Supporting Information. Shape-controlled metal-metal and metal-polymer Janus structures by thermoplastic embossing

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1 Supprting Infrmatin Shape-cntrlled metal-metal and metal-plymer Janus structures by thermplastic embssing Mlla Hasan, Nilfar Kahler, and Glden Kumar* Department f Mechanical Engineering, Texas Tech University, Lubbck 79409, TX *Crrespnding authr: Glden Kumar (glden.kumar@ttu.edu) S-1

2 Experimental details Thermplastic embssing experiments were perfrmed in air using a custm-built parallel plate setup illustrated in Figure S1. Tw steel plates (4 4 1 inches 3 ) equipped with cartridge heaters and thermcuples were installed n Instrn 5966 with a lad cell f 10 kn. The plates were lapped and mirrr plished t ensure parallelism. Fr cre-shell Janus structures, a silicn template (clsed r thrugh-etched) was placed n a lwer heating plate. Tw thermplastic materials with an verlapping supercled liquid temperature range were stacked n tp f the template (Figure S1-a). Figure S1: Schematic f c-embssing and additive embssing fabricatin f Janus structures frm thermplastic materials. Cre-shell Janus structure can nly be prduced using cembssing (a) whereas rd shaped Janus structures can be fabricated using bth c-embssing (b) and additive embssing (c). S-

3 The prcessing temperature and lad were selected based n previus knwledge f thermplastic frming f individual materials. The prcessing temperature was selected such that the viscsity f bth thermplastic materials drpped belw Pa.s. At these viscsities, a pressure in the range f MPa was sufficient fr template filling and jining f metallic glasses (MGs). Silicn templates were fabricated by phtlithgraphy and deep-reactive-inetching (DRIE). The silicn templates were disslved in 40% KOH slutin t release the free Janus structures. Rd shaped Janus structures were fabricated by bth c-embssing (Figure 1S-b) and additive embssing (Figure 1S-c). In c-embssing, a thrugh-etched silicn template was sandwiched between tw thermplastic materials. In additive embssing, the materials with a higher T g was embssed against a thrugh-etched template. The filling length can be cntrlled thrugh prcessing temperature and pressure values. Subsequently, the secnd thermplastic materials with a lwer T g was embssed frm the ther end f the template. This prvides an independent cntrl ver the fractins f tw thermplastic materials in Janus rds. Additive embssing was als used t prduce Janus structures frm thermplastic materials that have nn-verlapping supercled liquid temperature ranges. T achieve this, the material with higher T g was patterned with nanrds using prus alumina templates. After etching the alumina in KOH, the patterned sample was embssed n thrugh-etched silicn template. The silicn was cled t the prcessing temperature f lw-t g material which was then embssed frm the ppsite side f the partially filled template. During final embssing, the high-t g nanrds served as anchring sites fr the lw-t g material. Bth c-embssing and additive embssing prduced durable jints amng MGs and thermplastic plymers. Silicn based templates were handled with extreme care due t their fragile nature. Any ther template material can be used as lng as it can withstand the temperature and prcessing cnditins used in thermplastic embssing f metallic glasses. List f suitable template materials and their desirable prperties fr thermplastic frming f MGs are published elsewhere 1-4. Durability f jints Durability f jints frmed by c-embssing and additive embssing f thermplastic materials was tested by rm temperature tensile tests. As described in the manuscript, a strng jining was btained in bth c-embssing and additive embssing f dissimilar MGs. Similar strategy was applied fr jining f MGs and plymers. The bnding between MGs and plymers was less strng cmpared t MG-MG jints but stable Janus structures culd still be frmed. Figure S shws the SEM image f mechanical jint between nan-patterned Pt-based MG and PMMA. After separatin frm the MG, PMMA surface reveals the frmatin f deep hles and stretched ligaments. MG nanstructures did nt fracture during separatin as bserved in pulling f MG- MG jints. Despite weaker adhesin, the PMMA-Pt-based MG Janus pillars remained intact after all the prcessing and handling steps. Therefre, Janus structures frm MGs and plymers can be fabricated by mechanical interlcking f surface nanstructures. S-3

4 Figure S: Fracture f mechanical jint frmed between nan-patterned Pt-based MG and PMMA. After separatin, the PMMA surface shws the negative replica f nan-patterned Ptbased MG indicating the frmatin f intimate cntact between tw materials. Fabricatin f cre-shell Janus structures Cre-shell structures were fabricated using the cncept f blw-mlding f MGs 5-6. A thin sheet f MG can thermplastically inflate int a hllw shell when subjected t an air pressure difference (Figure S3-a). The final thickness and height f shell can be calculated frm the applied pressure, the prcessing time, the viscsity, and the strain-rate 4. The stress develped in hemispherical MG shell during defrmatin is given by: Pd Stress=, (S1) 4δ where P is the pressure difference acrss the MG1, d is the diameter f shell, and δ is the thickness f MG1 sheet. Thermplastic blwing cntinues as lng as the applied stress is higher than the Newtnian flw-stress f MG sheet: Flw stress = 3η 1 ε, (S) Figure S3: Schematic f thermplastic blw-mlding f MGs (a) and fabricatin f cre-shell Janus structures using the blw-mlding cncept (b). MG1 and MG crrespnd t tw different MGs. S-4

5 where η 1 is the viscsity f MG1 supercled liquid and ε is the strain-rate. The strain ε in a hemispherical shell is given by 7 : ε = ln( δ f / δ ), (S3) where δ f is the final thickness f hemispherical shell. The pressure and prcessing time (t) required t achieve the final thickness δ f can be calculated by cmbining Equatins (S1) t (S3): Pd 4δ 3η 1ε 3η 1 > = ln( δ f / δ ), t t 1 1 ηδ Pt > ln( δ f / δ ), (S4) d The shell height (h) can be calculated frm the thickness values using cnservatin f vlume. Equatin (S4) can be used t cntrl the thermplastic prcessing cnditins fr desirable dimensins f hllw MG shells. T fabricate cre-shell Janus structures, we replace air with a thermplastic material, MG (Figure S3-b). The additinal requirement is that the applied pressure shuld be sufficient t fill the cre with MG. The required pressure fr filling cylindrical feature f height h with MG can be estimated using Hagen-Piseuille law: 3η h P= t d δ f, (S5) where η is the viscsity f MG supercled liquid at the c-embssing temperature. Therefre, by using Equatins (S4) and (S5) ne can precisely cntrl the dimensins f cre-shell Janus structures. The initial thickness f MG1 has prfund effect n the utcme f c-embssing prcess but MG can be f any thickness as lng as sufficient amunt f material is supplied t fill the cre. Figure S4 shws examples f large number f cre-shell Janus structures with varying sizes and shapes fabricated by c-embssing f MGs. Figure S4: Examples f cre-shell Janus structure in large quantity (a), circular crss-sectin (b), and square crss-sectin (c). In images a & b, the cres and shells are made f Ni-based MG and Pd-based MG, respectively. In image c, the cre is Pd-based MG whilst the shell is Ni-based MG. S-5

6 Fabricatin f rd shaped Janus structures by c-embssing The rd shaped Janus structures were prduced by simultaneus filling f thrugh-etched templates with tw different thermplastic materials (Figure S5). The fractin f tw materials can be estimated using Hagen-Piseuille law. The pressure required t fill a cylindrical cavity with a Newtnian fluid is given by: 3ηh P=, (S6) td where P is the applied pressure, t is the embssing time, η is the viscsity f thermplastic material, h is the height and d is the diameter f cylindrical cavity. The embssing time t simultaneusly fill the cavity with tw different liquids A and B f viscsities η A and η B is (Figure S5): 3h t = Pd 1 η A + 1 η B. (S7) The fractin f filling lengths f tw liquids is related t their viscsity values as: h h A B η η B = and h h A + hb A =. (S8) Figure S5: Schematic f c-filling prcess using tw different thermplastic materials. Tw thermplastic materials, A and B, are simultaneusly embssed frm the ppsite ends f a thrugh-etched template. Accrding t these equatins under any embssing cnditins the liquid with lwer a viscsity will cnstitute the large fractin f the Janus rd. This limitatin can be alleviated by cntinuing the embssing peratin beynd t t displace the lw viscsity fluid with the high viscsity liquid. It allws cntrlling the lengths f materials in Janus structures despite the fixed rati f their viscsity values. In additin, prcessing temperature can als be used t alter the viscsity rati f c-flwing supercled liquids wing t their different fragility values. Effect f trapped air One f the factrs that may affect the jining f thermplastic materials in thrugh-etched templates is the presence f trapped air. Here, we cnsider the effect f trapped air theretically and experimentally. Initially the template cavities are filled with an air at atmspheric pressure S-6

7 (P i ~ 0.1 MPa) which remains trapped during the c-embssing f thermplastic materials A and B (Figure S6). With the increase in embssing pressure, the vlume (V f ) f trapped air decreases. Fr example, the vlume f trapped air will decrease t ~1/100 f its starting value (V i ) when the embssing pressure reaches 10 MPa (typical embssing pressure). Mrever, the trapped air is displaced twards the sidewalls f the template because f the parablic flw prfiles f thermplastic materials. Therefre, the presence f trapped air des nt severely affect the interface f thermplastic materials during c-embssing using a thrugh-etched template. Figure S6: Effect f trapped air n the interface during cembssing f tw thermplastic materials frm tw ppsite ends f a thrugh-etched template. The trapped air is squeezed twards the sidewalls because f parablic flw prfiles f thermplastic materials. The vlume f trapped air decreases t 1% f its initial vlume at an applied pressure f 10 MPa. T verify this hypthesis, Pt-based metallic glass was thermplastically embssed against a template cavity clsed at ne end. Figure S7 illustrates the prcess and the SEM image f metallic glass after remving frm the template. Despite the presence f trapped air, the tip f metallic glass faithfully replicates the bttm f the template except the edges. This suggests that the trapped air is pushed twards the crners f the template cavity during embssing. Because f parablic prfile f flwing metallic glass supercled liquid, it tuches the template bttm at the center and then expands utwards. At the final stage, majrity f the trapped air is squeezed in the crners. These theretical and experimental demnstratins indicate that the effect f trapped air is very small. Figure S7: An example f Pt-based metallic glass thermplastically embssed against a clsed template cavity. The SEM image shws that the metallic glass replicated the bttm f the template except edges, suggesting that the trapped air is squeezed twards the edges. S-7

8 C-filling f templates frm the same end Filling f templates can als be achieved by c-embssing f tw thermplastic materials flwing side-by-side (Figure S8). This can prduce Janus structures with vertical interface as shwn by the examples. When tw supercled liquids f different viscsities c-flw in a template cavity, the mre viscus fluid has a lwer velcity and fills t lwer depth. Hwever, the velcity prfile at the interface remains cntinuus due t balancing f the shear stress. Figure S8: Variant f cembssing when tw thermplastic materials flw side-by-side int a template cavity. The resulting Janus structures have vertical interface as shwn by example f pillars made frm Ni-based and Pd-based MGs. Multiphasic structures C-embssing and additive embssing can be cmbined t generate multiphasic structures as shwn in Figure S9. The materials with verlapping supercled temperature range are cembssed frm ne side f the thrugh-etched template resulting in frmatin f cre-shell mrphlgy. Subsequently, the material with a lwer prcessing temperature is filled frm the ppsite side f the template. Examples shw pillars cnsisting f three MGs rganised in different arrangements (Figure S9). Figure S9: Fabricatin f structures cmprised f three different thermplastic materials. Initially, materials A and B are c-embssed t frm cre-shell structures. Subsequently, third material C is added t make three-phase structures. Examples shw images f pillars made frm Pt-based, Pd-based, and Ni-based MGs. S-8

9 References (1) Kumar, G.; Desai, A.; Schrers, J., Bulk Metallic Glass: The Smaller The Better. Adv. Mater. 011, 3, () Schrers, J.; Pham, Q.; Desai, A., Thermplastic Frming f Bulk Metallic Glass - A Technlgy fr MEMS and Micrstructure Fabricatin. J. Micrelectrmech. Syst. 007, 16, (3) Schrers, J., Prcessing f Bulk Metallic Glass. Adv. Mater. 010,, (4) Sarac, B.; Kumar, G.; Hdges, T.; Ding, S. Y.; Desai, A.; Schrers, J., Three-Dimensinal Shell Fabricatin Using Blw Mlding f Bulk Metallic Glass. J Micrelectrmech. Syst. 011, 0, (5) Schrers, J.; Pham, Q.; Peker, A.; Patn, N.; Curtis, R. V., Blw Mlding f Bulk Metallic Glass. Scr. Mater. 007, 57, (6) Schrers, J.; Hdges, T. M.; Kumar, G.; Raman, H.; Barnes, A. J.; Quc, P.; Waniuk, T. A., Thermplastic Blw Mlding f Metals. Mater. Tday 011, 14, (7) Dealy, J. M., Official Nmenclature fr Material Functins Describing the Respnse f a Viscelastic Fluid t Varius Shearing and Extensinal Defrmatins. J. Rhel. 1995, 39, S-9