Negative / positive chemotaxis of a droplet : Dynamic response to a stimulant gas

We report here the repulsive/attractive motion of an oil droplet floating on an aqueous phase caused by the application of a stimulant gas. A cm-sized droplet of oleic acid is repelled by ammonia vapor. In contrast, a droplet of aniline on an aqueous phase moves toward hydrochloric acid as a stimulant. The mechanisms of these characteristic behaviors of oil droplets are discussed in terms of the spatial gradient of the interfacial tension caused by the stimulant gas.

Negative/positive chemotaxis of a droplet: Dynamic response to a stimulant gas Living organisms on Earth exhibit self-propelled motion or chemotactic behavior in response to external stimuli, where motion is driven by chemical energy under isothermal conditions.In other words, organisms can transduce information into active motion through a process in which external stimuli are recognized as being favorable or unfavorable.This phenomenon is even seen in single living cells that do not have a neural net.In contrast to these observations in living organisms, the ability of human technology to create an artificial chemical system that exhibits dynamic chemotactic behavior is still in its infancy.2][3][4][5][6][7][8] Various kinds of droplet motion driven by a gradient in interfacial tension have been reported, as well as the propelled motion caused by diffusiophoresis. 9,102][13][14][15][16] In addition, the motion of an oil droplet in an oil/water system has been shown to be chemosensitive. 17In measurements of the electrical potential at the oil-water interface in a similar system, the nature of the potential fluctuation and oscillation strongly depended on the chemical properties and the concentration. 18It was proposed that an oil-water system shows reactive spreading on a glass surface with recovery of the surface condition. 19,20The spontaneous motion of a selfpropelled oil droplet moving on a glass substrate under an aqueous phase has been shown to be limited to the region of the acid-treated glass surface. 21,22Here, we report an artificial model system consisting of an oil droplet floating on an aqueous layer that exhibits chemotactic behavior in response to a gas stimulus.
Figure 1(a) shows the experimental system in which a droplet of oleic acid is situated on an aqueous layer with a depth of 1 cm.In this system, ammonia vapor was applied by moving a cotton swab wetted with 28% NH 3 liquid (Wako Pure Chemical Industries) close to the oil droplet.negative chemotaxis.To analyze the motion of the droplet in a quantitative manner, we created a spatio-temporal diagram of the droplet image, as shown in Fig. 1(c).The vertical axis shows the distance from point x ¼ 0, which corresponds to the center of the original position.Careful inspection of the response of the droplet to ammonia vapor reveals that the footprint of the droplet initially expands while it remains at the initial position for ca.0.8 s.This increase in area implies that there is a decrease in interfacial tension at the oil-air and/ or oil-water interface.The droplet then accelerates and finally stops at ca. 4 s after application of the vapor.Such repulsive motion as exemplified in Fig. 1 was confirmed to be highly reproducible.We also noted that the lag-time before the appearance of the escaping motion, e.g., ca. 1 s in Fig. 1(c), tended to increase with an increase in the distance between the floating oil droplet and the cotton swab wetted with NH 3 liquid.
We performed a time derivation of the time-dependent change in the position of the center of the droplet and obtained the time course of the droplet velocity dx/dt, as shown in Fig. 2 where m [kg] is the mass of the droplet, n [Pa s m] is the viscosity coefficient, and f(t) [N] is the driving force.We can reasonably assume that the velocity profile at the point at which acceleration switches from positive to negative is given under the condition f ðtÞ % 0. Thus, from curve-fitting with a single exponent over the portion of the curve with a decrease in velocity (t > 1.2 s), we evaluated the effective viscosity n by using a value of m ¼ 22 Â 10 À6 kg for 25 ll oleic acid (density: 0.89 g/cm 3 ).With this value of n, we can evaluate the time-dependent change in the driving force, as shown in Fig. 2(c).In this figure, the apparent force f(t) is almost null except for the period between 0.5 and 1.0 s, which supports the validity of our analysis based on the rather simple kinetic equation in Eq. ( 1).Note that the actual friction has two different origins: usual fluidic friction at a mesoscopic scale and molecular friction due to absorbance and desorption at the interface. 23In a future study, it may be of interest to evaluate the origin of the apparent friction based on careful measurements.Next, to clarify the mechanism of the stimulus-induced vectorial motion of the droplet through observation of the aqueous layer, we monitored the motion of polyethylene beads floating on the surface to visualize surface flow (see Fig. 3( ) to distilled water to clearly visualize the system.To visualize the internal flow of the aqueous phase, the red dye was not thoroughly mixed in the water.In contrast to the profiles regarding surface flow, there is very little motion of the liquid inside the aqueous layer.Based on these observations, the driving force of the running droplet is most likely the force acting at the interface, i.e., the gradient of the surface tension should act as the driving force for the vectorial motion of the droplet.
Figure 4 shows a schematic representation of the mechanism by which the droplet moves away from the gas stimulus.With the application of ammonia vapor, the oleic acid located at the oil/air interface undergoes an acid-base reaction.The ionized molecules of oleic acid decrease the oil/air interfacial tension, and then cause flattening of the droplet as exemplified in the initial stage of vapor application at around 1 s.Successively, the negatively charged oleate molecules, through the binding of NH 3 to the carbonyl group, tend to spread onto the water/air interface, and this phenomenon would be more prominent on the side closer to the vapor source.Thus, the water/air interfacial tension decreases on this side.As a result, there is a large difference in interfacial tension between the sides of the droplet, and this propels the droplet to move away from the vapor source.The significant fluidic flow on the surface of the aqueous layer (Fig. 3(b)) and the stationary bulk phase (Fig. 3(d)) support the validity of this mechanism.
Figure 5 shows the attractive motion or positive chemotaxis of a 25 ll aniline droplet floating on an anilinesaturated aqueous phase.With the application of hydrochloric acid vapor, the droplet tends to approach the vapor source.In this experiment, we consider that aniline molecules tend to form a monolayer at the water/air interface in the absence of the vapor.The contact angle of the aniline droplet is relatively small compared to that in the experiment with oleic acid, as shown in Fig. 1.In this case, the interfacial tension close to the gas stimuli is considered to increase because the positively charged aniline complexed with hydrochloric acid tends to dissolve into the bulk aqueous layer, i.e., a gradient of interfacial tension is generated between the sides of the droplet.Thus, we can explain the positive chemotaxis of the aniline droplet caused by exposure to the acidic gas.
We have described the negative and positive chemotaxis of an oil droplet floating on an aqueous phase.Negative chemotaxis is observed for a droplet with a weakly acidic molecule, i.e., oleic acid, stimulated by the relatively strong alkaline vapor of NH 3 .In contrast, positive chemotaxis is generated for the combination of a weakly alkaline oil and the strong acidic vapor of HCl.Based on these findings, we strongly suspect that negative/positive chemotaxis could be controlled by considering the acidity/basicity of the droplet and the stimulating gas, together with the hydrophobicity of the chemical components of the droplet.It would be interesting to examine this hypothesis in future studies.
FIG. 4. Proposed mechanism of the negative chemotactic behavior of an oleic acid droplet.The gradient of interfacial tension drives the droplet away from the stimulating gas, resulting in negative chemotactic behavior.

Figure 1 (
Figure1(a) shows the experimental system in which a droplet of oleic acid is situated on an aqueous layer with a depth of 1 cm.In this system, ammonia vapor was applied by moving a cotton swab wetted with 28% NH 3 liquid (Wako Pure Chemical Industries) close to the oil droplet.Figure 1(b) shows an example of the repulsive motion of a 25 ll oleic acid droplet in response to ammonia vapor, indicating (a).Similarly, Fig. 2(b) shows the time-dependent change in acceleration d 2 x=dt 2 .We can analyze these time-dependent profiles by using the simple equation of motion in

FIG. 3 .
FIG. 2. characteristics of repulsive motion of the droplet shown inFig.1.(a) and (b) Profiles of the velocity and acceleration of the droplet evaluated through the time derivative and second-derivative, respectively, of the spatio-temporal diagram in Fig. 1(c).(c) Profiles of the propelling force evaluated from an analysis of the time-dependent changes in velocity and acceleration, by adapting a phenomenological kinetic equation: m d 2 x dt 2 þ n dx dt þ f ðtÞ ¼ 0.

FIG. 5 .
FIG. 5. Positive chemotactic behavior of an aniline droplet vs. HCl vapor.(a) Superimposed image of the aniline droplet moving toward the HCl vapor, supplied by a cotton swab wetted with hydrochloric acid solution (37%).(b) Proposed mechanism of the positive chemotactic behavior of the aniline droplet.