findings on overlapping topics, but differ with their work on a number of elements.…”

Section: Introductionsupporting

confidence: 85%

“…(ii) Our study focuses mostly on steady creeping flow, as Refs. [62,63], but also briefly considers finite Reynolds number and transient regimes. (iii) We examine in detail the vicinity of the symmetry breaking mechanism, where the influence of Marangoni flows is the strongest.…”

Section: Introductionmentioning

confidence: 99%

Role of Marangoni forces in the velocity of symmetric interfacial swimmers

et al. 2021

View full text Add to dashboard Cite

Interfacial swimmers are objects that self-propel at an interface by autonomously generating a gradient of surface tension, often through the continuous release of a surfactant. While the case of asymmetric swimmers has long been studied, experiments have shown that spontaneous motion is also possible for symmetric swimmers. The basic mechanism of symmetry-breaking is qualitatively well-established but one key aspect of the phenomenon that has proved particularly difficult to elucidate is the role of Marangoni effects in the self-propulsion. We address this question by numerical methods, which can fully handle the complex interplay between swimmer motion, fluid flow, surfactant distribution, and Marangoni stresses. Our swimmer is a disk releasing a soluble surfactant in a deep-layer fluid. We investigate how the swimming velocity, represented by a Péclet number Pe * depends on its characteristics, as encapsulated in the Marangoni number M. We analyze the properties of the swimming diagram Pe * (M) and compare with approximate models to understand their origin. We find that the low-Pe * regime exhibits a bistability region: spontaneous swimming involves a threshold Marangoni number, a discontinuity in velocity and possibly hysteresis. Those features are present only for a full description of the problem and reveal the subtle but key role of Marangoni flows. The large-Pe * regime features a robust asymptotic scaling law Pe * ∼ M α , whose exponent α 0.72 is close to the 3/4 value predicted by a simplified model, indicating a much weaker influence of Marangoni flows. While our results were obtained assuming a point-source swimmer in the Stokes flow regime, we show that the picture remains very similar when considering a spatially extended source size, finite Reynolds number, or a fixed concentration swimmer. We discuss our findings in relation to experiments.

show abstract

“…More information about these aspects can be found in Refs. [4,14,[18][19][20][21][22][23][24][25][26][27][28][29] of this Topical Issue.…”

mentioning

confidence: 99%

Editorial: Motile active matter

et al. 2021

View full text Add to dashboard Cite

Surfactant-loaded capsules as Marangoni microswimmers at the air–water interface: Symmetry breaking and spontaneous propulsion by surfactant diffusion and advection

Cited by 16 publications

References 69 publications

Self‐Propulsion of Droplets via Light‐Stimuli Rapid Control of Their Surface Tension

Self‐Propulsion of Droplets via Light‐Stimuli Rapid Control of Their Surface Tension

Role of Marangoni forces in the velocity of symmetric interfacial swimmers

Editorial: Motile active matter

Contact Info

Product

Resources

About