Self-propelled Oil Droplets Research Articles

Minimal model explanations of cognition

Active materials are self-propelled non-living entities which, in some circumstances, exhibit a number of cognitively interesting behaviors such as gradient-following, avoiding obstacles, signaling and group coordination. This has led to scientific and philosophical discussion of whether this may make them useful as minimal models of cognition (Hanczyc, 2014; McGivern, 2019). Batterman and Rice (2014) have argued that what makes a minimal model explanatory is that the model is ultimately in the same universality class as the target system, which underpins why it exhibits the same macrobehavior. We appeal to recent research in basal cognition (Lyon et al., 2021) to establish appropriate target systems and essential features of cognition as a target of modeling. Looking at self-propelled oil droplets, a type of active material, we do not find that organization alone indicates that these systems exhibit the essential features of cognition. We then examine the specific behaviors of oil droplets but also fail to find that these demonstrate the essential features of cognition. Without a universality class, Batterman & Rice’s account of the explanatory power of minimal models simply does not apply to cognition. However, we also want to stress that it is not intended to; cognition is not the same type of behavioral phenomena as those found in physics. We then look to the minimal cognition methodology of Beer (1996, 2020a, b) to show how active materials can be explanatorily valuable regardless of their cognitive status because they engage in specific behaviors that have traditionally been expected to involve internal representational dynamics, revealing misconceptions about the cognitive underpinnings of certain, specific behaviors in target systems where such behaviors are cognitive. Further, Beer’s models can also be genuinely explanatory by providing dynamical explanations.

Analysis of different self-propulsion types of oil droplets based on electrostatic interaction effects.

This study aims to investigate electrical effects in self-propelled oil droplets and analyze the motion caused by electrical interactions between two droplets. We examine the motion of self-propelled oil droplets in terms of electrical effects; no similar analyses have been reported to date. When an oleic acid droplet is introduced into a surfactant solution, oleic acid is adsorbed at the droplet surface; as a result, the surface becomes negatively charged. However, the electrostatic interaction between oil droplets has not been studied so far. We focus on the chargeability of self-propelled oil droplets, and propose that their motion can be controlled by changing the charge state at the oil/water interface. This is realized by controlling the pH of the surfactant solution. We prepared several aqueous solutions with slightly different pH values, and introduced two oil droplets into each solution to investigate the relationship between the pH value and the correlated motion of the oil droplets. In an aqueous solution at pH 12.00, the two oil droplets showed two types of motion (follow-up and parallel motion), while in an aqueous solution at pH 12.10 they exhibited only one type of motion, i.e., repulsive motion. In order to analyze the three types of motion, we investigated electrical effects on the surface of the oil droplets. First, we measured the zeta potentials of aqueous solutions at pH 12.00 and 12.10, and analyzed the relationship between pH and charge. Furthermore, we calculated the amount of charge and the repulsive force generated on the surface of the oil droplets. For the pH 12.10 solution, the charge of one oil droplet and the repulsive force generated by the charge were found to be −30.79 pC and 282 nN, respectively. Taking into account the driving forces controlling the motion of oil droplets reported in previous studies, this repulsive force could be the driving force of the observed repulsive motion. Based on the experimental results, we inferred that the three types of self-propulsive motion were controlled by electrical effects at the oil/water interface. To further examine these motions, we analyzed the time dependences of the total distance traveled by a droplet and of the distance between oil droplets, as well as the droplet velocities and images of internal convection of oil droplets.

Self-preserving mechanisms in motile oil droplets: a computational model of abiological self-preservation.

Recent empirical work has characterized motile oil droplets—small, self-propelled oil droplets whose active surface chemistry moves them through their aqueous environment. Previous work has evaluated in detail the fluid dynamics underlying the motility of these droplets. This paper introduces a new computational model that is used to evaluate the behaviour of these droplets as a form of viability-based adaptive self-preservation, whereby (i) the mechanism of motility causes motion towards the conditions beneficial to that mechanism’s persistence; and (ii) the behaviour automatically adapts to compensate when the motility mechanism’s ideal operating conditions change. The model simulates a motile oil droplet as a disc that moves through a two-dimensional spatial environment containing diffusing chemicals. The concentration of reactants on its surface change by way of chemical reactions, diffusion, Marangoni flow (the equilibriation of surface tension) and exchange with the droplet’s local environment. Droplet motility is a by-product of Marangoni flow, similar to the motion-producing mechanism observed in the lab. We use the model to examine how the droplet’s behaviour changes when its ideal operating conditions vary.

A reaction–diffusion particle model for clustering of self-propelled oil droplets on a surfactant solution

We experimentally and numerically investigate collective behaviors of oil droplets floating on a surfactant solution in a narrow circular channel. A closed environment where a glass cover is placed on the channel shown that ethyl salicylate droplets on the surface of sodium dodecyl sulfate solution exhibit transient oscillatory dynamics, leading to the formation of a single cluster via the merging of sub-clusters. When the glass cover is removed, oscillatory behavior resumes, and the cluster breaks up. To understand these experimental findings, we introduce a mathematical model that combines equations of motion for droplets with a reaction–diffusion system, where droplet dynamics and the chemical reactions are considered on the one-dimensional surface, and the diffusion of chemicals in the air phase and the water phase is treated in the two-dimensional region. Our model successfully reproduces transient oscillations and the characteristics of cluster formation, and the effect of the glass cover. We argue that the attractive long-range interaction due to the global concentration profile of the solution suffices for the cluster formation.

3D simulation of internal convection in a self-propelled oil droplet whose shape is fixed by a rigid frame

3D simulation of internal convection in a self-propelled oil droplet whose shape is fixed by a rigid frame

Behavior control of multiple self-propelled oil droplets using electrostatic interaction

Behavior control of multiple self-propelled oil droplets using electrostatic interaction

Temperature and Humidity Dependence of Marangoni Convection and Its Effect on the Self-propulsion of an Oil Droplet

This paper reports the temperature and relative humidity dependence of self-propelled oil droplets. Self-propelled oil droplets locomote by internal Marangoni convection, which is caused by gradien...

Directional and velocity control of active droplets using a rigid-frame.

This paper introduces a novel directional control method of self-propelled oil droplets. Oil droplets locomote spontaneously with surfactant action. This self-propulsion is caused by Marangoni convection within the oil droplet due to differences in the surfactant concentration at the droplet surface. Recent studies have reported that self-propelled oil droplets change their locomotion style depending on their shapes. We confirm that spherical oil droplets move randomly, including straight motion, bending motion, and rotation. In particular, we discover that boomerang-shaped oil droplets exhibit only straight motion. In this study, we introduce an exoskeleton for the directional and velocity control of oil droplets. A droplet shaped as a boomerang by an exoskeleton locomotes in the direction from a concave region to a convex region. Through experimental studies, we found that the stability of the velocity and locomotion direction depended on the boomerang shape. Self-propelled oil droplets with exoskeletons were then applied to a transporting robot driven only by the energy obtained from chemical reactions. We demonstrate the robot pushes and transports an object floating on water.

Direction control of self-propelled oil droplet focused on shape

Direction control of self-propelled oil droplet focused on shape

Erratum: Spontaneous change in trajectory patterns of a self-propelled oil droplet at the air-surfactant solution interface [Phys. Rev. E 91 , 032406 (2015)

Erratum: Spontaneous change in trajectory patterns of a self-propelled oil droplet at the air-surfactant solution interface [Phys. Rev. E <b>91</b> , 032406 (2015)

Locomotion Mode of Micrometer-Sized Oil Droplets in Solutions of Cationic Surfactants Having Ester or Ether Linkages.

Micrometer-sized self-propelled oil droplets under a far-from-equilibrium condition have drawn much attention because of their potential as a dynamic model for the chemical machinery in living organisms. To clarify the effect of interactions between the system components (surfactant, oil, and water) on the locomotion mode of droplets, we investigated the behaviors of oil droplets composed of n-heptyloxybenzaldehyde (HBA) in solutions of cationic surfactants having or not having an ester or an ether linkage. It was observed that in solutions of cationic surfactants having an ester or an ether linkage, spherical HBA droplets self-propelled by changing their direction frequently. On the other hand, when this functional group is absent, a slow self-propelled motion of the oil droplets concurrent with the evolution of aggregates on their surface was observed. From the results of measurement of interfacial tension and assessment of self-emulsification, we determined that the attractive interactions of cationic surfactants without an ester or an ether linkage with HBA are stronger than those having the linkage. The difference in the locomotion mode of oil droplets is probably explained from the viewpoint of the interactions among the system components.

Self-Propelled Oil Droplets and Their Morphological Change to Giant Vesicles Induced by a Surfactant Solution at Low pH.

Unique dynamics using inanimate molecular assemblies based on soft matter have drawn much attention for demonstrating far-from-equilibrium chemical systems. However, there are no soft matter systems that exhibit a possible pathway linking the self-propelled oil droplets to formation of giant vesicles stimulated by low pH. In this study, we conceived an experimental oil-in-water emulsion system in which flocculated particles composed of a imine-containing oil transformed to spherical oil droplets that self-propelled and, after coming to rest, formed membranous figures. Finally, these figures became giant vesicles. From NMR, pH curves, and surface tension measurements, we determined that this far-from-equilibrium phenomenon was due to the acidic hydrolysis of the oil, which produced a benzaldehyde derivative as an oil component and a primary amine as a surfactant precursor, and the dynamic behavior of the hydrolytic products in the emulsion system. These findings afforded us a potential linkage between mobile droplet-based protocells and vesicle-based protocells stimulated by low pH.

Deformable Self-Propelled Micro-Object Comprising Underwater Oil Droplets.

The self-propelled motion with deformation of micrometer-sized soft matter in water has potential application not only for underwater carriers or probes in very narrow spaces but also for understanding cell locomotion in terms of non-equilibrium physics. As far as we know, there have been no reports about micrometer-sized self-propelled soft matter mimicking amoeboid motion underwater. Here, we report an artificial molecular system of underwater oil droplets exhibiting self-propelled motion with deformation as an initial experimental model. We describe the heterogeneity in a deformable self-propelled oil droplet system in aqueous and oil phases and at their interface based on the behavior and interaction of surfactant and oil molecules. The current results have great importance for scientific frontiers such as developing deformable micro-swimmers and exploring the emergence of self-locomotion of oil droplet-type protocells.

Design of Reactive Surfactants that Control the Locomotion Mode of Cell-Sized Oil Droplets

The self-propelled motion of micrometer-sized oil droplets in nonequilibrium systems has attracted considerable attention as a primitive type of inanimate chemical machinery. We investigated the dynamics of n-heptyloxybenzaldehyde droplets in a cationic reactive surfactant solution with the objective of controlling the movement of self-propelled oil droplets. (1) We designed a novel molecular system that lengthened the locomotion time of oil droplets by using estercontaining cationic surfactants, (2) we evaluated the dynamics of oil droplets in a solution of a gemini cationic surfactant with various alkyl chain lengths in the linker to induce a locomotion mode change of the droplets and induce a conformational change in the flexible linked molecule, and (3) we examined the behavior of oil droplets in a microchannel in a solution containing a gemini cationic surfactant that has a carbonate linkage. These droplets exhibited two motions during the hydrolysis of the carbonate linkage, generating two surfactant species. Keywords: Cationic surfactant, hydrolysis, ion-exchange reaction, nonequilibrium system, oil droplet, self-propelled motion.

Molecular System for the Division of Self-Propelled Oil Droplets by Component Feeding.

Unique dynamics using inanimate molecular assemblies have drawn a great amount of attention for demonstrating prebiomimetic molecular systems. For the construction of an organized logic combining two fundamental dynamics of life, we demonstrate here a molecular system that exhibits both division and self-propelled motion using oil droplets. The key molecule of this molecular system is a novel cationic surfactant containing a five-membered acetal moiety, and the molecular system can feed the self-propelled oil droplet composed of a benzaldehyde derivative and an alkanol. The division dynamics of the self-propelled oil droplets were observed through the hydrolysis of the cationic surfactant in bulk solution. The mechanism of the current dynamics is argued to be based on the supply of "fresh" oil components in the moving oil droplets, which is induced by the Marangoni instability. We consider this molecular system to be a prototype of self-reproducing inanimate molecular assembly exhibiting self-propelled motion.

Spontaneous change in trajectory patterns of a self-propelled oil droplet at the air-surfactant solution interface

Trajectory-pattern formation of a self-propelled oil droplet floating on the surface of a surfactant solution in a circular dish is studied both experimentally and by simulation. The Marangoni effect induced by the dissolution of oil in the solution drives the droplet's motion. The trajectories spontaneously organize into several patterns including circular, knot-forming, back-and-forth, and irregular ones. They are either global patterns, whose center corresponds to the dish center, or other local patterns. Our simple model consisting of three forces, the driving force, the viscous resistance, and the repulsion from the boundary, successfully reproduces the global trajectory patterns including the power spectrum of the droplet speed.

Multiple-division of self-propelled oil droplets through acetal formation.

We demonstrate a novel system that exhibits both self-propelled motion and division of micrometer-sized oil droplets induced by chemical conversion of the system components. Such unique dynamics were observed in an oil-in-water emulsion of a benzaldehyde derivative, an alkanol and a cationic surfactant at a low pH.

PH-induced motion control of self-propelled oil droplets using a hydrolyzable gemini cationic surfactant.

Self-propelled motion of micrometer-sized substances has drawn much attention as an autonomous transportation system. One candidate vehicle is a chemically driven micrometer-sized oil droplet. However, to the best of our knowledge, there has been no report of a chemical reaction system controlling the three-dimensional motion of oil droplets underwater. In this study, we developed a molecular system that controlled the self-propelled motion of 4-heptyloxybenzaldehyde oil droplets by using novel gemini cationic surfactants containing carbonate linkages (2G12C). We found that, in emulsions containing sodium hydroxide, the motion time of the self-propelled oil droplets was longer in the presence of 2G12C than in the presence of gemini cationic surfactants without carbonate linkages. Moreover, in 2G12C solution, oil droplets at rest underwent unidirectional, self-propelled motion in a gradient field toward a higher concentration of sodium hydroxide. Even though they stopped within several seconds, they restarted in the same direction. 2G12C was gradually hydrolyzed under basic conditions to produce a pair of the corresponding monomeric surfactants, which exhibit different interfacial properties from 2G12C. The prolonged and restart motion of the oil droplets were explained by the increase in the heterogeneity of the interfacial tension of the oil droplets.

Mode Changes Associated with Oil Droplet Movement in Solutions of Gemini Cationic Surfactants

Micrometer-sized self-propelled oil droplets in nonequilibrium systems have attracted much attention, since they form stable emulsions composed of oil, water, and surfactant which represent a primitive type of inanimate chemical machinery. In this work, we examined means of controlling the movement of oil droplets by studying the dynamics of n-heptyloxybenzaldehyde droplets in phosphate buffers containing alkanediyl-α,ω-bis(N-dodecyl-N,N-dimethylammonium bromide) (nG12) with either tetramethylene (4G12), octaethylene (8G12), or dodecamethylene (12G12) chains in the linker moiety. Significant differences in droplet dynamics were observed to be induced by changes in the linker structure of these gemini cationic surfactants. In a phosphate buffer containing 30 mM 4G12, self-propelled motion of droplets concurrent with the formation of molecular aggregates on their surfaces was observed, whereas the fusion of oil droplets was evident in both 8G12 and 12G12 solutions. We also determined that the surface activities and the extent of molecular self-assembly of the surfactants in phosphate buffer were strongly influenced by the alkyl chain length in the linker moiety. We therefore conclude that the surface activities of the gemini cationic surfactant have important effects on the oil-water interfacial tension of oil droplets and the formation of molecular aggregates and that both of these factors induce the unique movement of the droplets.

PH-Sensitive Self-Propelled Motion of Oil Droplets in the Presence of Cationic Surfactants Containing Hydrolyzable Ester Linkages

Self-propelled oil droplets in a nonequilibrium system have drawn much attention as both a primitive type of inanimate chemical machinery and a dynamic model of the origin of life. Here, to create the pH-sensitive self-propelled motion of oil droplets, we synthesized cationic surfactants containing hydrolyzable ester linkages. We found that n-heptyloxybenzaldehyde oil droplets were self-propelled in the presence of ester-containing cationic surfactant. In basic solution prepared with sodium hydroxide, oil droplets moved as molecular aggregates formed on their surface. Moreover, the self-propelled motion in the presence of the hydrolyzable cationic surfactant lasted longer than that in the presence of nonhydrolyzable cationic surfactant. This is probably due to the production of a fatty acid by the hydrolysis of the ester-containing cationic surfactant and the subsequent neutralization of the fatty acid with sodium hydroxide. A complex surfactant was formed in the aqueous solution because of the cation and anion combination. Because such complex formation can induce both a decrease in the interfacial tension of the oil droplet and self-assembly with n-heptyloxybenzaldehyde and lauric acid in the aqueous dispersion, the prolonged movement of the oil droplet may be explained by the increase in heterogeneity of the interfacial tension of the oil droplet triggered by the hydrolysis of the ester-containing surfactant.