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Abstract

Magnetic field–induced atom actuation has admiring abundant analysis absorption in contempo years. However, accepted magnetic-controlled liquids depend primarily on alluring particles added to a droplet, which serves as the actuator on an accessible surface. These liquids accordingly ache from atom agreeable with the alluring particles or disengaging with the magnet, possibly arch to sample contamination, which acutely banned their carriage acceleration and activated applications. Here, we address a simple and additive-free adjustment to assemble alluring tubular microactuators for manipulating aqueous aerosol by magnetism-induced agee deformation, which generates an adjustable capillary force to actuate liquids. These alluring tubular microactuators can drive assorted aqueous aerosol with controllable acceleration and direction. A acceleration of 10 cm s−1 can be achieved, apery the accomplished acceleration of aqueous motion apprenticed by an alien stimulus–induced capillary force in a bankrupt tube activate so far.

INTRODUCTION

The controlled abetment of baby amounts of aqueous has admiring abundant analysis absorption (1–6). From the perspectives of accurate analysis and activated application, the ascendancy of aqueous beck or atom is acutely important, abnormally for the fields of biosensing (7), medical affection (8), biologic commitment (9, 10), and “lab-on-a-chip” DNA purifications (11). Appropriately far, there accept been abounding letters on actuating aqueous aerosol with alien thermal gradients, light, electric fields, and alluring fields (12–16). Alluring actuation is accustomed as one able adjustment for aqueous actuation based on its several advantages, including all-embracing action, ample ascendancy forces, and anemic alternation amid the alluring acreage and the aqueous droplet. However, to the best of our knowledge, about all antecedent letters on the alluring abetment of atom acclimated alluring particles added to the atom as the actuator on an accessible apparent (2, 17–19). These methods accordingly ache from atom agreeable with the alluring particles or disengaging with the magnet, possibly arch to sample contagion and essentially attached their abeyant use in accepted applications.

The self-propelled motion of a atom is about accomplished by atom aberration acquired by a actinic reaction, a temperature gradient, or an agee geometry (20, 21). For example, drops in cone-shaped tubes can self-propel against the attenuated arena because of the Laplace burden acclivity (21, 22). These self-propelled atom motions can action alone back the pinning force is negligible compared to the active force induced by the wettability acclivity or the Laplace burden gradient. Because the pinning force depends on the bulk of acquaintance bend hysteresis (CAH), it can be affected by designing tubular microactuators. Recently, Lv et al. (23) appear a action to dispense aqueous slugs by photoinduced agee anamorphosis of cross-linked aqueous bright polymer tubular microactuators, which induces capillary armament that can be acclimated for aqueous propulsion. Although this is a arresting adjustment for designing tubular microactuators, its about low carriage speed, claim for specific materials, and circuitous alertness action acutely bind its appliance in absolute life.

To boldness these limitations, here, we address a simple and reliable adjustment for fast aqueous carriage by application agee alluring tubular microactuators (MTMAs). The MTMAs are able from a atypical advised alluring polydimethylsiloxane (PDMS) abject bendable tube, assuming agee anamorphosis in a alluring acreage (Fig. 1A). The anamorphosis of the aerial allotment of the tube is induced by the alluring field, which leads to agee anamorphosis with account to the tube breadth absolute the aqueous droplet. As the allurement moves, this agee anamorphosis takes the appearance of a activating conical tube, bearing a Laplace burden acclivity that induces advanced aqueous transport. The MTMAs move aerosol against the narrower end, advantageous CAH via the agee anamorphosis of the tube induced by the alluring field. The acceleration of partially wetting liquids alcove 10 cm s−1, which is the accomplished acceleration of aqueous motion apprenticed by an alien stimulus–induced capillary force in bankrupt tubular microactuators activate so far. Because our microactuators do not crave the accession of any magnetic-sensitive particles into liquids, assorted aqueous aerosol can be auspiciously manipulated in the MTMA with controllable acceleration and administration and can alike run uphill. As far as we know, this is the aboriginal analysis into the alluring on-demand actuation of an additive-free aqueous in a bankrupt channel.

Download high-res angel Accessible in new tab Download Powerpoint Fig. 1 Design of MTMAs.

(A) Schematics assuming the motion of a aqueous atom of wetting aqueous bedfast in an MTMA apprenticed by a alluring field. (B) Ancillary bend of the agnate direct accompaniment of the aqueous atom in the acquired MTMAs. (C) Lateral photographs of the magnetic-induced motion of an booze atom in an MTMA anchored on a substrate. The allurement moves from larboard to right. The booze atom is propelled against the right; back the administration of the allurement is antipodal (bottom row), the administration of movement of the atom is additionally antipodal (movie S1).

RESULTSFabrication of MTMAs

MTMAs were bogus from bargain cross-linked PDMS and alluring carbonyl adamant powder. A simple alertness access for MTMAs is apparent in fig. S1. This access includes two PDMS abating processes. First, authentic PDMS (Sylgard 184, Dow Corning) was thoroughly alloyed with the abating reagent at a weight arrangement of 10:1. Then, the admixture was coated on animate capillaries with altered alien diameters, which were maintained angular at allowance temperature for altered continuance times, followed by thermal abating at 60°C for 20 min. From the aloft step, we can access a compatible PDMS animate capillary. Second, ferromagnetic adamant (Fe) particles were benumbed in PDMS prepolymer with active active to anatomy a mixture. Then, the admixture was coated on a apple-pie bottle accelerate application the tape-casting adjustment to anatomy a array of about 100 μm. The ahead acquired PDMS animate capillary was anxiously placed on the alluring PDMS layer, accoutrement the capillary with a attenuate band of alluring PDMS, and afresh thermally convalescent at 80°C for 60 min. A low temperature and a abbreviate time for abating were acclimated in the aboriginal abating process, acceptance the two PDMS layers to amalgamate actual able-bodied afterwards delamination. Last, the absolutely convalescent PDMS animate capillary was swelled in hexane and afresh bound demolded. Afterward the access of the combinations of altered capillaries and alluring band thicknesses (figs. S2 and S3), the bank thicknesses of the nonmagnetic band breadth and the alluring band were called at about 110 and 70 μm, respectively, for added study. Figure S4 illustrates that the alluring band can be able-bodied accumulated with the PDMS capillary.

Driving aqueous aerosol in MTMAs by agee deformation

Asymmetrical anamorphosis is acute for architecture tubular microactuators that abet a capillary force for affective liquids. Ancillary bend of the direct states (original state, activate to deform, final state) of a aqueous atom in the acquired MTMAs are apparent in Fig. 1B. Because of the wetting acreage with a CA of 25.8 ± 2.4° on the PDMS apparent in the air, a partially wetting aqueous (ethanol, absolute by rhodamine B) in the tube can anatomy a actual bright biconcave surface. Beneath a alluring acreage (a annular allurement with a bore of 4 mm and a apparent alluring acreage acuteness of 0.5 T), the booze askew with the MTMA in an agee manner, and these MTMAs can appropriately auspiciously dispense aqueous motion with a allurement (Fig. 1C and cine S1).

The anamorphosis of MTMAs with altered diameters beneath a alluring acreage is apparent in fig. S5 (B to D). The bulk of anamorphosis is potentiated as accretion the diameter. There was little anamorphosis back the bore was beneath than 0.8 mm, but apparent anamorphosis could be empiric back the bore is added to 2.0 mm. This is connected with the actuality that tubes with abate bore are added difficult to batter (24, 25). Unless declared otherwise, the MTMAs acclimated in this assignment were able with a bore of 2.0 mm. Back the atom askew with the MTMA due to the alluring force, the resultant agee anamorphosis constituted a minimum aperture bend of about 4° (fig. S5F). In the absence of a aqueous bead (fig. S5G), the anamorphosis induced by the alluring force produces a best aperture bend of about 10°. Therefore, with the compatible movement of the magnet, the connected anamorphosis of the MTMAs forms a activating conical capillary, which generates an adjustable capillary force to actuate liquids against the narrower end.

Propelling altered aggregate aqueous aerosol at altered velocities in MTMAs

Recent efforts accept been directed against the use of assorted alien stimuli (such as thermal gradients, light, electric fields, and chemicals) for aqueous transport. Although anniversary of these methods has altered advantages, the aloft obstacle they all appointment is hysteresis armament (26). Hysteresis armament generally advance to deceleration and stagnation of the droplet. Therefore, for accepted methods, the archetypal carriage speeds of the aqueous atom (ranging from μm s−1 to mm s−1) are generally too apathetic for activated applications in absolute activity (27). However, our MTMAs can auspiciously best this problem. As apparent in Fig. 2, the carriage action of a aqueous (ethanol, absolute by rhodamine B) with altered volumes was recorded application a agenda camera. Back the allurement beneath the atom was confused at a acceleration of 1 cm s−1, the booze atom can be propelled boring with the connected anamorphosis of MTMA. Back we added the aggregate of the booze atom from 10 to 25 μl, the atom still confused with the allurement (Fig. 2A and cine S2). Furthermore, the acceleration of the allurement is added from 4 to 10 cm s−1, and both a baby atom (10 μl) and a ample atom (25 μl) can move at a connected acceleration with the magnet, rather than abstract from the allurement (Fig. 2, B to D, and cine S2). This behavior indicates that the MTMA can accommodate a acceptable active force to affected the hysteresis armament for aqueous atom movement with aerial velocity. Upon added increases in the acceleration of the magnet, the direct alluring force cannot abet the anamorphosis of the MTMA (28–30). We additionally accede added factors that can affect the movement of the droplets, such as the bore of the tube, the accession of alluring nanoparticles to the tube, and the backbone of the magnet. The furnishings of these factors on the movement of aerosol are apparent in fig. S6, which can advice us added administer these MTMAs on demand. The movement acceleration of the booze alcove 10 cm s−1, which is the accomplished acceleration appear appropriately far in the abstract for a atom affective in tubular microactuators at allowance temperature. A abundant allegory is provided in table S1. The present adjustment achieves the accomplished acceleration and has several added advantages in alertness materials, activity consumption, and little sample contagion (15, 18, 23, 31–35).

Download high-res angel Accessible in new tab Download Powerpoint Fig. 2 Optical images assuming magnetic-controlled motion of ethanol.

Each alongside row shows the after-effects for a altered allurement speed, and the chicken arrows denote the allurement movement direction. (A) 1 cm s−1. (B) 3 cm s−1. (C) 7 cm s−1. (D) 10 cm s−1. In anniversary column, there is a altered aqueous volume: 10, 15, and 20 μl. The booze aerosol can be propelled afterwards disengagement from the magnet. Scale bars, 1 cm (movie S2).

Assorted aqueous aerosol propelled by alluring armament in MTMAs

Because these MTMAs do not crave the accession of any magnetic-sensitive particles in the apprenticed liquids, assorted aqueous aerosol could be auspiciously manipulated. Table 1 lists the wetting behaviors on a PDMS blur of a alternation of liquids with altered apparent tensions and viscosities. In our MTMAs, we advised a advanced ambit of partially wetting liquids, such as ethanol, ethyl acetate, hexadecane, isopropyl alcohol, and n-butyl booze (Fig. 3, A and B, and cine S3), and activate that our MTMAs can auspiciously drive these liquids at altered speeds. The specific capacity of magnetic-induced motion of baptize and booze admixture (ethanol, 80 aggregate %) are apparent in Fig. 3C. It can be apparent that the acceleration of the atom decreases with accretion baptize allotment (movie S4). This is to be expected, because beyond bulk of baptize makes the CAH access (from 6.82 ± 0.4° to 17.3 ± 0.1°), reinforcing the pinning force. However, the atom cannot be confused anymore back the baptize agreeable is added to 40%, agnate to a CA of 63.2 ± 2.1°, which abundantly broadens the ambit of the apprenticed aqueous (36). Atom motion on a abruptness is added difficult than that on a accumbent apparent because a ample active force is bare to affected the attrition of the slope. As apparent in Fig. 3D, our MTMAs can actuate an booze atom acclivous on an acclivity with a acceleration of 1 cm s−1 (movie S5). Considering the aftereffect of aqueous abscess on devices, a adherence analysis was agitated out on the compactness achievement of the MTMAs afterwards every 100 uses in aqueous transport. The breach breadth and adaptable modulus were activate to be abiding afterwards booze atom was transported 500 times (fig. S7, A and B). The abscess of PDMS by hexadecane was added austere than that by ethanol. However, in our MTMAs, the acceleration of the aqueous manual was abundant faster, acceptance the hexadecane to be transported in a actual abbreviate time. As apparent in fig. S7 (C and D), afterwards 300 cycles of aqueous transport, the breach breadth and adaptable modulus began to decline. In addition, the movement of the aqueous was synchronized with the about-face in the allurement movement direction, and there was no hysteresis during a quick affective administration switch, admitting added methods based on alien stimuli crave a continued aeon of time to change the aqueous affective direction.

Table 1 Backdrop of assorted delving liquids calm from the abstract or through class testing.

The ablaze dejected breadth represents the altitude that aerosol can be apprenticed in MAMTs by the alluring force, while the ablaze chicken breadth represents the altitude that aerosol cannot be apprenticed in MAMTs by the alluring force.

View this table:View popupView inline Download high-res angel Accessible in new tab Download Powerpoint Fig. 3 Assorted magnetic-controlled aqueous aerosol in MTMAs.

(A and B) Optical images assuming magnetic-controlled motion of nonviscous liquids, including butyl alcohol, ethyl acetate, 1,2-dichloroethane, and n-hexadecane (movie S3). In all rows, the chicken arrows denote the allurement motion direction. (C) Optical images assuming magnetic-controlled motion of a baptize and booze mixture. The admixture can be apprenticed back the booze agreeable is added than 60%, which corresponds to a CA of beneath than ~65° (movie S4). (D) Optical images assuming magnetic-controlled motion of an booze atom affective up an acclivity (movie S5).

DISCUSSION

To bigger accept the atom active behavior, we abutting assay the active apparatus for aqueous carriage in the MTMAs. Figure 4 illustrates the aqueous carriage behavior in the MTMAs. A aqueous bead in an MTMA with ambit r forms menisci with angles θ1 and θ2 (Fig. 4A). Afterwards the alluring force, the two menisci are symmetric (θ1 ≈ θ2), and the atom is in a anchored accompaniment of automated equilibrium. Back a alluring force is applied, the bendable tube will deform, causing a aberration in the Laplace burden (θR > θA), which will drive the atom against the narrower end (Fig. 4B). Figure 4C shows that the bore of the tube at altered positions declines with the access of alluring acreage (the afterpiece the magnet, the greater the alluring field), admitting the bore of the tube at altered positions afterwards the alluring acreage charcoal about constant. Therefore, the tube deforms to an agee cone-like geometry, which generates an adjustable capillary force to actuate the aqueous in the administration of the allurement movement. The curvature aberration of the menisci at anniversary end of the aqueous cavalcade and acme gives acceleration to a aberration in the Laplace burden (ΔP), which scales as (22, 23)(1)where γ is the aqueous apparent tension, θ is the CA of the liquid, α is the acme angle, x is the ambit of the forerunner from the apex, and L is the breadth of the precursor. In our case, the ambit amid the aqueous bang and the acme is abundant beyond than the breadth of the aqueous bang (x ≫ L). Therefore, the capillary force (driving force, FD) scales as (22, 23)(2)

Download high-res angel Accessible in new tab Download Powerpoint Fig. 4 Apparatus of the magnetic-controlled aqueous carriage behavior.

Menisci formed by a aqueous bead in an MTMA. (A) Abiding state. (B) Altitude for atom motion induced by the tube agee deformation. (C) Snapshot of a atom in its carriage action and the accord amid the accurate actuator and the simplified model.

This implies that the Laplace burden on the forerunner with a baby curvature is consistently beyond than that at the end of atom with a ample curvature; therefore, the resultant capillary force will drive the atom to move against the administration breadth the tube becomes narrower. If the allurement is fixed, the askew breadth of tube is additionally fixed; the atom will boring move to acquisition the automated calm accompaniment again. Now, if we move the allurement to the appropriate ancillary (a actual abbreviate distance), the askew allotment of the tube will be airy as the imposed alluring force becomes weaker. However, a new askew breadth of tube forms, and the resultant capillary force pushes the atom rightward. This indicates that if the allurement keeps moving, the atom additionally moves afterward the magnet. One could brainstorm that there would be an aerial acceleration limit, aloft which the tube and atom do not accept abundant time to deform, and the atom will stop afterward the magnet.

The aloft altercation is accurate for the case breadth the active capillary force dominates. This is accurate for nonviscous and absolutely wetting liquids, as the aggressive force there is negligible. As we all know, about all liquids do not absolutely wet on a PDMS film. The aqueous atom motion is resisted by not alone the adhering attrition (FV) but additionally the adhering force (FA) arising from the CAH. The adhering force is scaled as (22, 23)(3)where W is the breadth of the advancing acquaintance line, which is according to πR, and R is the bore of the MTMA. The appellation Δcosθ = cosθR − cosθA, breadth θR is the abbreviating CA and θA is the advancing CA. The adhering attrition force is accustomed by (22, 23)(4)where η is the accessory of active bendability of the aqueous and υ is the boilerplate acceleration of aqueous motion.

Only back the capillary active force is beyond than the sum of the adhering force and adhering attrition force can a partially wetting aqueous be propelled. For nonviscous liquids, such as ethanol, isopropyl alcohol, ethyl acetate, and water, the CAH was ahead admired as a attrition force for aqueous motion. In our case, the acme bend α of the MTMA is usually aural the ambit of 0.07 to 0.17 according to the aloft calculation. We performed calculations for a alternation of low-viscosity liquids with assorted wettabilities (Table 1) and activate that the beginning CA for atom motion is about 63.2 ± 2.1°. The adding after-effects in table S2 accede able-bodied with the beginning results, which appearance that a beneath adhering aqueous with a CA of beneath than ~65° in MTMAs can be propelled by allure (light dejected breadth in Table 1). Our MTMAs abundantly augment the ambit of apprenticed liquids afterwards acute added analysis of the apparent to accomplish complete wetting. It should additionally be acicular out that the actuation of a college bendability and added berserk aqueous with these MTMAs charcoal arduous (light chicken breadth in Table 1). Besides, due to the ample active force and direct acknowledgment of magnetism, our MTMAs can carriage liquids at a about aerial speed. Based on the aloft discussion, an operating diagram is apparent in fig. S8 for our aftereffect apropos the case of a 10-μl droplet. The diagram shows three audible operating regions: abiding atom transport, allurement disengagement, and arena in which the atom cannot be driven.

In summary, we accept developed a simple, effective, and bargain action to assemble agee alluring PDMS-based tubular microactuators. The MTMAs can auspiciously dispense aqueous motion with a magnet, breeding an adjustable capillary force to actuate a aqueous against the narrower end. Furthermore, the MTMAs do not crave the accession of alluring particles (content and properties); thus, MTMAs can bigger advance the backdrop of alteration the aerosol afterwards contagion and can anticipate the aqueous from agreeable from the alluring particles or disengaging from the magnet. Moreover, lower-viscosity liquids with a CA of beneath than ~65° can be apprenticed with controllable acceleration and direction, afterwards added analysis of the MTMA close bank to accomplish complete wetting. The movement of the aqueous atom can be controlled up to a acceleration of 10 cm s−1, which is, to our knowledge, a actual aerial acceleration of aqueous motion apprenticed by a magnetic-induced capillary force in a bankrupt tubular microactuator. We accept that the continued analysis accompanying to tubular microactuators based on a alluring force will afford new ablaze on aqueous manipulation, with a agnate abeyant for advanced applications.

MATERIALS AND METHODSPreparation of the MTMAs

First, the Sylgard monomer PDMS 184 and the abating abettor were alloyed in a 1:10 ratio. This PDMS prepolymer was coated on the animate capillary with an alien bore of 2 mm and kept in vertical administration at allowance temperature for 6 min, followed by thermal abating at 60°C for 20 min. Second, carbonyl adamant crumb (

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