Photochemically induced motion of liquid metal marblesXinke Tang, Shi-Yang Tang, Vijay Sivan, Wei Zhang, Arnan Mitchell, Kourosh Kalantar-zadeh, and Khashayar

Khoshmanesh Citation: Applied Physics Letters 103, 174104 (2013); doi: 10.1063/1.4826923 View online: http://dx.doi.org/10.1063/1.4826923 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/17?ver=pdfcov Published by the AIP Publishing

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Photochemically induced motion of liquid metal marbles

Xinke Tang, Shi-Yang Tang, Vijay Sivan, Wei Zhang, Arnan Mitchell,Kourosh Kalantar-zadeh,a) and Khashayar Khoshmanesha)

School of Electrical and Computer Engineering, RMIT University, Melbourne, Victoria, 3001, Australia

(Received 7 September 2013; accepted 9 October 2013; published online 24 October 2013)

We demonstrate photochemically induced actuation of liquid metal marbles, which are liquid

metal droplets encased in micro/nanoparticles. The WO3 nanoparticles coated marbles are placed

in H2O2 solution, and their surfaces are illuminated with UV light. The semiconducting WO3

coating behaves as a photocatalyst to trigger a photochemical reaction, generating oxygen bubbles

that propel the marble. The actuation of the marbles is investigated under different H2O2

concentrations, light intensities, and marble dimensions. Equations describing the fundamentals of

such actuations are presented. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4826923]

On demand actuation of small objects is essential in the de-

velopment of future consumer products. Such actuation has al-

ready been demonstrated in small scale mechanical systems

such as micromotors,1 micropumps,2,3 and micro/nano cargo-

towing carriers for drug delivery.4,5 Many studies have been

conducted to achieve this actuation using different mechanisms,

including light,6,7 electric field,8,9 magnetic field,10,11 chemical

interactions,1,12,13 and catalytic activities.5,14–16 In such

approaches, the actuation of objects relies on different symmetry

breaking mechanisms. Amongst them, catalytically or chemi-

cally induced motion, which is realized through the generation

of gas via intense catalytic/chemical reactions, has been widely

used for achieving the actuation of micro/nanoscale objects.

We have recently introduced “liquid metal marbles,”

which are formed by encasing liquid metal galinstan (which is

an eutectic alloy composed of 68.5% gallium, 21.5% indium,

and 10% tin) with coatings of functional nanoparticles.17 We

have shown remarkable abilities of these liquid metal marbles

for realizing soft electronic elements and sensors. We have

demonstrated that the nanoparticle coatings offer an extra

degree of freedom in comparison to bare liquid metal droplets,

adding extraordinary capabilities to the system. In addition, the

electrochemically induced actuation of liquid metal marbles in

aqueous solutions has been investigated by Tang et al.18

Particular recent attention has been given to chemical-

ly/catalytically powered micro/nano objects, including cylin-

ders and tubular rods19–23 and also spherical Janus

motors,1,5,14,24–26 which exhibit autonomous self-propulsion

in aqueous environments. The motion of these objects is

induced by gas bubbles, produced via chemical/catalytic

reactions between the objects themselves and their surround-

ing solutions. The actuation velocity achieved by these tech-

niques range from a few to several hundreds of body length

per second, and generally, the chemical actuations are faster

than their catalytic counterparts.

In chemically actuated objects, metals such as Ga/Al,

Mg, and Zn are commonly used as the fuel within the body

of the object since they chemically react with a wide range

of aqueous solutions. In these cases, metals are gradually dis-

solved into the solution through chemical reduction while

generating hydrogen gas bubbles, thus compromising the life

span of the object. It has been observed that the actuation ve-

locity is extremely sensitive to the size of the object, show-

ing a decrease in velocity with increasing object size.27 The

actuation ceases when the reacting materials are completely

consumed.1,22,25 Alternatively, materials such as Pt and Ti

are commonly used as catalysts to facilitate the dissociation

of hydrogen peroxide (H2O2), generating oxygen gas bub-

bles, without being consumed during the reactions. This

allows the actuations to have a much longer life span.14,21,23

Motion induced by the aforementioned chemical/cata-

lytic approaches takes relatively random trajectories. For

achieving any controlled actuation, external fields are

required. In this regard, light-induced chemical (photochemi-

cal) reactions using active photocatalysts, such as TiO2 and

WO3 in hydrogen peroxide, have been shown to be powerful

in generating gas bubbles.28,29 This phenomenon can poten-

tially be used for propelling small objects.

In this Letter, we demonstrate the actuation of liquid

metal marbles through photochemical reactions and present

it as an approach to induce controlled actuation. We

hypothesize that propulsion of the liquid metal marbles can

be achieved when a photocatalyst material such as WO3 is

used for coating the marble and stimulated with a light

source in a H2O2 aqueous solution.

A scanning electron microcopy (SEM) image of liquid

metal marble coated with WO3 nanoparticles is shown in Fig.

1(a). Since WO3 nanoparticles (particle size ca. 80 nm) are

semiconducting materials, with a band gap of �2.7 eV,30 they

show photocatalytic properties when exposed to a light source

with the wavelengths smaller than �460 nm. Applying a UV

light (320–380 nm) source to one side of the marble leads to

photo-excitation of the illuminated region. As a result, H2O2

is decomposed, leading to generation of oxygen bubbles

H2O2 þ 2e�cb þ 2Hþ ! 2H2O; (1)

H2O2 þ 2hþ�b ! 2Hþ þ O2; (2)

where e�cb is the exited electron in the conductive band, and

hþ�b is the hole in the valence band. This light-induced photo-

catalytic decomposition of H2O2 happens only on the illumi-

nated side of the marbles [Fig. 1(b)]. In the process, oxygen

a)Authors to whom correspondence should be addressed. Electronic addresses:

kourosh.kalantar-zadeh@rmit.edu.au and khashayar.khoshmanesh@rmit.edu.au

0003-6951/2013/103(17)/174104/4/$30.00 VC 2013 AIP Publishing LLC103, 174104-1

APPLIED PHYSICS LETTERS 103, 174104 (2013)

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bubbles are generated and gradually become more frequent

and larger. They eventually lift up the base of the marble, as

shown in Fig. 1(b). As a result, the marble experiences a

“rolling force” (as the red arrow shows) and tends to move

away from the light beam (as the green arrow shows). When

the stage is constantly moved (as the blue arrow shows), the

light beam approaches the marble from the left-hand side

with a constant speed along the channel.

When light impinges on the liquid metal marble, it gen-

erates bubbles. These bubbles produce actuation of the mar-

ble. If the generation of gas bubbles can induce an actuation

with equal velocity to the stage motion, the stage and the

marble move concurrently at the same speed. In this case,

the illuminated area of the marble is always about a quarter

of the total marble surface. By further increasing the stage

velocity, a speed is reached at which the marbles could not

be latched to the light source anymore and would fall behind.

Ideally, this velocity is called the “maximum speed” of the

marble actuation in this work and is used for assessing the

light induced actuation.

The experimental method and associated set up are pre-

sented in supplementary material.31 The stage was tethered

to a linear screw actuator with a velocity in the order of 0.1

to 50 mm/min, which pulled the microscope stage at the

desired speeds. An example of a moving WO3 coated marble

is shown in Fig. 2(a) and also in the Integrated Media. The

velocity curve is shown in Fig. 2(b). As can be seen, the gen-

eration of bubbles and the induced speed can be random at

some occasions. As a result, the maximum speed is used for

identifying the actuation of marbles.

The propelling force on the marble is provided by the

generation and growth of oxygen bubbles. Therefore, the av-

erage velocity of the marble can be related to the growth rate

of the bubbles as below

�Umarble ¼ r•bubble cos�h; (3)

FIG. 1. (a) SEM images of a liquid metal marble: galinstan droplet encapsulated

in a coating of 80 nm WO3 nanoparticles by rolling it on a powder bed. (b)

Schematic of the experimental setup showing the mechanism of light-induced

motion of a WO3 coated marble. The light beam passes through an objective

lens (that is also used for imaging) and is focused onto one side of the marble.

FIG. 2. (a) Sequential snapshots of the

motion of a 2 mm diameter WO3 coated

marble in 15% H2O2 solution when a

light intensity of 100% is approaching

the marble from the left-hand side with a

constant speed of 4.5 mm/min. The yel-

low arrows show the marble actuation

direction. The visible purple spot is the

light beam. The time interval

between two fames is 2 min. (b)

Time-displacement plots for both the

light spot and the marble. The marble is

solely propelled by the bubbles, which

are generated randomly by the photo-

chemical reaction. Therefore, the mar-

ble’s instantaneous speed can be slightly

different from that of the light beam, as

shown by the time-displacement plots.

However, the calculated average speed

of the marble is 4.38 mm/min, which is

approximately the same as the speed of

the light beam, thus we can assume that

the marble follows the stage with the

same speed. (c) Schematic of the marble

motion mechanism, motion is induced

by the growth of bubbles with different

latitude angles. The insert shows a sim-

plified model with an average latitude

angle of �h (enhanced online) [URL:

http://dx.doi.org/10.1063/1.4826923.1].

174104-2 Tang et al. Appl. Phys. Lett. 103, 174104 (2013)

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where �Umarble is the average velocity, r•bubble ¼ drbubble=dt is

the growth rate of the bubble, and �h is the average latitude

angle of the lines connecting the centers of the generated

bubbles to the center of the marble, as shown in Fig. 2(c).

In order to obtain r•bubble, we make three assumptions as

follows. First, we assume that the bubbles have a spherical

shape, and therefore, their mass can be expressed as

mbubble ¼ qbubble � 4=3 pr3bubble with both qbubble and rbubble

vary with time. Second, we assume that the oxygen enclosed

in the bubbles is as ideal gas and its density can be expressed

as qbubble ¼ P= �RT, where P is the internal pressure of the

bubble, �R is the universal gas constant, and T is the tempera-

ture. Third, considering that the bubbles are generated under-

neath the marble, we assume that their internal pressure is

governed by the weight of the marble as

P ¼ Mmarble g=p r2bubble þ Patm, where Mmarble is the mass of

the marble droplet, Patm is the pressure of the atmosphere,

and g is the gravitational acceleration. Combining these

equations and after some algebra (see supplementary mate-

rial31), r•bubble is obtained as follows:

r•bubble ¼9

16m• bubble

�RT

p qmarble g R3marble

: (4)

On the other hand, the rate of bubble generation is a

function of the applied light intensity (I), concentration of

H2O2 (CH2O2) and the surface area of the marble, which can

be expressed as m• bubble / Ia � CbH2O2

� p R2marble.

This yields to the following equation, which relates the

average velocity of the marble to the influential parameters

of the system

�Umarble /9

16

�RT

qmarble g

Ia CbH2O2

Rmarble

/Ia Cb

H2O2

Rmarble

: (5)

From this equation, it can be hypothesized that the maxi-

mum speed of approaching light beam, which can be also

regarded as the maximum speed of the marble, can be varied

by changing the concentration of H2O2, light intensity, and

marble dimension. A series of experiments are conducted to

investigate these variations, as described below.

First, we investigate the marble motion at different light

intensities of 12%, 25%, 50%, and 100% while setting the

marble diameter and H2O2 concentration to 2 mm and 15%,

respectively [Fig. 3(a)]. The results indicate that increasing

the light intensity leads to increasing the maximum speed of

the marble in a linear manner. According to Eq. (5), the

speed of the marble is proportional to the light intensity, Ia.By assuming a¼ 1, the marble’s speed varies linearly with

respect to the light intensity, as shown in Fig. 3(a). The pre-

dicted linear curve only matches the measured curve at low

light intensities. For higher light intensities, the difference

between the predicted and measured curves can be attributed

to the light scattering effect induced by the generation of

more bubbles or larger bubbles on the marble surface.

Decreasing the value of a to 0.5 provides an approximate fit-

ting to experimental results [Fig. 3(a)].

Next, we investigate the marble motion at different

H2O2 concentrations of 3%, 7.5%, 15%, and 30%, while set-

ting the marble diameter and light intensity to 2 mm and

100%, respectively [Fig. 3(b)]. The results show that increas-

ing the concentration of the H2O2 solution results in a larger

maximum actuation speed of the marble. This is in line with

Eq. (5), in which the motion speed of the marble is propor-

tional to the concentration of H2O2, CbH2O2

. By assuming

b¼ 1 the marble’s speed varies linearly with respect to the

light intensity, as shown in Fig. 3(b). The predicted linear

curve only matches the measured curve at low H2O2 concen-

trations. For higher H2O2 concentrations, the difference

between the predicted and measured curve can be again

attributed to the faster generation of bubbles or larger num-

ber of the bubbles, which locally reduces the H2O2 concen-

tration around the marble surface. Decreasing the value of b

FIG. 3. Characterization of the movement of the marbles at different operat-

ing conditions: (a) maximum speed of a 2 mm diameter marble as a function

of light intensity using a H2O2 concentration of 15%. Predictions are pro-

vided assuming that the speed of the marble is proportional to the light inten-

sity. (b) Maximum speed of a 2 mm diameter marble as a function of H2O2

concentration using a light intensity of 100%. Predictions are provided

assuming that the speed of the marble is proportional to the concentration of

H2O2. (c) Maximum speed of marble as a function of its diameter using a

15% H2O2 solution and a light intensity of 100%. The insets show top views

for each dimension of the marbles. Prediction obtained by assuming the

actuation speed of the marble is inversely proportional to the marble diame-

ter. (The data used for prediction plots are given in S3).31

174104-3 Tang et al. Appl. Phys. Lett. 103, 174104 (2013)

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to 0.5 provides a better agreement to the experimental results

[Fig. 3(b)].

Finally, we examine the marble motion by using different

marble diameters of 1–3 mm while setting the concentration

of H2O2 and light intensity to 15% and 100%, respectively

[Fig. 3(c)]. Accordingly, the gap between the two strips of

black tape is decreased to 1.1 mm (10% margin) and increased

to 3.3 mm (10% margin) for 1 mm marbles and 3 mm marbles,

respectively, with a view to confine a straight actuation path

for the marbles. Our results indicate that increasing the marble

diameter leads to smaller maximum actuation speed of the

marble. According to Eq. (5), the actuation speed of the mar-

ble is inversely proportional to the marble diameter 2Rmarble,

and the predicted curve matches the experimental results

obtained, as shown in Fig. 3(c).

In conclusion, we have demonstrated the photochemical

actuation of WO3 coated galinstan liquid metal marbles.

These marbles are placed in a H2O2 solution and are actuated

by applying a moving UV light. Based on our observations,

marbles experience a motion with an average speed approxi-

mately equal to that of the light beam. This motion could be

strongly affected by the concentration of H2O2, intensity of

the UV light, and the dimension of the marbles. This work

can be further extended by studying the photochemical

induced motion of marbles along arbitrary patterns. Adding

more control, the system can be equipped with a detector

and feedback controller to adjust the speed of the moving

beam with respect to the motion of the marble to ensure that

the light beam will not overtake the marble. Additionally, a

variety of experiments can also be conducted with different

coatings at various densities.

K. Khoshmanesh acknowledges the Australian Research

Council for funding under Discovery Early Career Researcher

Award (DECRA) scheme, (project DE120101402).

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detailed experimental method, an alternative set of equations with a differ-

ent initial assumption used in predicting the maximum speed of marble,

and the data used for prediction plots.

174104-4 Tang et al. Appl. Phys. Lett. 103, 174104 (2013)

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