Viscoelastic_JJ properties_NN2 of_IO oxide-coated_JJ liquid_JJ metals_NN2 I._NP1 INTRODUCTION_NN1 Small_NP1 droplets_NN2 of_IO simple_JJ liquids_NN2 tend_VV0 to_TO be_VBI spherical_JJ because_CS this_DD1 shape_NN1 minimizes_VVZ their_APPGE surface_NN1 area_NN1 and_CC surface_NN1 free_JJ energy_NN1 ._. 
Liquid_JJ droplets_NN2 ,_, and_CC even_RR bubbles_NN2 ,_, can_VM deviate_VVI from_II this_DD1 behavior_NN1 when_RRQ solids_NN2 are_VBR incorporated_VVN onto_II their_APPGE surface_NN1 ,_, thereby_RR "_" solidifying_VVG "_" the_AT liquid_JJ surface_NN1 Subramaniam_NP1 et_RA21 al_RA22 ._. 
2005_MC ;_; Xu_NP1 et_RA21 al_RA22 ._. 
2005_MC ._. 
A_AT1 similar_JJ realization_NN1 of_IO this_DD1 principle_NN1 occurs_VVZ when_RRQ the_AT outer_JJ surface_NN1 of_IO a_AT1 low-viscosity_JJ liquid_JJ metal_NN1 is_VBZ solidified_VVN ,_, in_II atmosphere_NN1 ,_, by_II an_AT1 oxide_NN1 ?_? 
To_TO quantify_VVI the_AT effect_NN1 of_IO oxidation_NN1 on_II material_NN1 properties_NN2 ,_, previous_JJ researchers_NN2 have_VH0 focused_VVN on_II surface_NN1 tension_NN1 measurements_NN2 Eustathopoulos_NN2 and_CC Drevet_VV0 1998_MC ;_; Ricci_NP1 et_RA21 al_RA22 ._. 
2005_MC ._. 
Such_DA measurements_NN2 have_VH0 limited_VVN usefulness_NN1 for_IF understanding_VVG the_AT mechanical_JJ properties_NN2 of_IO the_AT oxide_NN1 skins_NN2 in_II air_NN1 ,_, as_CSA demonstrated_VVN by_II recent_JJ experiments_NN2 of_IO flow_NN1 of_IO liquid_JJ metal_NN1 in_II microchannels_NN2 Dickey_NP1 et_RA21 al_RA22 ._. 
2008_MC ._. 
The_AT liquid-like_JJ characteristics_NN2 of_IO eutectic_JJ gallium_NN1 indium_NN1 EGaInwere_NN1 most_RGT apparent_JJ when_CS the_AT material_NN1 was_VBDZ injected_VVN at_II constant_JJ pressure_NN1 into_II a_AT1 micro_NN1 fluidic_JJ channel_NN1 that_CST became_VVD progressively_RR narrower_JJR ._. 
The_AT narrowing_NN1 of_IO the_AT channel_NN1 caused_VVD the_AT meniscus_NN1 of_IO the_AT EGaIn_JJ to_TO stop_VVI at_II a_AT1 position_NN1 that_CST depended_VVD on_II the_AT applied_JJ pressure_NN1 ._. 
Advancing_VVG the_AT meniscus_NN1 further_RRR required_VVN higher_JJR applied_JJ pressure_NN1 ._. 
A_AT1 force_NN1 balance_NN1 between_II the_AT pressure_NN1 and_CC the_AT surface_NN1 stress_NN1 in_II the_AT static_JJ meniscus_NN1 revealed_VVD the_AT maximum_JJ surface_NN1 stress_NN1 DY_NN1 that_CST the_AT oxide_NN1 skin_NN1 could_VM support_VVI without_IW flowing_JJ ._. 
For_IF EGaIn_NN1 ,_, DY_NP1 =0.630_FO N/m_ZZ1 ,_, a_AT1 value_NN1 that_CST is_VBZ in_II excellent_JJ agreement_NN1 with_IW a_AT1 separate_JJ measurement_NN1 of_IO an_AT1 apparent_JJ surface_NN1 tension_NN1 ,_, in_II atmosphere_NN1 ,_, obtained_VVD using_VVG the_AT pendant_NN1 drop_NN1 method_NN1 0.624_MC N/mZrnic_FU and_CC Swatik_NP1 1969_MC ._. 
This_DD1 result_NN1 implies_VVZ that_CST for_IF this_DD1 particular_JJ case_NN1 ,_, EGaIn_NN1 can_VM be_VBI characterized_VVN in_II31 terms_II32 of_II33 surface_NN1 tension_NN1 ,_, a_AT1 liquid-like_JJ characteristic_NN1 ._. 
In_II this_DD1 respect_NN1 ,_, EGaIn_NN1 is_VBZ not_XX unlike_JJ mercury_NN1 Hg_FO ,_, a_AT1 low-viscosity_JJ liquid_JJ metal_NN1 that_CST does_VDZ not_XX oxidize_VVI in_II atmosphere_NN1 :_: measurements_NN2 of_IO DY_NP1 obtained_VVD with_IW Hg_FO in_II the_AT micro_NN1 fluidic_JJ device_NN1 also_RR agree_VV0 with_IW literature_NN1 values_NN2 of_IO its_APPGE surface_NN1 tension_NN1 0.450_MC and_CC 0.480_MC N/m_NNU ,_, respectively_RR Dickey_NP1 et_RA21 al_RA22 ._. 
2008_MC ._. 
However_RR ,_, despite_II the_AT similarity_NN1 in_II how_RRQ these_DD2 two_MC materials_NN2 flow_VV0 into_II microchannels_NN2 ,_, EGaIn_JJ and_CC Hg_FO exhibited_VVN qualitatively_RR different_JJ behavior_NN1 when_CS the_AT pressure_NN1 was_VBDZ removed_VVN ._. 
II_MC ._. 
EXPERIMENTAL_JJ Our_APPGE experiments_NN2 were_VBDR conducted_VVN using_VVG three_MC types_NN2 of_IO stress-controlled_JJ rheometers_NN2 :_: the_AT AR-G2_NN1 TA_UH Instruments_NN2 ,_, the_AT C-VOR_NN1 ,_, and_CC the_AT Bohlin_NP1 Gemini_NP1 HRnano_NP1 rheometer_NN1 both_RR from_II Malvern_NP1 Instruments_NN2 ._. 
The_AT rheometers_NN2 were_VBDR equipped_VVN with_IW parallel-plate_JJ geometries_NN2 of_IO different_JJ size_NN1 ._. 
In_II this_DD1 configuration_NN1 ,_, the_AT bottom_JJ plate_NN1 is_VBZ stationary_JJ and_CC the_AT upper_JJ plate_NN1 is_VBZ rigidly_RR attached_VVN to_II a_AT1 low-friction_JJ bearing_NN1 ._. 
The_AT upper_JJ plate_NN1 is_VBZ rotated_VVN by_II a_AT1 motor_NN1 that_CST applies_VVZ a_AT1 torque_NN1 to_II the_AT bearing_NN1 ._. 
When_CS the_AT sample_NN1 is_VBZ loaded_VVN between_II the_AT two_MC plates_NN2 ,_, the_AT stress_NN1 in_II the_AT sample_NN1 is_VBZ assumed_VVN to_TO be_VBI proportional_JJ to_TO ,_, and_CC the_AT strain_NN1 is_VBZ assumed_VVN to_TO be_VBI proportional_JJ to_II the_AT bearing_NN1 displacement_NN1 ._. 
In_II most_DAT of_IO the_AT experiments_NN2 with_IW the_AT AR-G2_NN1 ,_, the_AT upper_JJ plate_NN1 material_NN1 was_VBDZ made_VVN of_IO hard_JJ anodized_JJ aluminum_NN1 ,_, and_CC a_AT1 homemade_JJ stainless_JJ steel_NN1 plate_NN1 covered_VVD the_AT lower_JJR plate_NN1 ._. 
In_II the_AT case_NN1 of_IO the_AT Malvern_NP1 rheometers_NN2 ,_, both_DB2 the_AT upper_JJ and_CC lower_JJR plates_NN2 consisted_VVN of_IO stainless_JJ steel_NN1 ._. 
Our_APPGE results_NN2 did_VDD not_XX appear_VVI to_TO depend_VVI significantly_RR on_II the_AT choice_NN1 of_IO the_AT materials_NN2 used_VVD ._. 
RESULTS_NN2 These_DD2 measurements_NN2 were_VBDR performed_VVN on_II EGaIn_NN1 using_VVG the_AT HRnano_NN1 equipped_VVN with_IW parallel_JJ plates_NN2 of_IO 20_MC and_CC 40_MC mm_NNU diameter_NN1 ._. 
The_AT rheometer_NN1 performed_VVD strain_NN1 oscillations_NN2 at_II =1_FO rad/s_FU of_IO first_MD ascending_JJ and_CC then_RT descending_JJ strain_NN1 amplitude_NN1 0_MC and_CC calculated_VVD apparent_JJ linear_JJ viscoelastic_JJ moduli_NN2 G_ZZ1 and_CC G._NP1 We_PPIS2 performed_VVD these_DD2 experiments_NN2 four_MC times_NNT2 for_IF each_DD1 plate_NN1 radius_NN1 ,_, with_IW each_DD1 test_NN1 preceded_VVN by_II a_AT1 strain-rate-controlled_NN1 pre-shear_VV0 (_( &plusmn;1.3_FO or_CC &plusmn;13_FO s-1_FO )_) ,_, applied_VVN for_IF 5_MC min_NNU with_IW no_AT equilibration_NN1 time_NNT1 before_II the_AT oscillatory_JJ strain_NN1 measurements_NN2 ._. 
B._NP1 Steady_JJ and_CC oscillatory_JJ shear_VV0 The_AT elastic_JJ characteristics_NN2 of_IO the_AT oxide_NN1 skin_NN1 are_VBR highly_RR dependent_JJ on_II the_AT shear_VV0 history_NN1 ._. 
Because_CS the_AT shear-history_JJ dependence_NN1 can_VM change_VVI GS_NP1 by_II more_DAR than_CSN an_AT1 order_NN1 of_IO magnitude_NN1 ,_, it_PPH1 can_VM potentially_RR lead_VVI to_II large_JJ discrepancies_NN2 between_II different_JJ measurements_NN2 of_IO the_AT mechanical_JJ properties_NN2 of_IO the_AT oxide_NN1 skin_NN1 ._. 
These_DD2 strain_VV0 history_NN1 effects_NN2 might_VM be_VBI present_JJ in_II other_JJ configurations_NN2 that_CST are_VBR spherically_RR designed_VVN to_II measure_NN1 surface_NN1 rheology_NN1 ._. 
To_TO control_VVI these_DD2 effects_NN2 during_II rheological_JJ measurements_NN2 ,_, it_PPH1 is_VBZ important_JJ to_TO understand_VVI the_AT mechanisms_NN2 that_CST cause_VV0 them_PPHO2 ._. 
We_PPIS2 seek_VV0 to_TO better_RRR understand_VVI the_AT difference_NN1 between_II the_AT stiff_JJ and_CC soft_JJ states_NN2 ,_, by_II first_MD considering_II the_AT conditions_NN2 under_II which_DDQ both_DB2 states_NN2 were_VBDR measured_VVN ._. 
Analysis_NN1 of_IO the_AT strain_NN1 oscillations_NN2 reveals_VVZ that_CST when_CS the_AT material_NN1 is_VBZ in_II the_AT stiff_JJ state_NN1 ,_, significant_JJ internal_JJ stresses_NN2 from_II the_AT pre-shear_VV0 are_VBR stored_VVN in_II the_AT oxide_NN1 film_NN1 ._. 
These_DD2 stresses_NN2 vanish_VV0 during_II the_AT large_JJ oscillations_NN2 and_CC are_VBR not_XX present_JJ in_II the_AT soft_JJ state_NN1 ,_, suggesting_VVG that_CST the_AT internal_JJ stress_NN1 plays_VVZ an_AT1 important_JJ role_NN1 in_II determining_VVG GS_NP1 ._. 
The_AT string_NN1 will_VM minimize_VVI its_APPGE stretching_NN1 by_II tracing_VVG the_AT path_NN1 of_IO shortest_JJT distance_NN1 between_II the_AT attachment_NN1 points_NN2 of_IO the_AT string_NN1 ._. 
If_CS this_DD1 path_NN1 is_VBZ projected_VVN onto_II one_MC1 of_IO the_AT plates_NN2 ,_, it_PPH1 will_VM not_XX follow_VVI the_AT arc_NN1 of_IO the_AT circles_NN2 but_CCB will_VM be_VBI a_AT1 chord_NN1 see_VV0 Fig.7_FO ._. 
If_CS the_AT same_DA parallel_JJ plates_NN2 are_VBR now_RT connected_VVN by_II a_AT1 thin_JJ elastic_JJ membrane_NN1 attached_VVN to_II the_AT outer_JJ rims_NN2 of_IO the_AT plates_NN2 ,_, then_RT relative_JJ rotation_NN1 of_IO the_AT plates_NN2 will_VM cause_VVI the_AT membrane_NN1 to_TO stretch_VVI ._. 
If_CS the_AT stretched_JJ elastic_JJ elements_NN2 of_IO the_AT membrane_NN1 are_VBR modeled_VVN as_II a_AT1 series_NN of_IO elastic_JJ strings_NN2 regularly_RR spaced_VVN around_II the_AT circumference_NN1 of_IO the_AT plates_NN2 ,_, then_RT upon_II shearing_VVG ,_, the_AT strings_NN2 will_VM trace_VVI the_AT path_NN1 of_IO shortest_JJT distance_NN1 between_II their_APPGE end_NN1 points_NN2 and_CC the_AT membrane_NN1 will_VM bow_VVI inward_RL ._. 
The_AT tendency_NN1 of_IO the_AT membrane_NN1 to_TO bow_VVI inward_RL when_CS sheared_VVN tends_VVZ to_TO decrease_VVI the_AT volume_NN1 enclosed_VVN by_II the_AT membrane_NN1 and_CC the_AT parallel_JJ plates_NN2 ._. 
However_RR ,_, if_CS this_DD1 space_NN1 is_VBZ occupied_VVN by_II an_AT1 incompressible_JJ fluid_NN1 ,_, then_RT the_AT fluid_NN1 can_VM accommodate_VVI the_AT bowing_NN1 of_IO the_AT outer_JJ membrane_NN1 by_II slightly_RR displacing_VVG the_AT parallel_JJ plates_NN2 away_II21 from_II22 each_PPX221 other_PPX222 ._. 
In_II our_APPGE experiments_NN2 ,_, this_DD1 displacement_NN1 is_VBZ detected_VVN by_II the_AT strain_NN1 gauges_NN2 ._. 
The_AT relaxation_NN1 of_IO the_AT tension_NN1 in_II the_AT elastic_JJ strings_NN2 is_VBZ accompanied_VVN by_II a_AT1 decrease_NN1 in_II the_AT measured_JJ pressure_NN1 p_NN1 of_IO the_AT bulk_NN1 ._. 
The_AT stress_NN1 relaxation_NN1 experiment_NN1 is_VBZ similar_JJ to_II the_AT strain-controlled_JJ oscillations_NN2 that_CST we_PPIS2 performed_VVD using_VVG two_MC different_JJ plate_NN1 size_NN1 with_IW the_AT HRnano_NN1 ._. 
The_AT oscillations_NN2 were_VBDR performed_VVN immediately_RR following_VVG a_AT1 pre-shear_VV0 and_CC without_IW allowing_VVG the_AT material_NN1 to_TO relax_VVI ._. 
In_II the_AT oscillatory_JJ experiments_NN2 ,_, the_AT steady_JJ stress_NN1 SE_NP1 vanished_VVD when_RRQ 0_MC became_VVD sufficiently_RR large_JJ ,_, which_DDQ occurred_VVD after_CS about_RG 15_MC min_NNU of_IO oscillations_NN2 ._. 
However_RR ,_, in_II the_AT stress_NN1 relaxation_NN1 experiment_NN1 in_II which_DDQ no_AT oscillations_NN2 were_VBDR performed_VVN ,_, the_AT steady_JJ stress_NN1 persisted_VVN for_IF several_DA2 hours_NNT2 ,_, in_II both_RR Ga_NP1 and_CC EGaIn_NN1 ._. 
This_DD1 result_NN1 indicates_VVZ that_CST the_AT large_JJ oscillations_NN2 do_VD0 indeed_RR play_VVI an_AT1 essential_JJ role_NN1 in_II causing_VVG SE_ND1 to_TO vanish_VVI during_II the_AT oscillatory_JJ tests_NN2 ._. 
E._NP1 Time-dependence_NN1 Our_APPGE analysis_NN1 is_VBZ performed_VVN on_II the_AT raw_JJ angular_JJ displacement_NN1 and_CC normal_JJ force_NN1 data_NN that_CST are_VBR provided_VVN by_II the_AT rheometer_NN1 during_II the_AT equilibration_NN1 period_NN1 at_II a_AT1 rate_NN1 of_IO 250_MC data_NN points_NN2 per_II second_NNT1 ._. 
We_PPIS2 convert_VV0 these_DD2 values_NN2 to_II and_CC p_ZZ1 ,_, respectively_RR ,_, and_CC observe_VV0 that_CST immediately_RR following_VVG the_AT cessation_NN1 of_IO the_AT pre-shear_VV0 ,_, the_AT rheometer_NN1 reverses_VVZ direction_NN1 and_CC exhibits_NN2 damped_VVD ringing_VVG about_II a_AT1 new_JJ equilibrium_NN1 position_NN1 ._. 
The_AT ringing_NN1 is_VBZ due_II21 to_II22 a_AT1 combination_NN1 of_IO sample_NN1 elasticity_NN1 and_CC rheometer_JJR inertia_NN1 ,_, and_CC the_AT damping_NN1 occurs_VVZ because_II21 of_II22 viscous_JJ dissipation_NN1 in_II the_AT sample_NN1 ._. 
The_AT slow_JJ elastic_JJ recoil_NN1 that_CST follows_VVZ the_AT pre-shear_VV0 persists_VVZ for_IF the_AT entire_JJ equilibration_NN1 period_NN1 ,_, even_CS21 when_CS22 the_AT equilibration_NN1 is_VBZ as_CS31 long_CS32 as_CS33 1_MC1 h_ZZ1 ._. 
The_AT recoil_NN1 indicates_VVZ that_CST the_AT elastic_NN1 stresses_VVZ that_CST were_VBDR stored_VVN during_II the_AT preceding_JJ pre-shear_VV0 are_VBR slowly_RR relaxing_VVG ._. 
Because_CS p_ZZ1 is_VBZ an_AT1 increasing_JJ function_NN1 of_IO S_ZZ1 ,_, we_PPIS2 expect_VV0 that_CST the_AT relaxation_NN1 of_IO S_ZZ1 should_VM correspond_VVI to_II a_AT1 decrease_NN1 in_II p_ZZ1 ._. 
We_PPIS2 test_VV0 this_DD1 hypothesis_NN1 by_II binning_VVG the_AT raw_JJ measurements_NN2 of_IO p_ZZ1 obtained_VVN from_II the_AT rheometer_NN1 into_II increments_NN2 that_CST are_VBR linearly_RR spaced_VVN in_II time_NNT1 and_CC averaging_VVG all_DB the_AT p_NN1 values_NN2 within_II each_DD1 bin_NN1 ._. 
We_PPIS2 then_RT plot_VV0 the_AT average_JJ p_NN1 value_NN1 from_II each_DD1 bin_NN1 as_II a_AT1 function_NN1 of_IO the_AT average_JJ time_NNT1 of_IO each_DD1 bin_NN1 ._. 
Free_JJ oscillations_NN2 typically_RR damp_VV0 out_RP until_CS the_AT material_NN1 effectively_RR arrests_VVZ at_II an_AT1 equilibrium_NN1 strain_NN1 ._. 
However_RR ,_, a_AT1 static_JJ equilibrium_NN1 strain_NN1 was_VBDZ not_XX observed_VVN in_II our_APPGE samples_NN2 ._. 
At_II times_NNT2 greater_JJR than_CSN about_RG 5_MC s_ZZ1 ,_, the_AT oscillations_NN2 did_VDD not_XX damp_VVI out_RP completely_RR but_CCB rather_RG continued_JJ to_TO vibrate_VVI throughout_II the_AT equilibration_NN1 time_NNT1 at_II a_AT1 small_JJ amplitude_NN1 that_CST varied_VVD in_II time_NNT1 ._. 
At_II times_NNT2 ,_, the_AT amplitude_NN1 of_IO the_AT vibrations_NN2 gradually_RR increased_VVN ,_, and_CC at_II other_JJ times_NNT2 it_PPH1 gradually_RR decreased_VVN ,_, as_CSA is_VBZ apparent_JJ in_II Fig._NN1 10a_FO ,_, and_CC in_II detail_NN1 in_II Fig._NN1 13a_FO ._. 
These_DD2 small_JJ oscillations_NN2 occur_VV0 because_CS the_AT sample_NN1 exhibits_VVZ a_AT1 combination_NN1 of_IO low_JJ viscosity_NN1 and_CC high_JJ elasticity_NN1 that_CST makes_VVZ it_PPH1 sensitive_JJ to_II minor_JJ perturbations_NN2 from_II either_RR the_AT environment_NN1 or_CC the_AT instrument_NN1 itself_PPX1 ._. 
The_AT time-dependence_NN1 of_IO GSduring_VVG the_AT equilibrium_NN1 period_NN1 indicates_VVZ that_CST a_AT1 sample_NN1 in_II the_AT stiff_JJ state_NN1 does_VDZ not_XX evolve_VVI to_II the_AT soft_JJ state_NN1 when_CS the_AT applied_JJ stress_NN1 is_VBZ zero_MC ._. 
This_DD1 observation_NN1 ,_, however_RR ,_, does_VDZ not_XX apply_VVI to_II relaxation_NN1 that_CST occurs_VVZ under_II constant_JJ strain_NN1 conditions_NN2 ._. 
At_II the_AT conclusion_NN1 of_IO the_AT stress_NN1 relaxation_NN1 experiment_NN1 ,_, p_ZZ1 had_VHD descended_VVN to_II 120_MC Pa_NP1 ,_, not_XX far_RR from_II the_AT value_NN1 of_IO about_RG 150_MC Pa_NNU that_DD1 is_VBZ associated_VVN with_IW soft_JJ state_NN1 ._. 
Although_CS we_PPIS2 did_VDD not_XX perform_VVI controlled_JJ oscillations_NN2 following_VVG the_AT stress_NN1 relaxation_NN1 ,_, we_PPIS2 were_VBDR able_JK to_TO collect_VVI about_RG 9_MC s_ZZ1 of_IO passive_JJ strain_NN1 vibrations_NN2 from_II raw_JJ data_NN that_CST was_VBDZ taken_VVN less_DAR than_CSN 2_MC min_NNU after_II the_AT conclusion_NN1 of_IO the_AT stress_NN1 relaxation_NN1 ,_, and_CC immediately_RR preceding_VVG the_AT onset_NN1 of_IO the_AT pre-shear_VV0 associated_VVN with_IW the_AT following_JJ oscillatory_JJ tests_NN2 ._. 
A_ZZ1 Fourier_NP1 transform_VV0 of_IO these_DD2 vibrations_NN2 indicates_VVZ that_CST G'S=1.70&plusmn;0.1_FO N/m_ZZ1 and_CC p=-146_FO Pa_NP1 ._. 
These_DD2 values_NN2 are_VBR representative_JJ of_IO the_AT soft_JJ state_NN1 and_CC indicate_VV0 that_CST the_AT soft_JJ state_NN1 can_VM be_VBI obtained_VVN by_II allowing_VVG stress_NN1 relaxation_NN1 to_TO occur_VVI under_II constant_JJ strain_NN1 conditions_NN2 ._. 
IV_MC ._. 
DISCUSSION_NN1 Our_APPGE results_NN2 suggest_VV0 the_AT possibility_NN1 of_IO rationalizing_VVG the_AT mechanism_NN1 responsible_JJ for_IF the_AT difference_NN1 between_II the_AT stiff_JJ and_CC soft_JJ states_NN2 in_II31 terms_II32 of_II33 our_APPGE simple_JJ visualization_NN1 of_IO Fig._NN1 7_MC that_CST treats_VVZ the_AT membrane_NN1 as_II a_AT1 series_NN of_IO elastic_JJ strings_NN2 ._. 
When_CS the_AT plates_NN2 are_VBR rotated_VVN and_CC the_AT strings_NN2 are_VBR stretched_VVN to_II their_APPGE maximum_JJ extension_NN1 about_II =0.02_FO ,_, they_PPHS2 exert_VV0 a_AT1 positive_JJ pressure_NN1 on_II the_AT bulk_NN1 liquid_NN1 ._. 
If_CS the_AT plates_NN2 are_VBR maintained_VVN at_II this_DD1 angular_JJ displacement_NN1 ,_, the_AT strings_NN2 can_VM only_RR relax_VVI their_APPGE tension_NN1 by_II increasing_VVG their_APPGE equilibrium_NN1 length_NN1 ._. 
In_II physical_JJ terms_NN2 ,_, this_DD1 may_VM correspond_VVI to_II additional_JJ oxidation_NN1 that_CST replaces_VVZ strained_JJ oxide_NN1 elements_NN2 with_IW unstrained_JJ elements_NN2 ._. 
Such_DA oxidation_NN1 would_VM likely_RR increase_VVI the_AT surface_NN1 area_NN1 of_IO the_AT oxide_NN1 film_NN1 ._. 
When_CS the_AT torque_NN1 that_CST was_VBDZ stretching_VVG the_AT film_NN1 is_VBZ subsequently_RR set_VVN to_TO zero_VVI ,_, the_AT film_NN1 will_VM be_VBI less_RGR taut_JJ because_CS it_PPH1 has_VHZ a_AT1 larger_JJR surface_NN1 area_NN1 ._. 
The_AT more_RGR "_" baggy_JJ "_" or_CC wrinkled_JJ film_NN1 will_VM exert_VVI less_DAR pressure_NN1 on_II the_AT bulk_NN1 liquid_NN1 ._. 
The_AT wrinkled_JJ film_NN1 might_VM also_RR exhibit_VVI a_AT1 lower_JJR apparent_JJ elastic_JJ modulus_NN1 because_CS it_PPH1 can_VM accommodate_VVI deformation_NN1 simply_RR by_II changing_VVG its_APPGE shape_NN1 rather_II21 than_II22 straining_VVG the_AT oxide_NN1 skin_NN1 ._. 
Because_II21 of_II22 this_DD1 ,_, we_PPIS2 expect_VV0 that_CST apparent_JJ elasticity_NN1 measured_VVN in_II the_AT soft_JJ state_NN1 does_VDZ not_XX represent_VVI the_AT intrinsic_JJ properties_NN2 of_IO the_AT material_NN1 but_CCB is_VBZ dependent_JJ on_II the_AT particular_JJ shape_NN1 that_CST the_AT material_NN1 has_VHZ achieved_VVN due_II21 to_II22 the_AT combination_NN1 of_IO both_RR stress_VV0 and_CC oxidation_NN1 in_II its_APPGE strain_NN1 history_NN1 ._. 
V._II CONCLUSIONS_NN2 The_AT dependence_NN1 of_IO our_APPGE measurements_NN2 on_II both_RR time_NNT1 and_CC strain_VV0 history_NN1 have_VH0 the_AT potential_NN1 to_TO significantly_RR complicate_VVI elasticity_NN1 measurements_NN2 ._. 
However_RR ,_, we_PPIS2 have_VH0 shown_VVN that_CST ,_, at_RR21 least_RR22 for_IF the_AT parallel_JJ plate_NN1 geometry_NN1 ,_, the_AT state_NN1 of_IO the_AT material_NN1 can_VM be_VBI reset_VVN through_II a_AT1 shear_VV0 rejuvenation_NN1 step_VV0 that_CST returns_VVZ GS_NP1 to_II an_AT1 average_JJ value_NN1 that_CST is_VBZ fairly_RR independent_JJ of_IO time_NNT1 ._. 
The_AT mechanism_NN1 by_II which_DDQ shear_VV0 can_VM bring_VVI about_RP these_DD2 effects_NN2 is_VBZ not_XX clear_JJ and_CC will_VM hopefully_RR be_VBI elucidated_VVN by_II future_JJ measurements_NN2 ._. 
We_PPIS2 have_VH0 rationalized_VVN both_RR the_AT time-dependant_JJ increase_NN1 and_CC decrease_VV0 in_II GS_NP1 in_II31 terms_II32 of_II33 additional_JJ oxidation_NN1 of_IO the_AT oxide_NN1 film_NN1 ._. 
Oxidation_NN1 may_VM decrease_VVI the_AT apparent_JJ elasticity_NN1 by_II increasing_VVG the_AT surface_NN1 area_NN1 of_IO the_AT oxide_NN1 film_NN1 thereby_RR increasing_VVG its_APPGE "_" bagginess_NN1 ._. "_" 
However_RR ,_, oxidation_NN1 might_VM also_RR reinforce_VVI the_AT oxide_NN1 film_NN1 ,_, thereby_RR increasing_VVG the_AT apparent_JJ elasticity_NN1 ._. 
If_CS oxidation_NN1 truly_RR is_VBZ responsible_JJ for_IF these_DD2 effects_NN2 ,_, then_RT we_PPIS2 expect_VV0 that_CST these_DD2 effects_NN2 may_VM also_RR be_VBI present_JJ in_II other_JJ geometries_NN2 that_CST are_VBR typically_RR used_VVN to_TO characterize_VVI surface_NN1 rheology_NN1 ._. 
Although_CS our_APPGE experiments_NN2 were_VBDR performed_VVN only_RR on_II liquid_JJ metals_NN2 ,_, similar_JJ behavior_NN1 may_VM extend_VVI to_II a_AT1 wider_JJR range_NN1 of_IO complex_JJ fluids_NN2 consisting_VVG of_IO liquids_NN2 coated_VVN by_II solid-like_JJ shells_NN2 ._. 
These_DD2 results_NN2 show_VV0 that_CST even_RR in_II a_AT1 simple_JJ geometry_NN1 ,_, under_II simple_JJ shear_VV0 ,_, solid-coated_JJ liquids_NN2 can_VM display_VVI complex_JJ mechanical_JJ behavior_NN1 ._. 
A_AT1 better_JJR understanding_NN1 of_IO the_AT mechanisms_NN2 governing_VVG the_AT properties_NN2 of_IO solid-coated_JJ liquids_NN2 will_VM facilitate_VVI their_APPGE rheological_JJ characterization_NN1 ,_, as_II31 well_II32 as_II33 the_AT use_NN1 of_IO these_DD2 materials_NN2 in_II advanced_JJ technologies_NN2 ._. 
