Smart Manipulation of Complex Optical Elements via Contact‐adaptive Dry Adhesives

Abstract The implementation of complex, high‐precision optical devices or systems, which have vital applications in the aerospace, medical, and military fields, requires the ability to reliably manipulate and assemble optical elements. However, this is a challenging task as these optical elements require contamination‐free and damage‐free manipulation and come in a variety of sizes and shapes. Here, a smart, contact‐adaptive adhesive based on magnetic actuation is developed to address this challenge. Specifically, the surface bio‐inspired adhesives made of fluororubber facilitate contamination‐free and damage‐free adhesion. The stiffness modulation of packaged magnetorheological grease based on the magnetorheological effect endows the smart adhesive with a high conformability to the optical elements in the soft state, a high grip force in the stiff state, and the ability to quickly release the optical elements in the recovered soft state. The smart adhesive provides a versatile solution for reliably and quickly manipulating and assembling multiscale optical elements with planar or complex 3D shapes without causing surface contamination or damage. These extraordinary capabilities are demonstrated by the manipulation and assembly of various optical elements, such as convex/concave/ball lenses and extremely complex‐shaped light guide plates. The proposed smart adhesive is a promising candidate for conventional optical element manipulation technologies.

Section S6.Influence of the magnetic induction on the shear yield stress of magnetorheological grease.
Section S7.Adhesion characterization of the smart adhesive for optical elements with various shapes.
Section S8.Adaption of the smart adhesive to flat glass with misaligned angles.Section S9.Gripping demonstrations of optical elements with various shapes.

Section S1. Fabrication details of the smart adhesive
The fabrication process of the smart adhesive is shown in Figure S1.First, a layer of bioinspired adhesives film made of FKM was prepared.Then, a silicone rubber box with an inner and outer frame was fabricated by cutting the silicone rubber bulk with a thickness of 4 mm, where the silicone rubber bulk was prepared by casting silicone rubber into a mold composed of glass and PET (Polyethylene terephthalate) substrates and subsequent curing.It should be noted that the use of silicone rubber with a 0.5 MPa elastic modulus rather than the more commonly used PDMS with a 2 MPa elastic modulus is to make the packaged smart adhesive's side walls more prone to deformation under the preload, resulting in better conformality with the optical elements.Subsequently, the bio-inspired adhesives film was pasted on the glass and PET substrates, followed by gluing the silicone rubber box onto the bio-inspired adhesives film with a thin spin-coated heat-resistant flexible glue.Then, the silicone rubber box with the bio-inspired adhesives film was placed in a 120℃ oven for 10 min to cure the flexible glue to enhance the bonding of the two (>0.74MPa, much higher than the adhesion strength (~200 kPa) of bio-inspired adhesives film), which ensures the sufficient sealing of the design and prevents the leaking of the magnetorheological grease during the manipulation process.Finally, the magnetorheological grease was poured into the open box, and the partially finished sample was encapsulated with more silicone rubber.After curing and cutting, a finished smart adhesive with a thickness of 4.2 mm was obtained.

Section S2. Fabrication details of the magnetorheological grease
The fabrication process of the magnetorheological grease is shown in Figure S2.First, PDMS base and the 3.6 µm carbonyl iron powder with a mass ratio of 1:5 were mixed in a beaker using a glass rod for an hour.Then, the mixture was degassed in a vacuum chamber for 10 min to obtain finished magnetorheological grease.One thing that should be mentioned is that the mass ratio plays a very important role in stiffness modulation effect of the magnetorheological grease.When the mass ratio is less than 1:3, the magnetic responsiveness of the magnetorheological grease is low, resulting in poor stiffness change.If the mass ratio is more than 1:7, the magnetorheological grease will have excessive viscosity and may even become semi-solid, limiting the stiffness modulation effect significantly.We chose the mass ratio of 1:5 to compromise between the magnetic responsiveness and viscosity to achieve a low stiffness at the soft state and a high stiffness at the stiff state, as far as possible.

Section S3. Fabrication details of the PP mold
The fabrication process of the PP mold is shown in Figure S3.First, a block of 5 mm thick bio-inspired adhesives made of PDMS based on our previous research was prepared.Then, a 4 mm thick PP board was placed on a hot plate (230°C).After the PP board was completely melted, the bio-inspired adhesives made of PDMS were pressed on it for 5 min.Finally, after cooling, the PP mold with inverted bio-inspired adhesive structures was obtained by slightly peeling off the bio-inspired adhesives made of PDMS.One thing to note is that the PP material was chosen as a mold because this crystalline plastic is difficult to dissolve in ethyl acetate.
The SEM pictures of bio-inspired FKM adhesive microstructures with three radii (16, 32.5, 40 µm) fabricated using PP molds are shown in Figure S4, exhibiting good uniformity and consistency.

Section S4. Adhesion and contact stiffness characterization details
The adhesion force test apparatus and test process for the bio-inspired adhesives are shown in   The scratch resistance of the prepared biomimetic FKM layer is also outstanding, and no damage, scratch or fracture is observed on the surface of the structure after 2000 repeated grip and release tests using the apparatus in Figure S6, as shown in the comparative SEM images before and after the tests in Figure S7.

carbon bonds
One reason for the extraordinary fatigue durability and contamination-free adhesion of bioinspired adhesives made of FKM is that the C-F bond energy of FKM is higher (485 KJ/mol) than other carbon bond energies (table S1).

Section S6. Influence of the magnetic induction on the shear yield stress of magnetorheological grease
According to Bingham fluid model, to enable magnetic chains to slip or bend, the applied shear stress τ must be higher than the shear yield stress τ y of magnetorheological grease as per the Eq. ( 1). [53] where, τ is the applied shear stress, τ y is the shear yield stress of magnetorheological grease, η is the plastic viscosity of magnetorheological grease,  is the shear rates.The shear yield stress τ y reflects the shearing stiffness of magnetorheological grease, with the following dependence on magnetic induction according to Fang's research: [52] 2 tanh where, α is related to the susceptibility of the magnetorheological grease and volume fraction or other analogous physical parameters, H represents the magnetic field, H c represents the critical magnetic field for magnetorheological grease.The hysteresis loop of the magnetorheological grease tested by MPMS is shown in Figure S8.
The sample weight is 55.28 mg.The magnetorheological grease can exhibit excellent magnetization properties with 123.46 A•m 2 /kg magnetization at 800.83 kA/m magnetic field strength, which is capable of inducing high shear yield stress.The coercive force and remanence of the magnetorheological grease are 0.8528 kA/m and -0.66269 A•m 2 /kg, respectively, demonstraing that the hysteresis of the magnetorheological grease is very weak.).In addition, at a constant frequency of 5 Hz, the magnetorheological grease behaves similarly to a viscous liquid because its loss modulus is much higher than storage modulus at various strains.The effect of the dispersed state of the magnetic particles in the magnetorheological grease on its response frequency and cycle stability is also discussed, as shown in Figures S10 and S11.
After applying a magnetic field of 200 mT by a permanent magnet, it is obvious that the magnetorheological effect of the magnetorheological grease with stirring for 1 h is slightly better than that with stirring for 1 min at the same response time from Figure S10 due to the better dispersed state of the magnetorheological grease with stirring for 1 h (Movies S1 to S2).Using the apparatus in Figure S6, 10 cycling tests of the magnetorheological effect of the magnetorheological grease are performed.The speed of the apparatus is set to the maximum, i.e. 1100 mm/min, and one cycle time is about 16 s.The magnetic inductions close to and away from the permanent magnet are 200 and 4 mT, respectively.As shown in Figure S11, the magnetorheological effect of the magnetorheological grease with stirring for 1 h is always stable (Movie S3).However, the magnetorheological effect of the magnetorheological grease with stirring for 1 min perfroms poorly in the first few cycles and better and better in the last few cycles due to the more uniform dispersed state induced by the magnetorheological effect (Movie S4).

Section S8. Adaption of the smart adhesive to flat glass with misaligned angles
To demonstrate the adaptability of the smart adhesive to flat glass with misaligned angles, we compared the adhesive force of the smart adhesive and bio-inspired adhesives (with a high stiffness backing) on flat glass with misaligned angles.
Figure S13a shows snapshots of misaligned angles (0°, 0.5°, 1°, 1.5°, and 2°) between flat glass and bio-inspired adhesives in the test processes.The force-time curves of bio-inspired adhesives testing processes on flat glass under varying misaligned angles and a 3-N preload force are shown in Figure S13b.As the misaligned angle grows, the contact area between bioinspired adhesives and flat glass decreases, reducing the adhesion force sharply.The adhesion force at 2° misaligned angle is reduced by 91.3% compared to that of 0° (Figure S13c).
Figure S14a shows snapshots of misaligned angles (0°, 1°, 2°, 3°, and 5°) between flat glass and smart adhesive in the test processes.At 0° misaligned angle, the smart adhesive's adhesive force as a function of the preload under 0 and 73 mT magnetic induction on flat glass is illustrated in Figure S14b, showing that the adhesive force increases with the preload for identical magnetic induction.Figure S14c illustrates the influence of misaligned angles on the adhesive force between smart adhesive and flat glass under 0 and 73 mT magnetic induction.
It demonstrates that due to the stiffness modulation of the smart adhesive, the reduction of adhesive force between the smart adhesive and flat glass is less than the reduction of adhesive force between bio-inspired adhesives and flat glass with the increase of misaligned angle.
When compared to 0° misaligned angle, the adhesion force under 0 and 73 mT magnetic induction at 2° misaligned angle reduced by 79.7% and 53%, respectively.

Figure S1 .
Figure S1.Fabrication process for the smart adhesive.

Figure S3 .
Figure S3.Fabrication process of PP mold.

Figure S4 .
Figure S4.SEM pictures of bio-inspired FKM adhesive microstructures with three radii.

Figure S5 .
Figure S5.Details of the adhesion and contact stiffness characterization.

Figure S6 .
Figure S6.Adhesion force test apparatus for the reusability test of the bio-inspired adhesives.

Figure S7 .
Figure S7.Comparative SEM images before and after 2000 repeated grip and release tests.Figure S8.Hysteresis loop of the magnetorheological grease.Figure S9.Viscoelastic response of the magnetorheological grease.Figure S10.Effect of the dispersed state of the magnetic particles in the magnetorheological grease on its response frequency.Figure S11.Effect of the dispersed state of the magnetic particles in the magnetorheological grease on its cycle stability.Figure S12.Adhesion characterization of the smart adhesive for optical elements with various

Figure S8 .
Figure S7.Comparative SEM images before and after 2000 repeated grip and release tests.Figure S8.Hysteresis loop of the magnetorheological grease.Figure S9.Viscoelastic response of the magnetorheological grease.Figure S10.Effect of the dispersed state of the magnetic particles in the magnetorheological grease on its response frequency.Figure S11.Effect of the dispersed state of the magnetic particles in the magnetorheological grease on its cycle stability.Figure S12.Adhesion characterization of the smart adhesive for optical elements with various

Figure S9 .
Figure S7.Comparative SEM images before and after 2000 repeated grip and release tests.Figure S8.Hysteresis loop of the magnetorheological grease.Figure S9.Viscoelastic response of the magnetorheological grease.Figure S10.Effect of the dispersed state of the magnetic particles in the magnetorheological grease on its response frequency.Figure S11.Effect of the dispersed state of the magnetic particles in the magnetorheological grease on its cycle stability.Figure S12.Adhesion characterization of the smart adhesive for optical elements with various

Figure S10 .
Figure S7.Comparative SEM images before and after 2000 repeated grip and release tests.Figure S8.Hysteresis loop of the magnetorheological grease.Figure S9.Viscoelastic response of the magnetorheological grease.Figure S10.Effect of the dispersed state of the magnetic particles in the magnetorheological grease on its response frequency.Figure S11.Effect of the dispersed state of the magnetic particles in the magnetorheological grease on its cycle stability.Figure S12.Adhesion characterization of the smart adhesive for optical elements with various shapes.

Figure S13 .
Figure S13.Effect of misaligned angles on the adhesive force between bio-inspired adhesives and flat glass.

Figure S14 .
Figure S14.Effect of misaligned angles on the adhesive force between smart adhesive and flat glass.

Figure S15 .
Figure S15.Illustration of the proposed smart adhesive reliably holding planar or complexshaped optical elements ranging from a few millimeters to tens of centimeters.Table S1.Comparison of bond energies between carbon-fluorine bonds and other carbon bonds.Movie S1.Magnetorheological effect of the magnetorheological grease with stirring for 1 h.

Figure S1 .
Figure S1.Fabrication process for the smart adhesive.(i) Casting a layer of dissolved FKM on the prepared PP mold, demolding after cross-linking of FKM and evaporation of solvent.(ii) Preparing a 4 mm thick silicone rubber bulk, curing, and cutting into a box with an inner and outer frame.(iii) Gluing the silicone rubber box onto the bio-inspired adhesives film using a thin spin-coated flexible glue.(iv) Casting the magnetorheological grease into the open box, and encapsulating the semi-finished sample with more silicone rubber, followed by cutting after curing to obtain a finished smart adhesive.

Figure S2 .
Figure S2.Fabrication process of the magnetorheological grease.(i) Adding 16.7wt% PDMS base into a beaker.(ii) Adding another 83.3wt% 3.6 µm carbonyl iron powder into the beaker.(iii) Stirring well for an hour to obtain finished magnetorheological grease.

Figure S3 .
Figure S3.Fabrication process of PP mold.(i) After pressing the bio-inspired adhesives made of PDMS onto the PP board at 230°C for 5 min, (ii) a PP mold is obtained by slightly peeling off the bio-inspired adhesives made of PDMS.

Figure S5a .
Figure S5a.The force test equipment (PT-1198GDP) is supplied by PERFECT Instrument Company, Dongguan, China.A flat glass (5 mm×5 mm) is connected to a load cell with a minimum accuracy of 2 mN through a steel rod.The linear stage of the tester lowers and raises the flat glass in displacement control, and the in-line load cell monitors force throughout the process.A tip/tilt stage is used to facilitate the alignment between the flat glass and the bio-inspired adhesives sample.During the adhesion force test process, the flat glass is pressed against the bio-inspired adhesives sample at a speed of 1 mm/min until the preload is attained, then held for 5 seconds before being pulled up at a speed of 1 mm/min.The contact stiffness test apparatus and test process for the smart adhesive are shown in Figure S5b.The force test equipment (PT-1198GDP) is supplied by PERFECT Instrument Company, Dongguan, China.A spherical probe (10 mm diameter) is connected to a load cell with a minimum accuracy of 2 mN through a steel rod.The linear stage of the tester lowers and raises the spherical probe in displacement control, and the in-line load cell monitors force throughout the process.A tip/tilt stage is used to facilitate the alignment between the spherical probe and the smart adhesive sample.An electromagnet is used to apply different magnetic induction for the smart adhesive sample (20 mm by 20 mm by 4.2 mm).During the contact stiffness test process, after applying different magnetic induction, the spherical probe is pressed against the smart adhesive sample at a speed of 1 mm/min until the preload is attained, then the contact stiffness is measured using K=F/ΔD (the preload force F divided by the pressing depth of spherical probe ΔD).The adhesion force test apparatus and test process for the smart adhesive are shown in FigureS5c.The force test equipment (PT-1198GDP) is supplied by PERFECT Instrument Company, Dongguan, China.The optical elements (convex lenses, concave lenses, ball lens and lens arrays) are connected to a load cell with a minimum accuracy of 2 mN through a steel rod.The linear stage of the tester lowers and raises the optical elements in displacement control, and the in-line load cell monitors force throughout the process.A tip/tilt stage is used to facilitate the alignment between the optical elements and the smart adhesive sample.During the adhesion force test process, the optical elements are pressed against the smart adhesive sample at a speed of 1 mm/min until the preload is attained, after which the magnetic field is triggered and held for 5 seconds before being pulled up at a speed of 1 mm/min.The test apparatus and test process for the adhesive force of the smart adhesive on flat glass with misaligned angles are shown in FigureS5d.The force test equipment (PT-1198GDP) is

Figure S5 .
Figure S5.Details of the adhesion and contact stiffness characterization.(a) Adhesion force test apparatus and test process for the bio-inspired adhesives.(i) Schematic and (ii) photography of the adhesion force test apparatus.(iii) Schematic illustration of the preload-pause-pulling up test process for the adhesion force of the bio-inspired adhesives.(b) The contact stiffness test apparatus and test process for the smart adhesive.(i) Schematic and (ii) photography of the contact stiffness test apparatus.(iii) Schematic illustration of the contact stiffness test process for the smart adhesive.(c) Adhesion force test apparatus and test process for the smart adhesive.(i) Schematic and (ii) photography of the adhesion force test apparatus.(iii) Schematic illustration of the preload-pause-pulling up test process for the adhesion force of the smart

Figure S6 .
Figure S6.Adhesion force test apparatus for the reusability test of the bio-inspired adhesives.Photography of the adhesion force test apparatus (ESM303, Mark-10, USA).

Figure S7 .
Figure S7.Comparative SEM images before and after 2000 repeated grip and release tests.
τ y possesses two limiting behaviors with respect to H: low H, τ y is proportional to H 2 , due to the local saturation of the magnetized particles, of H.For this work, applied magnetic field H (≤ 73 mT) is far below H c , thus τ y is proportional to H 2 .

Figure
FigureS9shows the viscoelastic response of the magnetorheological grease tested by MCR302.It can be seen that the viscosity of the magnetorheological grease exhibits an obvious shear thinning from 14200 Pa•s at 0.1 s -1 shear rate to 0.0106 Pa•s at 1000 s -1 shear rate (FigureS9a) (Inset: Magnified image of the viscosity response over the shear rate range of 0.1 s -1 to 104 s -1 ).In addition, at a constant frequency of 5 Hz, the magnetorheological

Figure S10 .
Figure S10.Effect of the dispersed state of the magnetic particles in the magnetorheological grease on its response frequency.

Figure S11 .
Figure S11.Effect of the dispersed state of the magnetic particles in the magnetorheological grease on its cycle stability.

Figure S13 .
Figure S13.Effect of misaligned angles on the adhesive force between bio-inspired adhesives and flat glass.(a) Snapshots of misaligned angles (0°, 0.5°, 1°, 1.5°, and 2°) between flat glass and bio-inspired adhesives in the test processes.(b) Force-time curves of bio-inspired adhesives testing processes on flat glass under varying misaligned angles and a 3-N preload force.(c) Influence of the misaligned angles on the adhesion force at a 3-N preload.

Figure S14 .
Figure S14.Effect of misaligned angles on the adhesive force between smart adhesive and flat glass.(a)Snapshots of misaligned angles (0°, 1°, 2°, 3°, and 5°) between flat glass and smart adhesive in the test processes.(b) Smart adhesive's adhesive force as a function of the preload for flat glass with 0° misaligned angle under 0, 73 mT magnetic induction.(c) Influence of misaligned angles on the adhesive force between smart adhesive and flat glass under 0 and 73 mT magnetic induction at a 3-N preload.