Filter On Indexed, Formatted DATE
You can't use the V_SPECIAL_SAMPLES with an existing index on the sample_date column because of the TO_CHAR in the view.  Your options are

1) Don't use the view and go directly against the sample table.

2) Create a function based index on sample_date that does both the TO_CHAR and the TO_DATE

Something like

CREATE INDEX fbi_sample_date
    ON sample( to_date( to_char( sample_date, 
                                 'mm/dd/yyyy HH24:MI AM' ),
                        'mm/dd/yyyy HH24:MI AM' ));


3) Modify the v_special_samples view to have both the sample_date as a VARCHAR2 and to have the raw DATE value and filter on the raw DATE value.